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Spin polarization in nonmagnetic materials originates from the break of inversion crystalline symmetry that could occur globally or locally. Centrosymmetric (CS) nonmagnetic materials with hidden spin polarization induced by non-CS site symmetries and spin-orbit coupling are promising candidates for spintronic applications, in light of the zero net spin polarization and modulatable spin effects hidden in the local structures. There is, however, an open issue regarding the possible spin splitting induced by broken inversion symmetry at the sample surface. Therefore, it is critical to perform combined experimental and theoretical studies on centrosymmetric TMDC materials such as 2H-MoTe2 to explore the mechanism of the hidden spin polarization. 2H-MoTe2 crystallizes in a trigonal prismatic arrangement with an in-plane lattice constant of 3.517 Å and an out-of-plane lattice constant of 13.962 Å. The unit cell of it is centrosymmetry, but the monolayer has non-centrosymmetric structure. With the virtue of low detection depth of spin-ARPES (&lt;6 Å with He-I), a large spin splitting of 236 meV and opposite spin polarizations up to 80% along out-of-plane direction (z axis) in K and K′ valleys were observed, consist with the density functional theory (DFT) calculations. We further found from the DFT calculations that a medium dipole field mimicked the surface symmetry breaking in ARPES measurements induces negligible variation of spin polarization, confirming that the measured spin effects originate from the intrinsic hidden spin polarization in 2H-MoTe2. The large spin splitting and net spin polarization found in spin-ARPES experiments also suggest that the hidden spin effects in inversion symmetric layered compounds can be used to generate large spin splitting on the surfaces in the absence of strong dipole field. Our combinatorial experimental and theoretical studies clarify the existence of hidden spin polarization in the centrosymmetric materials and opens the way of designing novel functional materials with coexisting hidden spin polarization and other hidden effects, such as hidden orbital polarization and hidden Berry curvature, for the energy efficient spintronics applications.

MMM 2020

Direct observation of hidden spin polarization in 2H-MoTe2

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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Spin polarization in nonmagnetic materials originates from the break of inversion crystalline symmetry that could occur globally or locally. Centrosymmetric (CS) nonmagnetic materials with hidden spin polarization induced by non-CS site symmetries and spin-orbit coupling are promising candidates for spintronic applications, in light of the zero net spin polarization and modulatable spin effects hidden in the local structures. There is, however, an open issue regarding the possible spin splitting induced by broken inversion symmetry at the sample surface. Therefore, it is critical to perform combined experimental and theoretical studies on centrosymmetric TMDC materials such as 2H-MoTe2 to explore the mechanism of the hidden spin polarization. 2H-MoTe2 crystallizes in a trigonal prismatic arrangement with an in-plane lattice constant of 3.517 Å and an out-of-plane lattice constant of 13.962 Å. The unit cell of it is centrosymmetry, but the monolayer has non-centrosymmetric structure. With the virtue of low detection depth of spin-ARPES (<6 Å with He-I), a large spin splitting of 236 meV and opposite spin polarizations up to 80% along out-of-plane direction (z axis) in K and K′ valleys were observed, consist with the density functional theory (DFT) calculations. We further found from the DFT calculations that a medium dipole field mimicked the surface symmetry breaking in ARPES measurements induces negligible variation of spin polarization, confirming that the measured spin effects originate from the intrinsic hidden spin polarization in 2H-MoTe2. The large spin splitting and net spin polarization found in spin-ARPES experiments also suggest that the hidden spin effects in inversion symmetric layered compounds can be used to generate large spin splitting on the surfaces in the absence of strong dipole field. Our combinatorial experimental and theoretical studies clarify the existence of hidden spin polarization in the centrosymmetric materials and opens the way of designing novel functional materials with coexisting hidden spin polarization and other hidden effects, such as hidden orbital polarization and hidden Berry curvature, for the energy efficient spintronics applications.

