Boosting n-type doping levels of Ge with co-doping by integrating plasma-assisted atomic layer deposition and flash annealing process
Baik, Seunghun, Hyeokjin Kwon, Chuck Paeng, He Zhang, Bodo Kalkofen, Jae Eun Jang, Y. S. Kim, Hyuk-Jun Kwon
IEEE Electron Device Letters 40 (9), pp 1507-1510 (2019)
abstractTo achieve a high concentration of dopants over 1 x 1020 cm(-3) on germanium (Ge), co-doping with phosphorus (P) and antimony (Sb) by plasma assisted atomic layer deposition (PALD) and a subsequent annealing process [rapid thermal annealing process (RTP) or flash lamp annealing process (FLP)] are proposed and investigated. We found that the PALD stacked co-doping (POx/SiOy and Sb2O5) films were uniformly deposited. Using the conventional RTP method led to a low doping concentration ( < 3 x 1019 cm−3). However, FLP with a Xenon (Xe) lamp (lamp duration: 3 ms; energy density: 56 J/cm(2)) raised the surface temperature to nearly 800 °C. Furthermore, high concentrations of both P and Sb ( > 1 x 1020 cm−3) were achieved at the surface. Our findings suggest that the FLP with high energy in a short amount of time ( ∼ 3 ms) can create the peak power effect and the co-doping effect. The evidence shows that these effects contribute to enhancing n-type doping levels in the Ge structure.
Magnetoelectric and HR-STEM investigations on eutectic CoFe2O4-Ba1−xSrxTiO3 composites
Breitenbach, Martin, Hakan Deniz, Stefan G. Ebbinghaus
Journal of Physics and Chemistry of Solids 135, pp 109076/1-8 (2019)
abstractMultiferroic Ba1−xSrxTiO3-CoFe2O4 (x = 0.03, 0.05) composites with rarely investigated 3-3 connectivity were prepared by eutectic crystallization in an optical floating zone furnace. High-resolution scanning transmission electron microscopy investigations of the CoFe2O4-BaTiO3 interface revealed an almost perfect connection between both components. These micrographs also showed that the impact of post-annealing in air was much larger than expected and resulted in formation of small BaTiO3 inclusions in the CoFe2O4 phase. The magnetoelectric coefficient αME was studied in detail with respect to its dependence on the static magnetic field, the frequency of the driving AC-field and temperature. Furthermore, the influence of different growth rates (5, 10 and 20 mm h−1), chemical composition, sample thickness and the alignment of electrical polarization and magnetic field (collinear or vertical) on the magnetoelectric properties were studied. The largest value of αME = 1.3 mV Oe−1 cm−1 was found for a sample grown at 5 mm h−1. For even slower growth rates, a higher Sr content was required to avoid the formation of impurity phases leading to a decrease of αME.
Lock-in thermography based local solar cell analysis for high efficiency monocrystalline hetero junction type solar cells
Breitenstein, O., D. Sontag
Solar Energy Materials and Solar Cells 193, pp 157-162 (2019)
abstractUntil now the Dark Lock-in Thermography (DLIT) based "Local I-V" method for analyzing the inhomogeneity of the two-diode parameters of solar cells was applied mostly to multicrystalline silicon cells. In this contribution it is applied to three high-efficiency monocrystalline cells of the hetero junction type (HJT), one showing very good and two showing slightly degraded cell parameters. It is shown that the degradation of the lower performing cells is mainly due to additional local J02-type current contributions, which are partly stemming from surface injuries and mainly degrade the fill factor. It can be estimated that the local inhomogeneous dark current contributions, which can be imaged and quantified by DLIT, degrade the efficiency of the best cell only by 0.1%, but that of the stronger degraded cells by 0.3% and 0.6% (absolute) respectively. Under reduced illumination intensity the differences between the cells are even higher. The possibilities and limitations of the "Local I-V" evaluation of solar cells for investigating high-efficiency cells are discussed.
Lock-in thermography for analyzing solar cells and failure analysis in other electronic components
Breitenstein, O., S. Sturm
Quantitative InfraRed Thermography Journal 2019, pp 1-15 (2019)
abstractLock-in thermography (LIT) is a dynamic variant of infrared thermography, where local heat sources are periodically pulsed and amplitude and phase images of the surface temperature modulation are obtained. If used in electronic device testing, this method enables the localization of very weak local heat sources below the surface. This contribution reviews the basics and application of LIT for local efficiency analysis of solar cells and for failure analysis in other electronic components like bare and encapsulated integrated circuits. In both application fields LIT has established as a reliable and easy-to-use standard method for failure analysis.
Enhanced spontaneous polarization in ultrathin SnTe films with layered antipolar structure
Chang, Kai, Thaneshwor Kaloni, Haicheng Lin, Amilcar Bedoya-Pinto, Avanindra Pandeya, Ilya Kostanovskiy, Kun Zhao, Yong Zhong, Xiaopeng Hu, Qi-Kun Xue, Xi Chen, Shuai-Hua Ji, Salvador Barraza-Lopez, Stuart S. Parkin
Advanced Materials 31 (3), pp 1804428/1-7 (2019)
abstract2D SnTe films with a thickness of as little as 2 atomic layers (ALs) have recently been shown to be ferroelectric with in-plane polarization. Remarkably, they exhibit transition temperatures (Tc) much higher than that of bulk SnTe. Here, combining molecular beam epitaxy, variable temperature scanning tunneling microscopy, and ab initio calculations, the underlying mechanism of the Tc enhancement is unveiled, which relies on the formation of γ-SnTe, a van der Waals orthorhombic phase with antipolar inter-layer coupling in few-AL thick SnTe films. In this phase, 4n - 2 AL (n = 1, 2, 3...) thick films are found to possess finite in-plane polarization (space group Pmn21), while 4n AL thick films have zero total polarization (space group Pnma). Above 8 AL, the γ-SnTe phase becomes metastable, and can convert irreversibly to the bulk rock salt phase as the temperature is increased. This finding unambiguously bridges experiments on ultrathin SnTe films with predictions of robust ferroelectricity in GeS-type monochalcogenide monolayers. The observed high transition temperature, together with the strong spin-orbit coupling and van der Waals structure, underlines the potential of atomically thin γ-SnTe films for the development of novel spontaneous polarization-based devices.
Standing waves induced by valley-mismatched domains in ferroelectric SnTe monolayers
Chang, Kai, Brandon J. Miller, Hao Yang, Haicheng Lin, Stuart S. Parkin, Salvador Barraza-Lopez, Qi-Kun Xue, Xi Chen, Shuai-Hua Ji
Physical Review Letters 122 (20), pp 206402/1-6 (2019)
abstractTwo-dimensional (2D) quasiparticle standing waves originate from the interference of coherent quantum states and are usually created by the scattering off edges, atomic steps, or adatoms that induce large potential barriers. We report standing waves close to the valence band maximum (EV), confined by electrically neutral domain walls of newly discovered ferroelectric SnTe monolayers, as revealed by spatially resolved scanning tunneling spectroscopy. Ab initio calculations show that this novel confinement arises from the polarization lifted hole valley degeneracy and a  ∼ 90° rotation of the Brillouin zones that render holes" momentum mismatched across neighboring domains. These results show a potential for polarization-tuned valleytronics in 2D ferroelectrics.
