Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session K54: Spin Hall and Topological Hall effectFocus Recordings Available
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Sponsoring Units: GMAG Chair: NILESH SALKE, University of Illinois Chicago Room: McCormick Place W-476 |
Tuesday, March 15, 2022 3:00PM - 3:12PM |
K54.00001: Observation of room temperature topological Hall effect in thin film magnetic insulator/heavy metal bilayers Sanyum Channa, Xin Yu Zheng, Zbigniew Galazka, Lauren Riddiford, Jacob J Wisser, Egecan Cogulu, Haowen Ren, Andrew D Kent, Yuri Suzuki Magnetic skyrmions are spin textures that are promising for future spintronics-based memory storage owing to their nanoscale structure, topological robustness and the ability to be manipulated with current densities as low as ~102 A/cm2. Therefore, from an application standpoint, it is vital to engineer materials that can stabilize skyrmions at room temperature. Magnetic insulators provide the ideal platform for skyrmion stabilization due to their absence of Joule heating and low Gilbert damping. In this talk, we demonstrate the observation of the topological Hall effect (THE) – a signature of magnetic skyrmions – in Li0.5Al1.0Fe1.5O4 (LAFO)/Pt bilayers. LAFO is a ferromagnetic insulating spinel that we have previously demonstrated to have ultra-low damping and high spin Hall angles (~0.1-0.6) when interfaced with Pt. By tuning LAFO’s magnetic anisotropy with thickness, we show that for < 4 nm films, we can observe THE even at room temperature and stabilize skyrmions for a large portion of the temperature-field phase space. These results highlight the potential for LAFO to act as a platform for current-driven skyrmion manipulation at room temperature, and combined with its low damping and high spin Hall angles, make LAFO a very promising material for future spin current based-applications. |
Tuesday, March 15, 2022 3:12PM - 3:24PM |
K54.00002: Resonance structures of direct and Hall currents in topological spin-Hall effect Andrei S Zadorozhnyi We study spin-dependent direct and Hall conductivities in the threshold region of Fermi energy, εF=2J, where J is the exchange integral between the conduction electron spins and the skyrmion spin texture. For εF at the threshold value and above the spin-down electrons are allowed to exist. We find the two in the direct and four narrow peaks in the Hall conductivities for Fermi energies slightly below the threshold value. The found effects are dramatic because the electric current changes by approximately eight times in the narrow range of gate voltages (∼4 meV). The values of the peaks strongly depend on skyrmion size. For small and very large skyrmion sizes the peak amplitudes are small compared to the conductivity absolute values. At the skyrmion radius a = 6 nm and very light conduction electrons, m*∼10-2me, the extrema are the most pronounced. The temperature evolution reveals the strong smearing effect where the peak-wise behavior completely disappears at room temperatures. Spin transistor could be considered for possible applications where in the narrow region of gate voltage the sharp conductivity change occurs. |
Tuesday, March 15, 2022 3:24PM - 3:36PM |
K54.00003: Theory of Hall effects with chiral magnetic textures and spin-orbit coupling Zachariah M Addison, Nishchhal Verma, Mohit Randeria Chiral magnetic materials offer a rich landscape of spin textures, including spirals and skyrmion phases, which influence the dynamics of itinerant electrons in the presence of spin-orbit coupling (SOC). Experiments are often analyzed as the sum of an anomalous Hall effect (AHE), dominated by momentum-space Berry curvature, and a topological Hall effect (THE) arising from the real-space Berry curvature of skyrmions, in addition to the ordinary Hall resistivity proportional to field. Here we present a theory that treats both momentum- and real-space Berry curvatures on equal footing. First, we will present results from a semi-classical Boltzmann approach with generalized phase space Berry curvatures. Apart from the AHE and THE, we find an additional contribution that depends on the SOC and on first-order gradients of the magnetic texture and can be non-zero even in the absence of all phase-space Berry curvatures. We discuss the relative strengths of the various contributions and find conditions under which the new Hall contribution vanishes. We also present results obtained from a complementary approach, valid in the regime of very long mean paths, using the Kubo formula and exact diagonalization in the magnetic unit cell. Finally, we comment on the question of using the Hall signal for detection of skyrmions. |
Tuesday, March 15, 2022 3:36PM - 3:48PM |