technical paper

In the spin Hall effect (SHE), a longitudinal charge-current induced transverse pure spin current has been extensively studied for the application of magnetization switching by spin-orbit torque. However, due to the strict right-hand rule among charge current, spin current, and spin polarization, this highly attractive switching scheme is not efficient and often accompanied by an unfavorable external magnetic field in a heterostructure with perpendicular magnetization (PM). In this work, we lift the restriction by demonstrating the magnetization-dependent spin Hall effect (MDSHE) in YIG/PM heterostructure, as shown in Fig. 1. Unlike the conventional SHE with the locked orientation of spin polarization, the spin current induced by the MDSHE can be arbitrarily and independently controlled by the magnetization direction of the magnetic layer. We reported about 3.6% of injected spins experience the effective spin rotation in PM layer of Pt(2)/Co(0.5)/Pt(2). Our approach provides an explicit route in exploring the spin-to-charge conversion with controllable orientations of spin polarization, which offers significant advantages in developing energy-efficient spintronic devices.

Controllable Orientations of Spin Polarization Beyond the Spin Hall Effect

Spin-torque ferromagnetic resonance (ST-FMR) is a common method used to measure spin-orbit torques (SOTs) in heavy metal/ferromagnet bilayer structures. In the course of a measurement, other resonant processes such as spin pumping (SP) and heating can cause spin current or heat flows between the layers, inducing additional resonant voltage signals via the inverse spin Hall effect (ISHE) and Nernst effects (NE). In the standard ST-FMR geometry, these extra artifacts exhibit a dependence on the angle of an in-plane magnetic field that is identical to the rectification signal from the SOTs. We show experimentally that the rectification and artifact voltages can be quantified separately by measuring the ST-FMR signal transverse to the applied current (i.e., in a Hall geometry) in addition to the usual longitudinal geometry. We find that in Pt (6 nm)/CoFeB samples the contribution from the artifacts is small compared to the SOT rectification signal for CoFeB layers thinner than 6 nm, but can be significant for thicker magnetic layers. We observe a sign change in the artifact voltage as a function of CoFeB thickness that we suggest may be due to a competition between a resonant heating effect and the SP/ISHE contribution.

Transverse and Longitudinal Spin-Torque Ferromagnetic Resonance for Improved Measurements of Spin-Orbit Torques

Using the first-principles non-equilibrium Green’s function technique [1] with supercell disorder averaging, we study the influence of interfacial intermixing on the spin-orbit torque in Co/Pt (111) bilayers. The interlayer distances are optimized, several models of intermixing are considered, and atomic potentials in the intermixed layers are obtained using the coherent potential approximation. The magnitude and thickness dependence of the damping-like torque are similar to earlier results for the Co/Pt (001) interface [1,2] and rather insensitive to intermixing. In contrast, the field-like torque, which is small in the case of an ideal interface, is strongly enhanced by intermixing.References: [1] K. D. Belashchenko, A. A. Kovalev, and M. van Schilfgaarde, Phys. Rev. Mater. 3, 011401 (2019). [2] K. D. Belashchenko, A. A. Kovalev, and M. van Schilfgaarde, arXiv:1908.02680.