The growth and phase distribution of ultrathin SnTe on graphene
Chang, Kai, Stuart S. Parkin
APL Materials 7 (4), pp 041102/1-7 (2019)
abstractRecently, a monolayer of SnTe was discovered to be a two-dimensional ferroelectric with an in-plane polarization, and, most dramatically, it exhibits a significant enhancement of the ferroelectric phase transition temperature compared to its bulk counterpart. This phenomenon is due to a structural phase transition from bulk-like α/β-SnTe, a topological crystalline insulator, to layered γ-SnTe as the thickness is decreased to a few atomic layers. A detailed understanding of the growth mechanism and phase distribution of ultrathin SnTe films are of great interest for potential applications. Here, we report detailed studies of the molecular beam epitaxial growth and in situ scanning tunneling microscopy characterization of ultrathin SnTe films on graphene substrates. By varying the growth conditions, SnTe can be prepared as either a continuous film or in the form of large rectangular plates. The rate of nucleation of SnTe was found to be highly sensitive to the substrate temperature. The coexistence and competition between the β and γ phases formed at room temperature was studied, and the phase diagram with respect to the average thickness of SnTe and the substrate temperature during growth is drawn
Low-field switching of noncollinear spin texture at La0.7Sr0.3MnO3-SrRuO3 interfaces
Das, S., A. D. Rata, I. V. Maznichenko, S. Agrestini, E. Pippel, N. Gauquelin, J. Verbeeck, K. Chen, S. M. Valvidares, H. Babu Vasili, J. Herrero-Martin, E. Pellegrin, K. Nenkov, A. Herklotz, A. Ernst, I. Mertig, Z. Hu, K. Dörr
Physical Review B 99 (2), pp 024416/1-11 (2019)
abstractInterfaces of ferroic oxides can show complex magnetic textures which have strong impact on spintronics devices. This has been demonstrated recently for interfaces with insulating antiferromagnets such as BiFeO3. Here, noncollinear spin textures which can be switched in very low magnetic field are reported for conducting ferromagnetic bilayers of La0.7Sr0.3MnO3-SrRuO3 (LSMO-SRO). The magnetic order and switching are fundamentally different for bilayers coherently grown in reversed stacking sequence. The SRO top layer forms a persistent exchange spring which is antiferromagnetically coupled to LSMO and drives switching in low fields of a few milliteslas. Density functional theory reveals the crucial impact of the interface termination on the strength of Mn-Ru exchange coupling across the interface. The observation of an exchange spring agrees with ultrastrong coupling for the MnO2/SrO termination. Our results demonstrate low-field switching of noncollinear spin textures at an interface between conducting oxides, opening a pathway for manipulating and utilizing electron transport phenomena in controlled spin textures at oxide interfaces.
Giant intrinsic spin Hall effect in W3Ta and other A15 superconductors
Derunova, E., Y. Sun, C. Felser, S. S. P. Parkin, B. Yan, M. N. Ali
Science Advances 5 (4), pp eaav8575/1-7 (2019)
abstractThe spin Hall effect (SHE) is the conversion of charge current to spin current, and nonmagnetic metals with large SHEs are extremely sought after for spintronic applications, but their rarity has stifled widespread use. Here, we predict and explain the large intrinsic SHE in β-W and the A15 family of superconductors: W3Ta, Ta3Sb, and Cr3Ir having spin Hall conductivities (SHCs) of -2250, -1400, and 1210 \frache (S/cm), respectively. Combining concepts from topological physics with the dependence of the SHE on the spin Berry curvature (SBC) of the electronic bands, we propose a simple strategy to rapidly search for materials with large intrinsic SHEs based on the following ideas: High symmetry combined with heavy atoms gives rise to multiple Dirac-like crossings in the electronic structure; without sufficient symmetry protection, these crossings gap due to spin-orbit coupling; and gapped crossings create large SBC.
Tetragonal Mn3Sn Heusler films with large perpendicular magnetic anisotropy deposited on metallic MnN underlayers using amorphous substrates
Ferrante, Yari, Jaewoo Jeong, Rana Saha, Sergey V. Faleev, Mahesh G. Samant, Teya Topuria, Hakan Deniz, Stuart S. Parkin
APL Materials 7 (3 ), pp 031103/1-8 (2019)
abstractTetragonal Heusler compounds that exhibit large perpendicular magnetic anisotropy are promising materials for advanced spintronic devices. A prerequisite are thin films whose tetragonal axis is oriented perpendicular to the plane of the films. Here we show that highly textured, (001) oriented, tetragonal Mn3Sn layers can be prepared using metallic zinc-blende (ZB) MnN as underlayers. Moreover, we show that these layers can be deposited on amorphous substrates using reactive magnetron sputtering. The ferrimagnetic Mn3Sn layers exhibit perpendicularly magnetized hysteresis loops with coercive fields of  ∼ 2 T. Stoichiometric ZB-MnN underlayers share an "equivalent" Mn-Mn layer at the interface with Mn3Sn, thus promoting their oriented growth. Other nitride underlayers are not effective due to their rock-salt (RS) crystal structure and the absence of Mn. Density functional theory calculations confirm that tetragonal Mn3Sn Heusler films are energetically stable when interfaced with ZB-MnN underlayers and not with any of the other RS nitride underlayers considered here. Such Heusler compounds have much promise as electrodes for magnetic tunnel junction memory elements for deeply scaled magnetic random access memories.
Luminescence based high resolution finite element simulation of inhomogeneous solar cells
Frühauf, Felix, J. Wong, Otwin Breitenstein
Solar Energy Materials and Solar Cells 189, pp 133-137 (2019)
abstractThe application of the model of independent diodes with an effective series resistance in units of Omega cm(2) leads to wrong predictions of the local diode voltages in solar cells, if their dark current is strongly inhomogeneous. Therefore, multicrystalline solar cells should be modelled by finite element methods such as that implemented in Griddler. Until now, the local parameters for this modelling were obtained for inhomogeneous solar cells by evaluating luminescence and dark lock-in thermography (DLIT) images. The inhomogeneous local diode parameters, in particular the saturation current density J(01), were obtained by the DLIT-based analysis (Local I-V), and the local emitter contact resistances were obtained by evaluating luminescence-based local diode voltages. This method, however, is experimentally demanding and shows a limited spatial resolution due to thermal blurring of the DLIT results. Previous luminescence-based methods for imaging J(01), based on the model of independent diodes, have been found to be erroneous. In this model it is assumed that each elementary cell region (image pixel) is connected to the terminals of the cell by an independent series resistance (Trupke et al., 2007) [1]. Meanwhile alternative luminescence evaluation methods are available for reliably imaging J(01) with high spatial resolution. This opens the way for a purely luminescence-based finite element simulation of inhomogeneous solar cells. In this contribution this method is described and applied to a multicrystalline PERC cell made from high performance silicon material. The results are compared to that of a luminescence plus DLIT based evaluation.
Non-filamentary non-volatile memory elements as synapses in neuromorphic systems
Fumarola, Alessandro, Y. Leblebici, P. Narayanan, R.M. Shelby, L.L. Sanchez, G.W. Burr, K. Moon, J. Jang, H. Hwang, S. Sidler
19th Non-Volatile Memory Technology Symposium (NVMTS), pp 8986194/1-6 (2019)
abstractCrossbar arrays of non-volatile memory (NVM) devices represent one possible path for implementing highly energy-efficient neuromorphic computing systems. For Deep Neural Networks (DNN), where information can be encoded as analog voltage and current levels, such arrays can represent matrices of synaptic weights, implementing the matrix-vector multiplication needed for algorithms such as backpropagation in a massively-parallel fashion. Previous research demonstrated a large-scale hardware-software implementation based on phase-change memories and analyzed the potential speed and power advantages over GPU-based training. In this proceeding we will discuss extensions of this work leveraging a different class of memory elements. Using the concept of jump-tables we simulate the impact of real conductance response of non-filamentary resistive devices based on Pr0.3Ca0.7MnO3 (PCMO). With the same approach as of [1], we simulate a three-layer neural network with training accuracy > 90% on the MNIST dataset. The higher ON/OFF conductance ratio of improved Al/Mo/PCMO devices together with new programming strategies can lead to further accuracy improvement. Finally, we show that the bidirectional programming of Al/Mo/PCMO can be used to implement high-density neuromorphic systems with a single conductance per synapse, at only a slight degradation to accuracy.