K54.00004: Metastable zero-field skyrmion lattice at room temperature in van der Waals (Fe0.5Co0.5)5GeTe2 Ying-Ting Chan, Hongrui Zhang, Rui Chen, Xiang Chen, Jie Yao, Robert J Birgeneau, Ramamoorthy Ramesh, Weida Wu Recently, zero-field skyrmions at room temperature were discovered in a van der Waals 50% Co-doped Fe5GeTe2 (FCGT) with a high Curie temperature (Tc = 365 K) and novel non-centrosymmetric structure. [1] Here, we visualize the magnetic domains near room temperatures in FCGT thin flakes using a variable-temperature magnetic force microscope. The skyrmion lattice can be induced from stripe domains by a small external magnetic field and persists as a metastable state in zero field at room temperature. Interestingly, the skyrmion lattice gradually “melt” into the stripe domains with annealing temperature above 320 K. The metastable skyrmion lattice and its annealing behavior can be explained by a phenomenological picture of the free energy landscape. |
Tuesday, March 15, 2022 3:48PM - 4:00PM |
K54.00005: Hall viscosity at angular momentum compensation point Bom Soo Kim Symmetries have served as an overarching theme to investigate interesting physical systems by connecting theoretical tools to experimental results. Chiral magnetic Skyrmions exist in materials without mirror (parity) symmetry. Hydrodynamic approaches, when parity symmetry is broken, uncover Hall viscosity, a universal and dissipationless transport coefficient. Previously we examined existing data to reveal that the Hall viscosity amounts 3\% - 5.4\% of skyrmion Hall effect by modeling Hall viscosity with a transverse velocity component of Thiele equation. This Hall viscosity plays role even in anti-ferromagnetic systems without skyrmion Hall effect. Here we propose different ways to reveal Hall viscosity in the angular momentum compensation point. |
Tuesday, March 15, 2022 4:00PM - 4:12PM |
K54.00006: Spin-polarized STM Study of Cr2Te3 Thin Films Joseph E Murray The topological Hall Effect (THE) is the manifestation of an effective magnetic field experienced by electrons interacting with a spatially-varying, non-coplanar spin texture. Studies over the past decade have demonstrated the robust presence of this effect – generally understood to be an indicator of an interfacial skyrmion lattice - in heavy metal/ferromagnet heterostructures which exhibit strong interfacial Dzyaloshinskii-Moriya interactions. Recent evidence also suggests the presence of a THE in quasi-2D thin film transition metal chalcogenides such as Cr2Te3. We report the results of our study of the latter using low-temperature, spin-polarized STM to complement previously performed transport measurements. |
Tuesday, March 15, 2022 4:12PM - 4:48PM |
K54.00007: Skyrmions, emergent magnetic monopoles, and their electromagnetic responses in cubic chiral magnets Invited Speaker: Yukako Fujishiro Topological spin crystals can give rise to versatile electromagnetic responses through the interplay of conduction electrons and scalar-spin chirality (SSC) [1]. Among them, emergent magnetic monopoles formed in a cubic chiral magnet MnGe exhibit intriguing transport phenomena due to static and dynamic SSC, which can be controlled by external magnetic field [2]. In particular, the dynamics of emergent magnetic monopoles generate unique electronic functions such as enhanced magneto-resistivity, magneto-Seebeck effect, or magneto-chiral effect. Furthermore, a gigantic anomalous Hall effect (AHE) showing a Hall conductivity of ~40,000 Ω-1cm-1 and a Hall angle of ~18 % is identified, which can hardly be explained by conventional mechanisms of intrinsic and extrinsic AHE [3]. All these unusual phenomena will be discussed in the light of magnetic fluctuations and novel electron scattering process upon unwinding of the dense topological spin singularities. The formation mechanism of emergent magnetic monopoles will also be discussed, highlighting the role of conduction-electron mediated exchange interactions (e.g. RKKY and related higher-order spin interactions), which is also supported by the experimental observation of skyrmion-monopole transition induced by chemical pressure control in MnSi1-xGex [4]. |
Tuesday, March 15, 2022 4:48PM - 5:00PM |
K54.00008: Distinguishing Hump-shape Hall Effects withKarplus-Luttinger and Topological Origins Zhi Shiuh Lim, May Inn Sim, Lin Er Chow, Ganesh Ji Omar, Anjan Soumyanarayanan, Ariando Ariando Using SrRuO3-based thin film heterostructures, we aim to resolve the two debated interpretations that distinguish between the artefactual humps produced from overlapping double Karplus-Luttinger Anomalous Hall Effects (KL-AHE)[1] and the Geometrical/Topological Hall Effect (THE)[2], without magnetic imaging. Firstly, we prepared two heterostructures with similar hump-shape Hall features which have similar structures to earlier publications with magnetic force microscopy imaging[3,4]. Next, we performed θ-rotation of magnetic field from out-of-plane to in-plane in Hall measurement (Rxy(θ, H)) as the critical differentiation tool. The first heterostructure showing field-position of Hall hump (Hhump) diverging with ~1/cos(θ) can be correctly simulated using double Langevin loops, thus agrees with the expected behaviour of KL-AHE for topological-trivial magnetic domains. Yet, the second one showing constant Hhump with increasing θ agrees with a micromagnetic simulation with Néel-Skyrmions[5,6], thus can be convincingly assigned as Skyrmion-induced THE. The behaviour in