Effect of interfacial intermixing on spin-orbit torque in Co/Pt bilayers

Weyl semimetals combine both topological and semimetallic effects in their transport of fermions through both bulk and surface states, making them excellent candidates for transverse thermomagnetic transport via the Nernst effect. The Nernst effect is a thermoelectric phenomenon requiring a magnetic field. A temperature gradient is applied to a sample, and the sample is placed in an external magnetic field orthogonal to the temperature gradient. The charge carriers, moving parallel to the temperature gradient, now experience a Lorentz force due to the cross product of the magnetic field and the temperature gradient. An electric field is produced in the mutually perpendicular direction. Single-crystal NbP has shown an unprecedentedly large Nernst thermopower at high magnetic fields, exceeding 800 μV K-1 at 109 K and 9 T [1]. Bulk polycrystalline NbP maintains a large Nernst thermopower, although its magnitude is decreased by approximately a factor of 8 under similar circumstances. Due to the high electrical conductivity of the material, polycrystalline NbP’s Nernst power factor is determined to be 35 x 10-4 W m-1 K-2 at 9 T and 136 K, which is comparable to that of state-of-the-art thermoelectric materials [2]. This high conversion of thermal-to-electrical energy in NbP has been determined as due to the chemical potential’s location near linear, Dirac band crossing points (called Weyl points), giving access to many mobile holes and electrons. These initial results of transverse thermomagnetic transport in both single-crystal and polycrystalline NbP motivate further study to elucidate Weyl fermion transport and the length scales over which it is effective and dominant. Here, we present a grain size study on bulk polycrystalline samples of NbP, where different annealing times result in different grain sizes. Via microstructural characterization and thermomagnetic transport measurements, we can begin to determine the effects of grain size on transport in Weyl semimetals.References: [1] S. J. Watzman et al. Phys. Rev. B 97(16), 161404(R) (2018). [2] C. Fu et al. Energy Environ. Sci. 11(10), 2813-2830 (2018).

Grain Size Dependence of Transverse Thermomagnetic Transport in the Weyl Semimetal NbP

Weyl semimetals (WSMs) are a newly discovered class of quantum materials which can host a number of massless quasiparticles called Weyl fermions. One of the most unique properties of WSMs is the chiral anomaly – a pair of Weyl nodes of opposite chirality acts as source and drain of electrons in the presence of non-perpendicular electric and magnetic fields. To date, the most remarkable phenomenon induced by the chiral anomaly is the longitudinal negative magnetoresistance, which is a linear response effect. In this work, we theoretically investigate the transport properties of WSMs in the nonlinear regime and predict a novel nonlinear Hall effect in tilted non-centrosymmetric Weyl semimetals. Intuitively, a steady-state density difference between a pair of Weyl nodes is established when the chiral pumping and internode relaxation reach a balance, which conspires with the anomalous velocity to give rise to this nonlinear Hall effect. Taking the semiclassical Boltzmann approach, we find that the nonlinear Hall conductivity scales linearly with both the electric and magnetic fields, and depends critically on the tilting of the Weyl cones in both type-I and type-II WSMs. For a pair of Weyl cones that are un-tilted or oppositely tilted, the chiral-anomaly-induced nonlinear Hall current vanishes because it is forbidden by a mirror symmetry. We also show that this effect does not rely on a finite Berry curvature dipole, in contrast to the intrinsic quantum nonlinear Hall effect that was proposed to occur in time-reversal invariant materials [1].References: [1] I. Sodemann and L. Fu, Phys. Rev. Lett. 115, 216806 (2015).

Chiral-Anomaly-Induced Nonlinear Hall Effect in Tilted Weyl Semimetals

We have investigated various electronic transport properties such as the anisotropic magnetoresistance, planar Hall effect, and anomalous Hall effect in kagome Fe3Sn2, which has been predicted to be a type II Weyl metal. Based on magnetortransport, we determine the spin reorientation transition to peak at 120 K and coexistence of both out of plane and in plane phases at temperatures around the spin reorientation, indicative of a first order phase transition. In planar Hall effect, we discover a field antisymmetric contribution in addition to the standard field symmetric contribution. This field antisymmetric planar Hall effect has an in-plane 3-fold rotational symmetry, which is distinctively different from the symmetric counterpart. Furthermore, the temperature and field dependence of the field antisymmetric contribution is different from both the symmetric planar Hall effect and anomalous Hall effect, pointing to a different origin. A linear and antisymmetric planar Hall effect is predicted to be present in the case of non-magnetic Weyl semimetals. The experimental signatures in our case fit well with the prediction in addition to reflecting contributions from its ferromagnetism. As the magnetization of Fe3Sn2 has a 3-fold rotational degeneracy, the associated band structure and Weyl nodes determined by the magnetization direction owing to the spin-orbit coupling will have accordingly a 3-fold rotational degeneracy giving rise to the 3-fold antisymmetric PHE. We thus propose that the antisymmetric PHE that we discovered is the realization of an antisymmetric and linear planar Hall effect due to the presence of Weyl nodes near the Fermi level in a time reversal symmetry broken system. Our finding offers a promising route for macroscopically probing Weyl systems.References: [1] Neeraj Kumar, Y. Soh, Yihao Wang, et al., Anomalous planar Hall effect in a kagome ferromagnet, arXiv:2005.14237 (2020). [2] Nandy, S., G. Sharma, A. Taraphder, et al., Chiral Anomaly as the Origin of the Planar Hall Effect in Weyl Semimetals. Phys Rev Lett, 119(17): p. 176804 (2017). [3] M. Yao, H. Lee, N. Xu, et al., Switchable Weyl nodes in topological Kagome ferromagnet Fe3Sn2, arXiv:1810.01514 (2018).