Electrical writing, deleting, reading, and moving of magnetic skyrmioniums in a racetrack device
Göbel, Börge, Alexander Schäffer, Jamal Berakdar, Ingrid Mertig, Stuart S. Parkin
Scientific Reports 9, pp 12119/1-8 (2019)
abstractA magnetic skyrmionium (also called 2 π-skyrmion) can be understood as a skyrmion - a topologically nontrivial magnetic whirl - which is situated in the center of a second skyrmion with reversed magnetization. Here, we propose a new optoelectrical writing and deleting mechanism for skyrmioniums in thin films, as well as a reading mechanism based on the topological Hall voltage. Furthermore, we point out advantages for utilizing skyrmioniums as carriers of information in comparison to skyrmions with respect to the current-driven motion. We simulate all four constituents of an operating skyrmionium-based racetrack storage device: creation, motion, detection and deletion of bits. The existence of a skyrmionium is thereby interpreted as a `1' and its absence as a `0' bit.
Atomic layer deposition of metals: Precursors and film growth
Hagen, Dirk, M. E. Pemble, M. Karppinen
Applied Physics Reviews 6 (4), pp 5087759/1-63 (2019)
abstractThe coating of complex three-dimensional structures with ultrathin metal films is of great interest for current technical applications, particularly in microelectronics, as well as for basic research on, for example, photonics or spintronics. While atomic layer deposition (ALD) has become a well-established fabrication method for thin oxide films on such geometries, attempts to develop ALD processes for elemental metal films have met with only mixed success. This can be understood by the lack of suitable precursors for many metals, the difficulty in reducing the metal cations to the metallic state, and the nature of metals as such, in particular their tendency to agglomerate to isolated islands. In this review, we will discuss these three challenges in detail for the example of Cu, for which ALD has been studied extensively due to its importance for microelectronic fabrication processes. Moreover, we give a comprehensive overview over metal ALD, ranging from a short summary of the early research on the ALD of the platinoid metals, which has meanwhile become an established technology, to very recent developments that target the ALD of electropositive metals. Finally, we discuss the most important applications of metal ALD.
Author Correction: `Lattice strain-enhanced exsolution of nanoparticles in thin films (vol 10, 1471, 2019)´
Han, Hyeon, Jucheol Park, Sang Yeol Nam, Kun Joong Kim, Gyeong Man Choi, Stuart S. Parkin, Hyun Myung Jang, John T. S. Irvine
Nature Communications 10, pp 2083 (2019)
Lattice strain-enhanced exsolution of nanoparticles in thin films
Han, Hyeon, Jucheol Park, Sang Yeol Nam, Kun Joong Kim, Gyeong Man Choi, Stuart S. Parkin, Hyun Myung Jang, John T. S. Irvine
Nature Communications 10, pp 1471/1-8 (2019)
abstractNanoparticles formed on oxide surfaces are of key importance in many fields such as catalysis and renewable energy. Here, we control B-site exsolution via lattice strain to achieve a high degree of exsolution of nanoparticles in perovskite thin films: more than 1100 particles μm−2 with a particle size as small as similar to 5 nm can be achieved via strain control. Compressive-strained films show a larger number of exsolved particles as compared with tensile-strained films. Moreover, the strain-enhanced in situ growth of nanoparticles offers high thermal stability and coking resistance, a low reduction temperature (550 °C), rapid release of particles, and wide tunability. The mechanism of lattice strain-enhanced exsolution is illuminated by thermodynamic and kinetic aspects, emphasizing the unique role of the misfit-strain relaxation energy. This study provides critical insights not only into the design of new forms of nanostructures but also to applications ranging from catalysis, energy conversion/storage, nano-composites, nano-magnetism, to nano-optics.
Nonlinear magnetization dynamics driven by strong terahertz fields
Hudl, Matthias, Massimilia d' Aquino, Matteo Pancaldi, See-Hun Yang, Mahesh G. Samant, Stuart S. Parkin, Hermann A. Dürr, Claudio Serpico, Matthias C Hoffmann, Stefano Bonetti
Physical Review Letters 123 (19), pp 197204/1-6 (2019)
abstractWe present a comprehensive experimental and numerical study of magnetization dynamics in a thin metallic film triggered by single-cycle terahertz pulses of  ∼ 20 MV/m electric field amplitude and  ∼ 1 ps duration. The experimental dynamics is probed using the femtosecond magneto-optical Kerr effect, and it is reproduced numerically using macrospin simulations. The magnetization dynamics can be decomposed in three distinct processes: a coherent precession of the magnetization around the terahertz magnetic field, an ultrafast demagnetization that suddenly changes the anisotropy of the film, and a uniform precession around the equilibrium effective field that is relaxed on the nanosecond time scale, consistent with a Gilbert damping process. Macrospin simulations quantitatively reproduce the observed dynamics, and allow us to predict that novel nonlinear magnetization dynamics regimes can be attained with existing tabletop terahertz sources.
Robust antiskyrmion phase in bulk tetragonal Mn-Pt(Pd)-Sn Heusler system probed by magnetic entropy change and AC-susceptibility measurements
Jamaluddin, Sk, Subhendu K Manna, Bimalesh Giri, P. V. Prak Madduri, Stuart S. Parkin, Ajaya K. Nayak
Advanced Functional Materials 29 (24), pp 1901776/1-6 (2019)
abstractMagnetic skyrmions, topologically protected chiral spin textures having potential applications in data storage, are stabilized in certain magnetic materials with broken inversion symmetry. The existence of magnetic antiskyrmions has been recently demonstrated in thin plates of a tetragonal Heusler material with D2d crystal symmetry. Here, the robust nature of the antiskyrmion phase in bulk tetragonal Mn-Pt(Pd)-Sn compounds by utilizing magnetic entropy change and AC-susceptibility measurements is shown. It is found that the formation of the antiskyrmion phase is accompanied by a positive magnetic entropy change, which is supported by the concomitant observation of an anomaly in AC-susceptibility measurements. Supporting these findings, no anomalies are found in AC-susceptibility and magnetic entropy change measurements for a Mn-Pt(Pd)-Sn compound that is stabilized in the cubic phase by slight changes in chemical composition, thereby showing the robustness of the antiskyrmion phase to the D2d crystal structure.
From an atomic layer to the bulk: Low-temperature atomistic structure and ferroelectric and electronic properties of SnTe films
Kaloni, Thaneshwor, Kai Chang, Brandon J. Miller, Qi-Kun Xue, Xi Chen, Shuai-Hua Ji, Stuart S. Parkin, Salvador Barraza-Lopez
Physical Review B 99 (13), pp 134108/1-8 (2019)
abstractSnTe hosts ferroelectricity that competes with its weak nontrivial band topology: in the high-symmetry rocksalt structure-in which its intrinsic electric dipole is quenched-this material develops metallic surface bands, but in its rhombic ground-state configuration-which hosts a nonzero spontaneous electric dipole-the crystalline symmetry is lowered, and the presence of surface electronic bands is not guaranteed. Here, the type of ferroelectric coupling and the atomistic and electronic structure of SnTe films ranging from 2 to 40 atomic layers (ALs) are examined on freestanding samples, to which atomic layers were gradually added. Four-AL SnTe films are antiferroelectrically coupled, while thicker freestanding SnTe films are ferroelectrically coupled. The electronic band gap reduces its magnitude in going from 2 to 40 ALs, but it does not close due to the rhombic nature of the structure. These results bridge the structure of SnTe films from the monolayer to the bulk.
