Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session M35: Novel Functional Magnetic Materials and Structures |
Hide Abstracts |
Sponsoring Units: GMAG Chair: Shinji Miwa, Univ of Tokyo Room: 507 |
Wednesday, March 4, 2020 11:15AM - 11:27AM |
M35.00001: Interplay of local moment and itinerant magnetism in cobalt-based Heusler ferromagnets: Co2TiSi, Co2MnSi and Co2FeSi Guanhua Qin, Wei Ren, David Singh Heusler ferromagnets based on Co are important materials for spintronics due to the exceptional combinations of high Curie temperature and strong spin polarization. We investigate the electronic structure, magnetism and spin excitations in the cobalt-based full Heusler compounds, Co2TiSi, Co2MnSi and Co2FeSi using first principles calculations. Co2TiSi and Co2MnSi are half-metals, while Co2FeSi is not. The trends in the Curie temperature of Co2MnSi and Co2FeSi are well reproduced by the calculated spin wave dispersions. Remarkably, Co2TiSi is a very itinerant magnet, while both Co2FeSi and Co2MnSi show local moment behavior regarding the Fe and Mn, while retaining the itinerancy of the Co magnetism. Although our results do not support the half-metallic character proposed for Co2FeSi, consistent with the reported high Curie temperature, we find a very strong positive transport spin polarization, which combined with the large moment, cubic structure and high Curie temperature supports the further investigation of Co2FeSi as a material for spintronic applications. |
Wednesday, March 4, 2020 11:27AM - 11:39AM |
M35.00002: Effect of V substitution on structural, magnetic, transport and mechanical properties of the half-metal-type Heusler alloy Co2FeGe Rabin Mahat, Shambhu K KC, Daniel Wines, Sudhir Regmi, UPAMA KARKI, Fatih Ersan, Can Ataca, Arunava Gupta, Patrick LeClair Half-metallic ferromagnets are ideal candidates for spintronic applications due to their ∼100% spin polarization. High Curie temperature Tc is one of the important prerequisite from the application point of view. Co2Fe- based Heusler alloys in L21 structure have attracted great interest for this reason. We present a combined experimental and theoretical study of quaternary Heusler alloys Co2-xVxFeGe and Co2Fe1-xVxGe (0 ≤ x ≤ 1) as promising candidates for spintronic applications. Single phase microstructures for arc-melted alloys are observed for V substitution for Co from x = 0.25 to x = 0.625, but when V substituites for Fe only x = 0.25 and 0.375 yield single phase microstructures. All single phase samples show fcc structures with L21 ordering, corroborated by X-ray diffraction. Low-temperature saturation magnetic moments agree well with our theoretical results and obey the Slater-Pauling rule, a prerequisite for half metallicity. All alloys are soft ferromagnets with high Tcs, allowing for applications at room temperature and above. Transport measurements are performed to elucidate the electronic structure of the alloys. Relatively high mechanical hardness values are also observed. |
Wednesday, March 4, 2020 11:39AM - 11:51AM |
M35.00003: Half-metallicity in CrAl-terminated Co2CrAl thin film Juliana C Herran, Ryan Carlile, Parashu R. Kharel, Pavel Lukashev Half-metals with high Curie temperature are ideal candidates for applications in spintronics. Many half-metallic materials have been predicted theoretically, and some have been confirmed experimentally. At the same time, in thin-film geometry the electronic structure of these materials may change due to the potential presence of surface/interface states, which could limit practical applications of these materials in nano-size devices. Here, from first principles we study a full Heusler compound, Co2CrAl in thin film geometry. This material has been studied extensively, and it has been reported that it exhibits half-metallic properties in the bulk. We show that this material retains 100% spin polarization in CrAl-terminated thin film geometry (Co-termination results in destroyed half-metallicity) as well. Moreover, we confirm that under biaxial strain Co2CrAl retains half-metallicity for a practically feasible range of considered pressure. The magnetic alignment of Co2CrAl is confirmed to be ferromagnetic, with the non-integer total magnetic moment of Co-terminated cell, and the integer total magnetic moment of CrAl-terminated cell. If confirmed experimentally, these results may have an important impact in spin-based electronics. |
Wednesday, March 4, 2020 11:51AM - 12:03PM |