Anomalous planar Hall effect in predicted type II Weyl Fe3Sn2

YbPtBi is a half Heusler alloy with claimed properties of topological Weyl nodes and chiral anomaly; antiferromagnetism from the RKKY coupling of localized Yb f-electrons at low temperatures; and heavy fermion behaviour as those localized f-electrons interact with the conduction bands [1,2]. So far only bulk crystals have been grown of YbPtBi with no published work on thin films. By using c-plane sapphire the half Heusler alloy has an epitaxially relationship of YbPtBi(111)||Al2O3(0001) and YbPtBi(110)||Al2O3(1100). With thin films we could have to option of induced strain, device fabrication and many other thin films manipulation techniques. In this work, we present a study of epitaxially grown YbPtBi thin films that grew as triangular islands with matched faceted edge orientations. The structure is confirmed to be epitaxial from x-ray diffraction and cross-sectional transmission electron microscopy. We also measure the electrical transport properties and find a non-metallic ρ(T) similar to other semimetals RePtBi (Re=Gd, Ce, Sm) half Heusler Alloy, although different from bulk YbPtBi. We measured a negative longitudinal magnetoresistance and the temperature dependence of an apparent chiral anomaly (Fig: a,b). Both of which are indicators of a possible Weyl semimetal state. We also obversed a large hysteretic MR at very low temperatures, 250 mK, not previously seen in YbPtBi.References: [1] C. Y. Guo, F. Wu, Z. Z. Wu, M. Smidman, C. Cao, A. Bostwick, C. Jozwiak, E. Rotenberg,Y. Liu, F. Steglich, and H. Q. Yuan, Nature Communications. 9, 4622 (2018) [2] B. G. Ueland, A. Kreyssig, E. D. Mun, J. W. Lynn, L. W. Harriger, D. K. Pratt, K. Prokeš, Z. Hüsges, R. Toft-Petersen, S. Sauerbrei, S. M. Saunders, Y. Furukawa, S. L. Bud’ko, R. J.McQueeney, P. C. Canfield, and A. I. Goldman, Phys. Rev. B. 99, 184431 (2019)

Epitaxial Growth Of The Weyl Semimetal YbPtBi (111) On C-plane Sapphire By Co-Sputtering

The magnetic properties of van der Waals magnetic topological insulators MnBi2Te4 and MnBi4Te7 are investigated by magneto-transport measurements. We evidence that the relative strength of the inter-layer exchange coupling J to the uniaxial anisotropy K controls a transition from an A-type antiferromagnetic order to a ferromagnetic-like metamagnetic state. A bi-layer Stoner-Wohlfarth model allows us to describe this evolution, as well as the typical angular dependence of specific signatures, such as the spin-flop transition of the uniaxial antiferromagnet and the switching field of the metamagnet. In micron-size magnets, the single-domain switching-field astroid are however partly truncated by the nucleation of domain walls along the easy-axis direction.References: A. Tan, V. Labracherie, N. Kunchur, A. U. B. Wolter, J. Cornejo, J. Dufouleur, B. Büchner, A. Isaeva and R. Giraud Metamagnetism of Weakly Coupled Antiferromagnetic Topological Insulators. Phys. Rev. Lett. 124, 197201. (2020)