RTSim: A cycle-accurate simulator for racetrack memories
Khan, Asif Ali, Fazal Hameed, Robin Bläsing, Stuart S. Parkin, Jeronimo Castrillon
IEEE Computer Architecture Letters 18 (1), pp 43-46 (2019)
abstractRacetrack memories (RTMs) have drawn considerable attention from computer architects of late. Owing to the ultra-high capacity and comparable access latency to SRAM, RTMs are promising candidates to revolutionize the memory subsystem. In order to evaluate their performance and suitability at various levels in the memory hierarchy, it is crucial to have RTM-specific simulation tools that accurately model their behavior and enable exhaustive design space exploration. To this end, we propose RTSim, an open source cycle-accurate memory simulator that enables performance evaluation of the domain-wall-based racetrack memories. The skyrmions-based RTMs can also be modeled with RTSim because they are architecturally similar to domain-wall-based RTMs. RTSim is developed in collaboration with physicists and computer scientists. It accurately models RTM-specific shift operations, access ports management and the sequence of memory commands beside handling the routine read/write operations. RTSim is built on top of NVMain2.0. offering larger design space for exploration.
Shiftsreduce: Minimizing shifts in racetrack memory 4.0
Khan, Asif Ali, Fazal Hameed, Robin Bläsing, Stuart S. Parkin, Jeronimo Castrillon
ACM Transactions on Architecture and Code Optimization 16 (4), pp 56/1-23 (2019)
abstractRacetrack memories (RMs) have significantly evolved since their conception in 2008, making them a serious contender in the field of emerging memory technologies. Despite key technological advancements, the access latency and energy consumption of an RM-based system are still highly influenced by the number of shift operations. These operations are required to move bits to the right positions in the racetracks. This article presents data-placement techniques for RMs that maximize the likelihood that consecutive references access nearby memory locations at runtime, thereby minimizing the number of shifts. We present an integer linear programming (ILP) formulation for optimal data placement in RMs, and we revisit existing offset assignment heuristics, originally proposed for random-access memories. We introduce a novel heuristic tailored to a realistic RM and combine it with a genetic search to further improve the solution. We show a reduction in the number of shifts of up to 52.5%, outperforming the state of the art by up to 16.1%.
Facet-dependent in situ growth of nanoparticles in epitaxial thin films: The Role of interfacial energy
Kim, Kun Joong, Hyeon Han, Thomas Defferriere, Daseob Yoon, Suenhyoeng Na, Sun Jae Kim, Amir Masou Dayaghi, Junwoo Son, Tae-Sik Oh, Hyun Myung Jang, Gyeong Man Choi
Journal of the American Chemical Society 141 (18), pp 7509-7517 (2019)
abstractNucleation of nanoparticles using the exsolution phenomenon is a promising pathway to design durable and active materials for catalysis and renewable energy. Here, we focus on the impact of surface orientation of the host lattice on the nucleation dynamics to resolve questions with regards to "preferential nucleation sites". For this, we carried out a systematic model study on three differently oriented perovskite thin films. Remarkably, in contrast to the previous bulk powder-based study suggesting that the (110) surface is a preferred plane for exsolution, we identify that other planes such as (001)- and (111)-facets also reveal vigorous exsolution. Moreover, particle size and surface coverage vary significantly depending on the surface orientation. Exsolution of (111)-oriented film produces the largest number of particles, the smallest particle size, the deepest embedment, and the smallest and most uniform interparticle distance among the oriented films. Based on classic nucleation theory, we elucidate that the differences in interfacial energies as a function of substrate orientation play a crucial role in controlling the distinct morphology and nucleation behavior of exsolved nanoparticles. Our finding suggests new design principles for tunable solid-state catalyst or nanoscale metal decoration.
Extremely high conductivity observed in the triple point topological metal MoP
Kumar, Nitesh, Yan Sun, Michael Nicklas, Sarah J. Watzman, Olga Young, Inge Leermakers, Jacob Hornung, Johannes Klotz, Johannes Gooth, Kaustuv Manna, Vicky Suess, Satya N. Guin, Tobias Förster, Marcus Schmidt, Lukas Müchler, Binghai Yan, Peter Werner, Walter Schnelle, Uli Zeitler, Jochen Wosnitza, Stuart S. Parkin, Claudia Felser, Chandra Shekhar
Nature Communications 10, pp 2475/1-6 (2019)
abstractWeyl and Dirac fermions have created much attention in condensed matter physics and materials science. Recently, several additional distinct types of fermions have been predicted. Here, we report ultra-high electrical conductivity in MoP at low temperature, which has recently been established as a triple point fermion material. We show that the electrical resistivity is 6 nΩ cm at 2 K with a large mean free path of 11 microns. de Haas-van Alphen oscillations reveal spin splitting of the Fermi surfaces. In contrast to noble metals with similar conductivity and number of carriers, the magnetoresistance in MoP does not saturate up to 9 T at 2 K. Interestingly, the momentum relaxing time of the electrons is found to be more than 15 times larger than the quantum coherence time. This difference between the scattering scales shows that momentum conserving scattering dominates in MoP at low temperatures.
Current-induced magnetization switching by the high spin hall conductivity α-W
Liao, Wei-Bang, Tian-Yue Chen, Yari Ferrante, Stuart S. Parkin, Chi-Feng Pai
Physica Status Solidi RRL 2019, pp 1900408/1-7 (2019)
abstractThe spin Hall effect originating from 5d heavy transition-metal thin films such as Pt, Ta, and W is able to generate efficient spin-orbit torques that can switch adjacent magnetic layers. This mechanism can serve as an alternative to conventional spin-transfer torque for controlling next-generation magnetic memories. Among all 5d transition metals, W in its resistive amorphous phase typically shows the largest spin-orbit torque efficiency ≈ 0.20-0.50. In contrast, its conductive and crystalline alpha phase possesses a significantly smaller efficiency of ≈ 0.03 and no spin-orbit torque switching is realized using α-W thin films as the spin Hall source. Herein, through a comprehensive study of high-quality W/CoFeB/MgO and the reversed MgO/CoFeB/W magnetic heterostructures, it is shown that although amorphous-W has a greater spin-orbit torque efficiency, the spin Hall conductivity of α-W (|σSHα−W| = 3.71x105−1 m−1) is ≈ 3.5 times larger than that of amorphous W (|σSHamorphousW| = 1.05x105−1 m−1). Moreover, spin-orbit torque-driven magnetization switching using a MgO/CoFeB/α-W heterostructure is demonstrated. The findings suggest that the conductive and high spin Hall conductivity α-W is a potential candidate for future low-power consumption spin-orbit torque memory applications.