M35.00004: Anomalous Hall effect in a two sub-lattice ferrimagnet. Nishchal Thapa Magar, Dina Michel, Nirmal Ghimire The anomalous Hall effect (AHE) is a characteristic of ferromagnets where a finite Hall effect proportional to the magnetization appears in zero external magnetic field. It is now known that the AHE can also arise in non-collinear and non-coplanar antiferromagnets due to spin chirality and in materials consisting of topologically non-trivial electronic structure such as Weyl semimetals. Here we will present the observation of an anomalous Hall effect in a hexagonal ferrimagnet TbMn6Sn6 consisting of two spin sub-lattices due to Mn and Tb ordering. Both spin sublattices order simultaneously above room temperature and show a spin reorientation below 310 K from the basal plane towards c-axis. The AHE that is observed at temperatures well below the spin reorientation temperature does not follow the underlying net magnetization, indicating the formation of a non-coplanar or complex chiral spins textures |
Wednesday, March 4, 2020 12:03PM - 12:15PM |
M35.00005: Itinerant ferromagnetism and intrinsic anomalous Hall effect in amorphous iron-germanium Dinah Simone Bouma, Neal Reynolds, Zhanghui Chen, Frank Bruni, Binhua Zhang, Michael Flatté, Robert Streubel, Lin-Wang Wang, Ruqian Wu, Frances Hellman The amorphous iron-germanium system (a-FexGe1-x) lacks long-range structural order and hence lacks a meaningful Brillouin zone. The magnetization of a-FexGe1-x is well explained by the Stoner model for x ≥ 0.4, indicating that the local order of the amorphous structure preserves the spin-split density of states of the Fe-3d states sufficiently to polarize the electronic structure despite k being a bad quantum number. Measurements show an enhanced anomalous Hall resistivity relative to crystalline FeGe; comparison to density functional theory calculations of the anomalous Hall conductivity (AHC) resolves its underlying mechanisms. The intrinsic mechanism, typically understood as the Berry curvature integrated over occupied k-states but equivalent to the density of curvature (DOC) integrated over occupied energies, dominates the AHC of a-FexGe1-x (0.38 ≤ x ≤ 0.61). The DOC is the sum of spin-orbit correlations of local orbital states and can hence be calculated with no reference to k. This result and the accompanying Stoner-like model for the intrinsic AHC establish a unified understanding of the physics of the anomalous Hall effect in both crystalline and disordered systems. |
Wednesday, March 4, 2020 12:15PM - 12:27PM |
M35.00006: Unusual Friedel and RKKY responses in 3D quadratic band-touching Luttinger semimetals Louis J. Godbout, Sergueï Tchoumakov, William Witczak-Krempa We study the response of Luttinger semimetals, such as alpha-Sn, HgSe, HgTe, YPtBi and Pr2Ir2O7, to charge and magnetic impurities. In these materials the strong spin-orbit coupling and quadratic band-touching lead to unusual Friedel and Ruderman-Kittel-Kasuya-Yoshida (RKKY) oscillations, as well as an asymmetry in the response upon electron or hole doping. We also examine the magnetic Pauli and Landau susceptibilities, which differ from those of regular metals. These results motivate the study of Kondo physics in Luttinger semimetals, where novel quantum phases can arise due to spin-orbit coupling. |
Wednesday, March 4, 2020 12:27PM - 12:39PM |
M35.00007: Search for new permanent magnet materials: methodology and applications. Olga Vekilova Permanent magnets (PMs) are of vital importance for sustainable industry and in particular, are parts of most of the sources of “green” energy. The theory-assisted search for new PMs, especially those that are free from expensive rare-earth elements, has now become an active field of research. We could recently suggest a number of promising PMs using the ab initio-based analyses and high-throughput approach [1,2]. One of key target properties is the proper ferromagnetic state with high Curie temperature. To be able to effectively describe that, especially in the course of high-throughput calculations, we have introduced a novel theoretical method, combining molecular dynamics with the magnetic state search [3]. The implementation and examples showing the power of the method will be considered. |
Wednesday, March 4, 2020 12:39PM - 12:51PM |