Meta-Magnetism of Weakly-Coupled Antiferromagnetic Topological Insulators

The rare earth nitride series of intrinsic ferromagnetic semiconductors offers a rich selection of spin-orbit systems in which to study electrical transport and magnetic phenomenon, the various Hall effects being one of the many meeting places of these[1,2]. The central member of the series GdN (8S7/2) has a strong spin only magnetisation and relatively simple band structure, dominated by the Gd 5d conduction band and N 2p valence band. Electron doping is routinely achieved via the inclusion of nitrogen vacancies. Here we report on 'anomalous' Hall effect data in heavily doped GdN, which show an apparent change of sign of the dominant charge carrier as the sample is cooled through the ferromagnetic transition, while the anomalous Hall effect retains the negative sign of electron doped GdN. We will discuss these in the context of the calculated band structures of stoichiometric and heavily doped GdN.References: [1] W. F. Holmes-Hewett, R. G. Buckley, B. J. Ruck, F. Natali and H. J. Trodahl, 4f conduction in the magnetic semiconductor NdN, Phys. Rev. B. 100, 195119 (2019) [2] W. F. Holmes-Hewett, F. H. Ullstad, B. J. Ruck, F. Natali and H. J. Trodahl, Anomalous Hall effect in SmN: Influence of orbital magnetism and 4f band conduction, Phys. Rev. B. 98, 235201 (2018)

Anomalous Hall effects in rare earth nitrides

How strong correlations drive quantum states of matter with nontrivial topology is an outstanding issue in contemporary condensed matter physics. This talk will provide an overview and discuss the latest development on Weyl-Kondo semimetal, which has been concurrently advanced by theoretical [1,2] and experimental [3,4,5] studies. The theoretical work addressed a nonsymmorphic Anderson lattice model that breaks the inversion symmetry while preserving the time-reversal symmetry, whereas the experimental effort focused on the non-magnetic non-centrosymmetric and non-symmorphic heavy fermion semimetal Ce3Bi4Pd3. The salient signatures of the Weyl-Kondo semimetal include a correlation enhanced specific heat C=ΓT3 [3], and a giant zero-magnetic-field Hall response and even-in-field Hall effects [4]. The latest high magnetic field experiments observed a two-stage topological phase transition, from Weyl-Kondo semimetal to Kondo insulator to heavy-fermion metal [5]. To bridge the latest experiments and our theoretical work, we investigated the aforementioned Anderson lattice model in the presence of a Zeeman coupling. Tuning both the Zeeman coupling and the inversion-symmetry-breaking potential controls the position and number of Weyl nodes in the Brillouin zone, and several topologically distinct phases emerge in this model.References: [1] H.-H. Lai, S. E. Grefe, S. Paschen, and Q. Si, Proc. Natl. Acad. Sci. U.S.A. 115, 93 (2018). [2] S. E. Grefe, H.-H. Lai, S. Paschen, and Q. Si, Phys. Rev. B, 101, 075138 (2020). [3] S. Dzsaber, L. Prochaska, A. Sidorenko, G. Eguchi, R. Svagera, M. Waas, A. Prokofiev, Q. Si, and S. Paschen, Phys. Rev. Lett. 118, 246601 (2017). [4] S. Dzsaber, X. Yan, G. Eguchi, A. Prokofiev, T. Shiroka, P. Blaha, O. Rubel, S. E. Grefe, H.-H. Lai, Q. Si, and S. Paschen., arXiv preprint arXiv:1811.02819, (2018). [5] S. Dzsaber, D. A. Zocco, A. McCollam, F. Weickert, R. McDonald, M. Taupin, X. Yan, A. Prokofiev, L. M. K. Tang, B. Vlaar, et al., arXiv preprint arXiv:1906.01182 (2019).

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Direct observation of hidden spin polarization in 2H-MoTe2

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