Dirac nodal arc semimetal PtSn4: an ideal platform for understanding surface properties and catalysis for hydrogen evolution
Li, Guowei, Chenguang Fu, Wujun Shi, Lin Jiao, Jiquan Wu, Qun Yang, Rana Saha, Machteld E Kamminga, Abhay K. Srivastava, Enke Liu, Aliza N. Yazdani, Nitesh Kumar, Jian Zhang, Graeme R. Blake, Xianjie Liu, Mats Fahlman, Steffen Wirth, Gudrun Auffermann, Johannes Gooth, Stuart S. Parkin, Vidya Madhavan, Xinliang Feng, Yan Sun, Claudia Felser
Angewandte Chemie International Edition 58 (37), pp 13107-13112 (2019)
abstractConductivity, carrier mobility, and a suitable Gibbs free energy are important criteria that determine the performance of catalysts for a hydrogen evolution reaction (HER). However, it is a challenge to combine these factors into a single compound. Herein, we discover a superior electrocatalyst for a HER in the recently identified Dirac nodal arc semimetal PtSn4. The determined turnover frequency (TOF) for each active site of PtSn4 is 1.54 H2s−1 at 100 mV. This sets a benchmark for HER catalysis on Pt-based noble metals and earth-abundant metal catalysts. We make use of the robust surface states of PtSn4 as their electrons can be transferred to the adsorbed hydrogen atoms in the catalytic process more efficiently. In addition, PtSn4 displays excellent chemical and electrochemical stabilities after long-term exposure in air and long-time HER stability tests.
In situ modification of delafossite-type PdCoO2 bulk single crystal for reversible hydrogen sorption and fast hydrogen evolution
Li, Guowei, Seunghyun Khim, Celesta S. Chang, Chenguang Fu, Nabhanila Nandi, Fan Li, Qun Yang, Graeme R. Blake, Stuart S. Parkin, Gudrun Auffermann, Yan Sun, David A. Muller, Andrew P. Mackenzie, Claudia Felser
ACS Energy Letters 4 (9), pp 2185-2191 (2019)
abstractThe observation of extraordinarily high conductivity in delafossite-type PdCoO2 is of great current interest, and there is some evidence that electrons behave like a fluid when flowing in bulk crystals of PdCoO2. Thus, this material is an ideal platform for the study of the electron transfer processes in heterogeneous reactions. Here, we report the use of bulk single crystal PdCoO2 as a promising electrocatalyst for hydrogen evolution reactions (HERs). An overpotential of only 31 mV results in a current density of 10 mA cm−2, accompanied by excellent long-term stability. We have precisely determined that the crystal surface structure is modified after electrochemical activation with the formation of strained Pd nanoclusters in the surface layer. These nanoclusters exhibit excellent hydrogen sorption/desorption reversibility, creating more active sites for hydrogen access. The bulk PdCoO2 single crystal with ultra-high conductivity, which acts as a natural substrate for the Pd nanoclusters, provides a high-speed channel for electron transfer.
Surface states in bulk single crystal of topological semimetal Co3Sn2S2 toward water oxidation
Li, Guowei, Qiunan Xu, Wujun Shi, Chenguang Fu, Lin Jiao, Machteld E Kamminga, Mingquan Yu, Harun Tueysuez, Nitesh Kumar, Vicky Süss, Rana Saha, Abhay K. Srivastava, Steffen Wirth, Gudrun Auffermann, Johannes Gooth, Stuart S. Parkin, Yan Sun, Enke Liu, Claudia Felser
Science Advances 5 (8), pp eaaw9867/1-9 (2019)
abstractThe band inversion in topological phase matters bring exotic physical properties such as the topologically protected surface states (TSS). They strongly influence the surface electronic structures of the materials and could serve as a good platform to gain insight into the surface reactions. Here we synthesized high-quality bulk single crystals of Co3Sn2S2 that naturally hosts the band structure of a topological semimetal. This guarantees the existence of robust TSS from the Co atoms. Co3Sn2S2 crystals expose their Kagome lattice that constructed by Co atoms and have high electrical conductivity. They serves as catalytic centers for oxygen evolution process (OER), making bonding and electron transfer more efficient due to the partially filled orbital. The bulk single crystal exhibits outstanding OER catalytic performance, although the surface area is much smaller than that of Co-based nanostructured catalysts. Our findings emphasize the importance of tailoring TSS for the rational design of high-activity electrocatalysts.
Optimization of catalytic active sites in non-collinear antiferromagnetic Mn3Pt bulk single-crystal
Li, G., Q. Yang, K. Manna, C. Fu, Hakan Deniz, Jagannath Jena, Fan Li, Stuart S. Parkin, G. Auffermann, Y. Sun, Claudia Felser
Materials Today Physics 10, pp 100137/1-6 (2019)
abstractElectrons in non-collinear antiferromagnets exhibit abundant transfer properties of interest to next-generation innovative devices. As two of the most important properties of electrons, both charge and spin must be simultaneously transferred. This will certainly influence many surface reaction processes like the hydrogen evolution reaction (HER). We grow a Mn3Pt bulk single-crystal that having a room-temperature long-range magnetic order at the Mn sites, which showed Pt-like activity and excellent stability as a catalyst for HER. Experiments and density-functional-theory calculations reveal that the electronic structure can be modified owing to the spin polarization of the Mn atoms. This further affects the adsorption energy of the reaction intermediate by tailoring the arrangement and filling of d-electrons. With this strategy, a similar Gibbs free energy for hydrogen adsorption was obtained between Mn-Mn hollow sites and Pt sites. In other words, more actives sites beyond Pt are created. This study paves the way for the design of high-efficiency electrocatalysts through the interplay between the spin states and the adsorption-desorption behaviors.
Dirac surface states in intrinsic magnetic topological insulators EuSn2As2 and MnBi2nTe3n+1
Li, Hang, Shun-Ye Gao, Shao-Feng Duan, Yuan-Feng Xu, Ke-Jia Zhu, Shang-Jie Tian, Jia-Cheng Gao, Wen-Hui Fan, Zhi-Cheng Rao, Jie-Rui Huang, Jia-Jun Li, Yu Yan, Zheng-Tai Liu, Wan-Ling Liu, Yao-Bo Huang, Yu-Liang Li, Yi Liu, Guo-Bin Zhang, Peng Zhang, Takeshi Kondo, Shik Shin, He-Chang Lei, You-Guo Shi, Wen-Tao Zhang, Hong-Ming Weng, Tian Qian, Hong Ding
Physical Review X 9, pp 041039/1-10 (2019)
abstractIn magnetic topological insulators (TIs), the interplay between magnetic order and nontrivial topology can induce fascinating topological quantum phenomena, such as the quantum anomalous Hall effect, chiral Majorana fermions, and axion electrodynamics. Recently, a great deal of attention has been focused on the intrinsic magnetic TIs, where disorder effects can be eliminated to a large extent, which is expected to facilitate the emergence of topological quantum phenomena. Despite intensive efforts, experimental evidence of the topological surface states (SSs) remains elusive. Here, by combining first-principles calculations and angle-resolved photoemission spectroscopy (ARPES) experiments, we reveal that EuSn2As2 is an antiferromagnetic TI with the observation of Dirac SSs consistent with our prediction. We also observe nearly gapless Dirac SSs in antiferromagnetic TIs MnBi2nTe3n+1 (n = 1 and 2), which are absent in previous ARPES results. These results provide clear evidence for nontrivial topology of these intrinsic magnetic TIs. Furthermore, we find that the topological SSs show no observable changes across the magnetic transition within the experimental resolution, indicating that the magnetic order has a quite small effect on the topological SSs, which can be attributed to weak hybridization between the localized magnetic moments, from either 4f or 3d orbitals, and the topological electronic states. This finding provides insights for further research that the correlations between magnetism and topological states need to be strengthened to induce larger gaps in the topological SSs, which will facilitate the realization of topological quantum phenomena at higher temperatures.