M35.00008: The Voltage-Controlled Magnetic Anisotropy Effect Under High Electric Field Bowei Zhou, Meng Xu, Pravin Khanal, Yu Zhang, Weigang Wang The discovery of voltage-controlled magnetic anisotropy (VCMA) effect in perpendicular magnetic tunneling junction (pMTJ) has attracted considerable interests for low-power and high-speed memory application. In a recent DFT study1, the VCMA effect is expected to diverge from commonly observed linear PMA vs E-field relation and exhibit nonlinearity under high E-field. However, limitations of established methods make high E-field unattainable. |
Wednesday, March 4, 2020 12:51PM - 1:03PM |
M35.00009: Energy-efficient stochastic computing with superparamagnetic tunnel junctions Matthew W Daniels, Advait Madhavan, Philippe Talatchian, Alice Mizrahi, Mark Stiles We design an efficient stochastic bitstream generator based on superparamagnetic tunnel junctions, which can produce low energy, truly random bits, in turn drastically reducing cross-correlation. This bitstream generator allows us to address an outstanding issue in stochastic computing: that it has been limited by the inaccuracies introduced by correlations between the pseudorandom bitstreams used in the calculations. This bitstream generator gives us the freedom of not having to design around correlations and allows us to propose a low-energy approach to stochastic computing. To demonstrate the effectiveness of this approach, we incorporate it into an efficient CMOS neural network design. Our simulations of this network reach error rates comparable to recent work in stochastic-computing-based neural networks at nearly an order of magnitude lower energy expenditure. |
Wednesday, March 4, 2020 1:03PM - 1:15PM |
M35.00010: Atomic scale magnetic and structural imaging by achromatic electron microscopy Zechao Wang, Jing Zhu The atomic-level knowledge of local spin configuration of the magnetic materials is of great importance to predict and control their physical properties, in order to meet the challenges of ever-increasing demands on performance of functional materials. However, it is highly challenging to experimentally characterize magnetic properties of such materials with atomic scale spatial resolution. |
Wednesday, March 4, 2020 1:15PM - 1:27PM |
M35.00011: Electrical transport in FIB-microstructures of single-crystalline Mn1.4PtSn Moritz Winter, Sandra Hamann, Marc Uhlarz, Jacob Gayles, Praveen Vir, Markus Koenig, Claudia Felser, Joachim Wosnitza, Toni Helm Mn1.4PtSn is a half-Heusler compound with tetragonal crystal structure that hosts a D2d symmetry and strong Dzyaloshinskii-Moriya interactions. These are key-ingredients for the possible presence of Antiskyrmion phases. Recently, Antiskyrmions were observed well above room temperature by Lorentz transmission microscopy (LTEM) in polycrystalline nanoscaled ingots of the material [1]. Such a magnetic texture may cause a topological Hall effect (THE) leading to a significant deviation from the expected anomalous Hall effect. Indeed, a topological component of high magnitude was revealed by Hall measurements, conducted on bulk single crystals [2]. It however, only was observed below the spinreorientation transition at TSR = 160 K associated with the formation of a non-coplanar spin structure. We investigate transport devices fabricated by the application of focused ion beams (FIB) from high-quality single crystals. Assisted by FIB we are able to conduct experiments on devices with sub-micron feature sizes. Our study of the temperature, field, and thickness dependent transport reveals clear signatures of THE originating from both, the non-coplanar spin structure as well as Antikyrmions, with apparent differences. |
Wednesday, March 4, 2020 1:27PM - 1:39PM |
M35.00012: Valence instability across magnetostructural transition in USb2 Zachary Brubaker, Yuming Xiao, Paul Chow, Curtis Kenney-Benson, Jesse Smith, Hyunchae Cynn, Christopher Reynolds, Nicholas Butch, Rena Zieve, Jason R Jeffries Pressure dependent transport measurements on USb2 indicate an abrupt antiferromagnetic–ferromagnetic (AFM–FM) transition near P = 8 GPa, accompanied by a reduction in ordering temperature by more than 100 K. The AFM–FM transition has been the subject of several recent theoretical calculations, though only few experimental probes have been reported. To elucidate the behavior near this magnetic transition, we performed pressure dependent X-ray diffraction and resonant X-ray emission spectroscopy experiments on USb2. The magnetic transition coincides with a tetragonal to orthorhombic structural transition resulting in a 17% volume collapse as well as a transient f-occupation enhancement. The AFM–FM transition width determined from structural and spectroscopic measurements agrees well with previously published transport measurements and suggests a sluggish transition from about 8 to 11 GPa. Except for the enhancement across the transition region, the f-occupation decreases steadily with pressure, demonstrating the multiconfigurational nature of actinide materials. |