the latter case can be explained by the Ginzburg-Landau framework of 3q spin-waves superposition for Skyrmion-lattice[7]. We further extended our investigation scope by Rxy(θ, H) to other reported heterostructures with Hall-humps such as the SrRuO3/ferroelectric in (001)-orientation showing 2q spin-waves[8] and Pt/Tm3Fe5O12(111)[9], and summarize the trends. We will provide magnetic imaging evidence as a strong support. We envision such Rxy(θ, H) scheme could establish as a protocol for identifying Skyrmions. Finally, in the continuous Rxy(θ) curves measured at Hpeak for the second heterostructure, we discuss the finding of several periodic step-functions at θ=(n+1/2)*π/3 as a possible signature of “mirror anomaly”[10,11] arising from massive Dirac Fermions in the k-space band structure of a Skyrmion-lattice[12]. |
Tuesday, March 15, 2022 5:00PM - 5:12PM |
K54.00009: Magnetoresistance in Helical Antiferromagnet Eu Metal Thin Films Narendra Shrestha Europium (Eu) metal has a body centered cubic crystal structure which, upon a paramagnetic-to-helical magnetic transition, undergoes a body centered tetragonal distortion. The magnetic helix appears below Néel temperature (TN) of ~90 K, and the presence of a magnetic field gives rise to conical helimagnet. We have prepared Eu metal thin films on Si (001) substrates using Eu metal as a target by pulsed laser deposition (PLD). The transport measurement shows the helical antiferromagnetic transition at 88 K, and a thermal hysteresis reveals the first-order nature of the phase transition. We present the magnetoresistance (MR) of Eu thin films when the magnetic field (B) is applied both out-of-plane and in-plane. We found, at temperatures below TN, the MR is positive whereas above TN the MR is negative. We also observed the hysteretic oscillation above 30 K and below the Néel temperature. The hysteretic oscillations observed in the MR is believed to be associated with the change in the distribution of the antiferromagnetic domains in Eu caused by the interplay of the applied magnetic field and the strain in the films. |
Tuesday, March 15, 2022 5:12PM - 5:24PM |
K54.00010: Spin Dynamics in Fe3Sn2 frustrated kagome ferromagnet for room-temperature Skyrmion based technology Nirel Bernstein, Ranen Ben-Shalom, Elichai Frohlich, Hang Li, Wenhong Wang, Amir Capua Magnetic skyrmions are attractive for ultra-dense data storage and distribution applications such as the racetrack memory (RM) [1]. Conventionally, the skyrmion’s topological protection is achieved by the Dzyaloshinskii-Moriya interaction (DMI). Recently, frustration in ferromagnetic (FM) crystals was predicted of also being capable of providing the topological protection of the skyrmion without relying on the DMI and the effect was immediately discovered in Fe3Sn2 bulk crystals [2,3]. Surprisingly, these skyrmions survive even at room temperature and are controllable by electrical current [4]. Interestingly, the kagome lattice of the Fe3Sn2 system is expected to possess a non-zero spin Berry curvature thereby giving rise to a large spin Hall effect (SHE) which is central to the skyrmion’s motion. In this work, we explore the underlying dynamics of the Fe3Sn2 system using a time-resolved optically probed Spin-Torque Ferromagnetic Resonance method. Our results indicate a significant SHE that is accompanied by a rich nonlinear response stemming from the unique crystals structure. |
Tuesday, March 15, 2022 5:24PM - 6:00PM |
K54.00011: Topological Hall and Nernst effects in spin-chiral magnets Invited Speaker: Max Hirschberger We are interested in the effect of spin-chirality on the electronic band structure of solids, where topological electronic states may be generated without explicitly invoking spin-orbit coupling. To study the impact of spin-chirality on the Bloch wave functions, it is ideal to choose materials where the length scale of a spin-twist is comparable to the size of a crystallographic unit cell. Highly symmetric intermetallics with tiny skyrmion vortices, representing quantized units of spin chirality, are a perfect playground in this respect [1,2]. In Gd2PdSi3 with triangular lattice of magnetic moments and with skyrmions smaller than 2.8 nanometers in diameter, our meticulous transport experiments revealed giant Hall and Nernst responses [3]. These indicate the importance of Berry-phase theory in a momentum-space picture, beyond the conventional real-space picture suitable to larger-sized skyrmion lattice materials. At temperatures significantly above the transition to long-range order, Dzyaloshinskii-Moriya interactions can imprint themselves as a chiral habit of dynamical spin fluctuations [4]. We demonstrate the modification of Bloch wavepackets and their band dispersion due to thermal fluctuations in a trimer-based ferromagnet with Dzyaloshinskii-Moriya interactions. Intriguing features, such as the comparable size of Hall and thermoelectric Nernst conductivities when measured in fundamental units, are discussed in light of recent advances in ab-initio theoretical modeling. |
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