Magnetic Weyl semimetal phase in a Kagomé crystal
Liu, D. F., A. J. Liang, E. K. Lui, Q. N. Xu, Y. W. Li, C. Chen, D. Pei, W. J. Shi, S. K. Mo, T. Kim, C. Cacho, G. Li, Y. Sun, L. X. Yang, Z. K. Liu, Stuart S. Parkin, C. Felser, Y. L. Chen
Science 365 (6459), pp 1282-1285 (2019)
abstractWeyl semimetals are crystalline solids that host emergent relativistic Weyl fermions and have characteristic surface Fermi-arcs in their electronic structure. Weyl semimetals with broken time reversal symmetry are difficult to identify unambiguously. In this work, using angle-resolved photoemission spectroscopy, we visualized the electronic structure of the ferromagnetic crystal Co3Sn2S22 and discovered its characteristic surface Fermi-arcs and linear bulk band dispersions across the Weyl points. These results establish Co3Sn2S2 as a magnetic Weyl semimetal that may serve as a platform for realizing phenomena such as chiral magnetic effects, unusually large anomalous Hall effect and quantum anomalous Hall effect.
Theoretical and experimental evidence for the intrinsic three-dimensional Dirac state in Cu2HgSnSe4
Lv, Yang-Yang, Lin Cao, Qian-Qian Yuan, Si-Si Chen, Zhi-Qiang Shi, Qi-Yuan Li, Y. B. Chen, Shu-Hua Yao, Jian Zhou, Huaiqiang Wang, Haijun Zhang, Shao-Chun Li, Defa Liu, Yan-Feng Chen
Physical Review B 100 (19), pp 195147/1-7 (2019)
abstractThree-dimensional (3D) Dirac and Weyl semimetals are quantum states that have emerged in physics recently. But their intrinsic transport properties are quite elusive because of either the coexistence of Schrodinger fermions or the deviation of linear dispersion at Fermi level in previously proposed Dirac and Weyl semimetals. Here, we provide the theoretical and experimental evidences of an intrinsic Dirac state in the quaternary chalcogenide Cu2HgSnSe4 that has bared linear dispersions in conduction bands. Scanning tunneling spectroscopy reveals the quadratic energy-dependent density of states within an extremely large energy range ( ∼ 400 meV) on conduction bands of Cu2HgSnSe4, which is self-consistent with linear dispersion detected by angle-resolved photoemission spectroscopy. In electron-doped Cu2HgSnSe4, positive magnetoresistance at low magnetic field B( < 2.5 T) and negative magnetoresistance under high B are observed, which is attributed to the chiral anomaly effect. However, conventional negative magnetoresistance is observed in hole-doped Cu2HgSnSe4, which is attributed to weak localization broken by B. Remarkably, the carrier mobility has a 105-fold decrease when the Fermi level is adjusted from conduction to valence bands. Our results suggest that Cu2HgSnSe4 not only provides a playground for exploring intrinsic properties of 3D Dirac fermions but also is promising for developing high-speed, low-dissipation electronic devices.
Proposal to detect dark matter using axionic topological antiferromagnets
Marsh, David. J., Kin Chung Fong, Erik W. Lentz, Libor Šmejkal, Mazhar N. Ali
Physical Review Letters 123, pp 121601/1-7 (2019)
abstractAntiferromagnetically doped topological insulators (ATI) are among the candidates to host dynamical axion fields and axion polaritons, weakly interacting quasiparticles that are analogous to the dark axion, a long sought after candidate dark matter particle. Here we demonstrate that using the axion quasiparticle antiferromagnetic resonance in ATIs in conjunction with low-noise methods of detecting THz photons presents a viable route to detect axion dark matter with a mass of 0.7 to 3.5 meV, a range currently inaccessible to other dark matter detection experiments and proposals. The benefits of this method at high frequency are the tunability of the resonance with applied magnetic field, and the use of ATI samples with volumes much larger than 1 #8201; #8201;mm3.
Protecting private communications in cyber-physical systems through physical unclonable functions
Perez-Jimenez, Marina, Borja Bord Sanchez, Andrea Migliorini, Ramon Alcarria
Electronics 8 (4), pp 390/1-22 (2019)
abstractCyber-physical systems (CPS) are envisioned to change the whole of society. New engineered systems joining physical and digital solutions are being employed in industry, education, etc. These new systems are networked by default, and private information is shared among the different components related to users, critical infrastructures, or business operations. In this context, it is essential to encrypt those communication links to protect such information. However, even most complicated schemes based on hybrid (asymmetric and symmetric) solutions, finally require physical devices to store a secret key. This approach is cryptographically weak, as any person with physical access to the device could obtain that key. Therefore, in this paper we propose the use of physical unclonable functions (PUF) to generate secret keys for lightweight encryption schemes. Using PUFs, any attempt to capture the key is changing the original secret stream, and even manufacturers are not able to build two identical PUFs. The proposed key generator is based on magnetic materials and lightweight pseudorandom number generators to meet the low-cost and small size requirements of CPS. In particular, materials with an activated exchange-bias effect are employed, together with simple copper coils. The encryption process can be based on a simple XOR gate because of the robustness of the proposed key generator. In order to evaluate the performance of the proposed technology, an experimental validation based on simulation scenarios is also provided.
A bismuth triiodide monosheet on Bi2Se3(0001)
Polyakoy, Andrey, Katayoon Mohseni, German R. Castro, Juan Rubio-Zuazo, Alexander Zeugner, Anna Isaeva, Ying-Jiun Chen, Christian Tusche, Holger L. Meyerheim
Scientific Reports 9, pp 4052/1-9 (2019)
abstractA stable Bil(3) monosheet has been grown for the first time on the (0001) surface of the topological insulator Bi2Se3 as confirmed by scanning tunnelling microscopy, surface X-ray diffraction, and X-ray photoemision spectroscopy. Bil(3) is deposited by molecular beam epitaxy from the crystalline BiTel precursor that undergoes decomposition sublimation. The key fragment of the bulk Bil3 structure, a2 [l-Bi-l] layer of edge-sharing Bil6 octahedra, is preserved in the ultra-thin film limit, but exhibits large atomic relaxations. The stacking sequence of the trilayers and alternations of the Bi-l distances in the monosheet are the same as in the bulk Bil3 structure. Momentum resolved photoemission spectroscopy indicates a direct band gap of 1.2 eV. The Dirac surface state is completely destroyed and a new flat band appears in the band gap of the Bil3 film that could be interpreted as an interface state.
Intrinsic stability of magnetic anti-skyrmions in the tetragonal inverse Heusler compound Mn1.4Pt0.9Pd0.1Sn
Saha, Rana, Abhay K. Srivastava, Tianping Ma, Jagannath Jena, Peter Werner, Vivek Kumar, Claudia Felser, Stuart S. Parkin
Nature Communications 10, pp 5305/1-7 (2019)
abstractMagnetic anti-skyrmions are one of several chiral spin textures that are of great current interest both for their topological characteristics and potential spintronic applications. Anti-skyrmions were recently observed in the inverse tetragonal Heusler material Mn1.4Pt0.9Pd0.1Sn. Here we show, using Lorentz transmission electron microscopy, that anti-skyrmions are found over a wide range of temperature and magnetic fields in wedged lamellae formed from single crystals of Mn1.4Pt0.9Pd0.1Sn for thicknesses ranging up to  ∼ 250 nm. The temperature-field stability window of the anti-skyrmions varies little with thickness. Using micromagnetic simulations we show that this intrinsic stability of anti-skyrmions can be accounted for by the symmetry of the crystal lattice which is imposed on that of the Dzyaloshinskii-Moriya exchange interaction. These distinctive behaviors of anti-skyrmions makes them particularly attractive for spintronic applications.