Wednesday, March 4, 2020 1:39PM - 1:51PM |
M35.00013: Effect of Co, Ni, and Co-Ni doping on magnetic and electronic properties of MnBi by first-principles calculations Chandani Nandadasa, Bipin Lamichhane, Dinesh Thapa, Minyeong Choi, Yang-Ki Hong, Seong-Gon Kim While Manganese Bismuth (MnBi) is a well-studied rare-earth free permanent magnetic material, its magnetic and electronic properties in the presence of dopants have not been discovered extensively. We studied the magnetic and electronic properties of MnBi doped with Co, Ni, and Co-Ni using the Generalized Gradient Approximation (GGA) method within Density Functional Theory (DFT). GGA+U method is used to get electronic correlation in 3d electrons in transition metal alloys. We considered both interstitial and substitutional sites in the MnBi lattice when adding Co, Ni, and Co-Ni. The addition of Co, Ni, and Co-Ni into interstitial sites has yielded a significant increase in the magnetization with dopant concentration. However substitutional doping into Mn site decreased its magnetization with dopant concentration. Our results show that individual Co and Ni doping prefers parallel spin configurations with Mn site while Co-Ni doping prefers both parallel and antiparallel configurations. We further calculated magnetic anisotropy energy (MAE) and magnetic anisotropy constant (Ku) of pure MnBi and doped-MnBi. Electronic properties were examined to describe exchange interactions between the dopants and the MnBi lattice. |
Wednesday, March 4, 2020 1:51PM - 2:03PM |
M35.00014: Kardar-Parisi-Zhang Universality in the Infinite Temperature spin-half Heisenberg Chain Nicholas Sherman, Joel Moore There are two simple paradigms of how conserved quantities transport through a system: thermalizing systems with effectively random collisions in the system leading to diffusion, and particles moving freely in a material giving rise to ballistic transport. In the spin-half XXZ model in one dimension, both of these behaviors have been shown rigorously. Recent studies have suggested at the isotropic point of the model, the spin-half Heisenberg model, there is a third behavior at infinte temperature given by the stochastic classical Kardar-Parisi-Zhang (KPZ) universality class. In this work, I computed the dynamical structure factor at infinite temperature using matrix product states for the spin-half Heisenberg, and show that it exhibits the scaling function from the KPZ universality class, strengthening the claim that KPZ dynamics are present in quantum spin systems. |
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M35.00015: Mechanically Induced Thermal Breakdown in Magnetic Shuttle Structures Olya A. Ilinskaya, Sergeij I. Kulinich, Ilya V. Krive, Robert Shekhter, Hee Chul Park, Mats Jonson A theory of a thermally induced single-electron “shuttling" instability in a magnetic nanomechanical device subject to an external magnetic field is presented in the Coulomb blockade regime of electron transport. The model magnetic shuttle device considered comprises a movable metallic grain suspended between two magnetic leads, which are kept at different temperatures and assumed to be fully spin polarized with antiparallel magnetizations. For a given temperature difference shuttling is found to occur for a region of external magnetic fields between a lower and an upper critical field strength, which separate the shuttling regime from normal small-amplitude “vibronic" regimes. We find that (i) the upper critical magnetic field saturates to a constant value in the high temperature limit and that the shuttle instability domain expands with a decrease of the temperature; (ii) the lower critical magnetic field depends not only on the temperature independent phenomenological friction coefficient used in the model but also on intrinsic friction caused by magnetic exchange forces and electron tunneling between the quantum dot and the leads. The feasibility of using thermally driven magnetic shuttle systems to harvest thermal breakdown phenomena is discussed. |
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