Electric field control of phase transition and tunable resistive switching in SrFeO2.5
Saleem, Muhammad S, Bin Cui, Cheng Song, Yiming Sun, Youdi Gu, Ruiqi Zhang, Muhammad U Fayaz, Xiaofeng Zhou, Peter Werner, Stuart S. Parkin, Feng Pan
ACS Applied Materials and Interfaces 11 (6), pp 6581-6588 (2019)
abstractSrFeOx (SFOx) compounds exhibit ionic conduction and oxygen related phase transformation, having potential applications in solid oxide fuel cells, smart windows, and memristive devices. The phase transformation in SFOx typically requires a thermal annealing process under various pressure conditions, hindering their practical applications. Here, we have achieved a reversible phase transition from brownmillerite (BM) to perovskite (PV) in SrFeO2.5 (SFO2.5) films through ionic liquid (IL) gating. The real-time phase transformation is imaged using in situ high resolution transmission electron microscopy. The magnetic transition in SFO2.5 is identified by fabricating an assisted La0.7Sr0.3MnO3 (LSMO) bottom layer. The IL-gating-converted PV phase of a SrFeO3−δ (SFO3−δ) layer shows a ferromagnetic-like behavior but applies a huge pinning effect on LSMO magnetic moments, which consequently leads to a prominent exchange bias phenomenon, suggesting an uncompensated helical magnetic structure of SFO3−δ. On the other hand, the suppression of both magnetic and exchange coupling signals for a BM-phased SFO2.5 layer elucidates its fully compensated G-type antiferromagnetic nature. We also demonstrated that the phase transition by IL gating is an effective pathway to tune the resistive switching parameters, such as set, reset, and high/low-resistance ratio in SFO2.5-based resistive random-access memory devices.
In-plane ferroelectric tunnel junction
Shen, Huitao, Junwei Liu, Kai Chang, Liang Fu
Physical Review Applied 11 (2), pp 024048/1-6 (2019)
abstractFerroelectric materals are an important platform for the realization of nonvolatile memories. So far, existing ferroelectric memory devices have utilized out-of-plane polarization in ferroelectric thin films. In this paper, we propose a type of random-access memory (RAM) based on ferroelectric thin films with in-plane polarization, called an "in-plane ferroelectric tunnel junction."" Apart from nonvolatility, lower power usage, and a faster writing operation compared with traditional dynamic RAMs, our proposal has the advantage of a faster reading operation and a nondestructive reading process, thus overcoming the write-after-read problem that exists widely in current ferroelectric RAMs. The recent discovered room-temperature ferroelectric IV-VI semiconductor thin films are a promising material platform for the realization of our proposal.
Observation of robust Néel skyrmions in metallic PtMnGa
Srivastava, Abhay K., Parul Devi, Ankit K. Sharma, Tianping Ma, Hakan Deniz, Holger L. Meyerheim, Claudia Felser, Stuart S. Parkin
Advanced Materials 32 (7), pp 1904327/1-6 (2019)
abstractOver the past decade the family of chiral noncollinear spin textures has continued to expand with the observation in metallic compounds of Bloch-like skyrmions in several B20 compounds, and antiskyrmions in a tetragonal inverse Heusler. Néel like skyrmions in bulk crystals with broken inversion symmetry have recently been seen in two distinct nonmetallic compounds, GaV4S8 and VOSe2O5 at low temperatures (below ≈ 13 K) only. Here, the first observation of bulk Néel skyrmions in a metallic compound PtMnGa and, moreover, at high temperatures up to ≈ 220 K is reported. Lorentz transmission electron microscopy reveals the chiral Néel character of the skyrmions. A strong variation is reported of the size of the skyrmions on the thickness of the lamella in which they are confined, varying by a factor of 7 as the thickness is varied from ≈ 90 nm to ≈ 4 μm. Moreover, the skyrmions are highly robust to in-plane magnetic fields and can be stabilized in a zero magnetic field using suitable field-cooling protocols over a very broad temperature range to as low as 5 K. These properties, together with the possibility of manipulating skyrmions in metallic PtMnGa via current induced spin-orbit torques, make them extremely exciting for future spintronic applications.
Epitaxial growth, structural characterization, and exchange bias of noncollinear antiferromagnetic Mn3Ir thin films
Taylor, James M., Edouard Lesne, Anastasios Markou, Fasil Kida Dejene, Benedikt Ernst, Adel Kalache, Kumari Gau Rana, Neeraj Kumar, Peter Werner, Claudia Felser, Stuart S. Parkin
Physical Review Materials 3 (7), pp 074409/1-12 (2019)
abstractAntiferromagnetic materials are of great interest for spintronics. Here we present a comprehensive study of the growth, structural characterization, and resulting magnetic properties of thin films of the noncollinear antiferromagnet Mn3Ir. Using epitaxial engineering on MgO (001) and Al2O3 (0001) single-crystal substrates, we control the growth of cubic gamma-Mn3Ir in both (001) and (111) crystal orientations, and discuss the optimization of growth conditions to achieve high-quality crystal structures with low surface roughness. Exchange bias is studied in bilayers, with exchange bias fields as large as -29 mT (equivalent to a unidirectional anisotropy constant of 0.115 erg cm−2 or 11.5 nJ cm−2) measured in Mn3Ir (111)/Permalloy heterostructures at room temperature. In addition, a distinct dependence of blocking temperature on in-plane crystallographic direction in Mn3Ir (001)/Permalloy bilayers is observed. These findings are discussed in the context of antiferromagnetic domain structures, and will inform progress towards chiral antiferromagnetic spintronic devices.
Magnetic and electrical transport signatures of uncompensated moments in epitaxial thin films of the noncollinear antiferromagnet Mn3Ir
Taylor, James M., Edouard Lesne, Anastasios Markou, Fasil Kida Dejene, Pranava Ke Sivakumar, Simon Poellath, Kumari Gau Rana, Neeraj Kumar, Chen Luo, Hanjo Ryll, Florin Radu, Florian Kronast, Peter Werner, Christian Back, Claudia Felser, Stuart S. Parkin
Applied Physics Letters 115 (6), pp 062403/1-5 (2019)
abstractNoncollinear antiferromagnets, with either an L12 cubic crystal lattice (e.g., Mn3Ir and Mn3Pt) or a D019 hexagonal structure (e.g., Mn3Sn and Mn3Ge), exhibit a number of phenomena of interest to topological spintronics. Among the cubic systems, for example, tetragonally distorted Mn3Pt exhibits an intrinsic anomalous Hall effect (AHE). However, Mn3Pt only enters a noncollinear magnetic phase close to the stoichiometric composition and at suitably large thicknesses. Therefore, we turn our attention to Mn3Ir, the material of choice for use in exchange bias heterostructures. In this letter, we investigate the magnetic and electrical transport properties of epitaxially grown, face-centered-cubic γ-Mn3Ir thin films with (111) crystal orientation. Relaxed films of 10 nm thickness exhibit an ordinary Hall effect, with a hole-type carrier concentration of (1.500 ± 0.002) x 1023cm−3. On the other hand, TEM characterization demonstrates that ultrathin 3 nm films grow with significant in-plane tensile strain. This may explain a small net magnetic moment, observed at low temperatures, shown by X-ray magnetic circular dichroism spectroscopy to arise from uncompensated Mn spins. Being of the order of 0.02 μB/atom, this dominates electrical transport behavior, leading to a small AHE and negative magnetoresistance. These results are discussed in terms of crystal microstructure and chiral domain behavior, with spatially resolved XML(C)D-PEEM supporting the conclusion that small antiferromagnetic domains, < 20nm in size, with differing chirality account for the absence of observed Berry curvature driven magnetotransport effects.
Anomalous and topological Hall effects in epitaxial thin films of the noncollinear antiferromagnet Mn3Sn
Taylor, James M., Anastasios Markou, Edouard Lesne, Pranava Ke Sivakumar, Chen Luo, Florin Radu, Peter Werner, Claudia Felser, Stuart S. Parkin
Physical Review B 101 (9), pp 094404/1-7 (2019)
abstractNoncollinear antiferromagnets with a D019 (spacegroup = 194, P63/mmc) hexagonal structure have garnered much attention for their potential applications in topological spintronics. Here, we report the deposition of continuous epitaxial thin films of such a material, Mn3Sn, and characterize their crystal structure using a combination of x-ray diffraction and transmission electron microscopy. Growth of Mn3Sn films with both (0001) c-axis orientation and (4043) texture is achieved. In the latter case, the thin films exhibit a small uncompensated Mn moment in the basal plane, quantified via magnetometry and x-ray magnetic circular dichroism experiments. This cannot account for the large anomalous Hall effect simultaneously observed in these films, even at room temperature, with magnitude σxy0H = 0 T) = 21 Ω−1 cm−1 and coercive field μ0Hc = 1.3 T. We attribute the origin of this anomalous Hall effect to momentum-space Berry curvature arising from the symmetry-breaking inverse triangular spin structure of Mn3Sn. Upon cooling through the transition to a glassy ferromagnetic state at around 50 K, a peak in the Hall resistivity close to the coercive field emerges. This indicates the onset of a topological Hall effect contribution, arising from a nonzero scalar spin chirality that generates a real-space Berry phase. We demonstrate that the polarity of this topological Hall effect, and hence the chiral nature of the noncoplanar magnetic structure driving it, can be controlled using different field-cooling conditions.
Band structure engineering of chemically tunable LnSbTe (Ln = La, Ce, Pr)
Weiland, Ashley, David G. Chaparro, Maia G. Vergniory, Elena Derunova, Jiho Yoon, Iain W. H. Oswald, Gregory T. McCandless, Mazhar Ali, Julia Y. Chan
APL Materials 7 (10), pp 101113/1-6 (2019)
abstractThe ZrSiS family of compounds has garnered interest as Dirac nodal-line semimetals and offers an approach to study structural motifs coupled with electronic features, such as Dirac crossings. CeSbTe, of the ZrSiS/PbFCl structure type, is of interest due to its magnetically tunable topological states. The crystal structure consists of rare earth capped square nets separating the magnetic Ce-Te layers. In this work, we report the single crystal growth, magnetic properties, and electronic structures of LnSb1−xBixTe (Ln = La, Ce, Pr; x  ∼ 0.2) and CeBiTe, adopting the CeSbTe crystal structure, and the implication of tuning the electronic properties by chemical substitution.
Chiral exchange drag and chirality oscillations in synthetic antiferromagnets
Yang, See-Hun, Chirag Garg, Stuart S. Parkin
Nature Physics 15 (6), pp 543-548 (2019)
abstractLong-range interactions between quasiparticles give rise to a "drag" that affects the fundamental properties of many systems in condensed matter physics. Drag typically involves the exchange of linear momentum between quasiparticles and strongly influences their transport properties. Here, we describe a kind of drag that involves the exchange of angular momentum between two current-driven magnetic domain walls. The motions of the domain walls are correlated and determined by the strength of the drag. When the drag is below a threshold value, the domain walls move together at a constant intermediate velocity with a steady leakage of angular momentum from the faster to the slower wall. However, we find that when the drag exceeds a threshold value, a different dynamic can take place in which the faster domain wall"s magnetization oscillates synchronously with a precessional motion of the slower domain wall"s magnetization, and angular momentum is continuously transferred between them. Our findings demonstrate a method for delivering spin angular momentum remotely to magnetic entities that otherwise could not be manipulated directly by current, for example, by coupling domain walls or other non-collinear spin textures in metallic and insulating media.
Novel stable 3D stainless steel-based electrodes for efficient water splitting
Zhang, Haojie, Juliana Ma de Souza e Silva, Xubin Lu, Cristine S de Oliveira, Bin Cui, Xiaopeng Li, Chao Lin, Stefan L. Schweizer, A. Wouter Maijenburg, Michael Bron, Ralf B. Wehrspohn
pp 1900774/1-10 (2019)
abstractThe stability of electrocatalysts grown on substrates is a significant challenge for the construction of 3-dimensional (3D) stainless-steel (SS)-based electrodes for highly efficient water splitting. This paper presents an efficient and universal process to enhance the interfacial interaction between SS and highly active electrocatalysts for the preparation of 3D electrodes through the formation of an interfacial network of carbon nanotubes (CNTs) on the SS. Nanoscale X-ray computed tomography and focused ion beam are used to visualize the interface between CNTs and SS, and 3D structure of CNT/SS electrodes. The strongly interconnected CNTs network increases the surface area of the SS support that benefits the modification of highly active electrocatalysts and also serves as an electron/charge-conductive highway between electrocatalysts and support. The electrocatalysts on CNT/SS further improve hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performances, respectively, of the 3D electrodes. While compared to the SS-based electrodes reported recently, Pt/OxCNT/SS shows the best HER activity over wide pH range and RuO2/OxCNT/SS exhibits a comparable OER performance in neutral and alkaline electrolyte. An efficient approach is reported to combine highly active electrocatalysts with SS for the preparation of active and stable 3D electrodes that can be further explored in various areas.
Effect of interfacial insertion layers on the spin-orbit torque in W(O)|CoFeB heterostructures
Zhang, Jie, Timothy Phung, Brian P. Hughes, See-Hun Yang, Chirag Garg, Yong Jiang, Stuart S. Parkin
Applied Physics Express 12 (3), pp 033001/1-5 (2019)
abstractWe report on an experimental investigation of spin-orbit torque (SOT) in W(O)|CoFeB heterostructures where a thin insertion layer with negligible spin-orbit coupling is inserted at the W(O)|CoFeB interface. The SOT is found to be suppressed with the addition of the insertion layer, contrary to estimates using the transparency formalism. In addition, the SOT, as quantified by the spin Hall angle remains constant at -50% for W(O) thicknesses down to 2 nm. Our data is thus consistent with an interfacial SOT mechanism in the W(O)|CoFeB system.
A hydrated crystalline calcium carbonate phase: Calcium carbonate hemihydrate
Zou, Zhaoyong, Wouter J. Habraken, Galina Matveeva, Anders C. Jensen, Luca Bertinetti, Matthew A. Hood, Chang-yu Sun, Pupa U. P. Gilbert, Iryna Polishchuk, Boaz Pokroy, Julia Mahamid, Yael Politi, Steve Weiner, Peter Werner, Sebastian Bette, Robert Dinnebier, Ute Kolb, Emil Zolotoyabko, Peter Fratzl
Science 363 (6245), pp 396-400 (2019)
abstractAs one of the most abundant materials in the world, calcium carbonate, CaCO3, is the main constituent of the skeletons and shells of various marine organisms. It is used in the cement industry and plays a crucial role in the global carbon cycle and formation of sedimentary rocks. For more than a century, only three polymorphs of pure CaCO3-calcite, aragonite, and vaterite-were known to exist at ambient conditions, as well as two hydrated crystal phases, monohydrocalcite (CaCO3·1H2O) and ikaite (CaCO3·6H2O). While investigating the role of magnesium ions in crystallization pathways of amorphous calcium carbonate, we unexpectedly discovered an unknown crystalline phase, hemihydrate CaCO3·1/2H2O, with monoclinic structure. This discovery may have important implications in biomineralization, geology, and industrial processes based on hydration of CaCO3.