Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session Y28: Antiferromagnets, Dynamics, and Spin Texture |
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Sponsoring Units: GMAG Chair: Alan Farhan, Lawrence Berkeley National Lab Room: 291 |
Friday, March 17, 2017 11:15AM - 11:27AM |
Y28.00001: Polarization-dependent antiferromagnetic domain wall motion Weichao Yu, Jin Lan, Jiang Xiao In antiferromagnet, the polarization freedom of spin waves is fully unlocked. Here, we investigate the antiferromagnetic domain wall motion driven by linearly polarized spin waves. Following momentum conservation laws for domain wall system, we establish a rigid domain wall model for antiferromagnetic domain wall motion driven by spin wave with arbitrary polarizations. We discover that, originating from the distinct transmission behaviors of two linear spin wave polarizations when passing through the domain wall, the domain wall can be pushed or dragged by spin wave depending on the carried linear polarization direction. This theoretical prediction on polarization-dependent domain wall motion is confirmed by magnetic simulations. Steering antiferromagnetic domain wall motion by simply tuning the polarization of inject spin wave, as demonstrated here, offers new designing principles for building domain-wall based magnetic processing components. [Preview Abstract] |
Friday, March 17, 2017 11:27AM - 11:39AM |
Y28.00002: Antiferromagnetic domain wall as spin wave polarizer Jin Lan, Weichao Yu, Jiang Xiao Spin waves are collective excitations of local magnetizations that can effectively propagate information even in magnetic insulators. In antiferromagnet, spin waves are endowed with additional polarization freedom. Here we propose that the antiferromagnetic domain wall can act as a spin wave polarizer, which perfectly passes one linearly polarized spin wave while substantially reflects the perpendicular one. We show that the polarizing effect lies in the suppression of one linear polarization inside domain wall, in close analogy to the wire-grid optical polarizer. Our finding opens up new possibilities in magnonic processing by harnessing spin wave polarization in antiferromagnet. [Preview Abstract] |
Friday, March 17, 2017 11:39AM - 11:51AM |
Y28.00003: How to move topological solitons in antiferromagnets Sayak Dasgupta, Oleg Tchernyshyov We study the dynamics of topological solitons in antiferromagnets using a Lagrangian formalism constructed in terms of collective coordinates representing the soft modes of the defect. We adopt a procedure by which we can effectively study the interplay of internal exchange fields and external fields (magnetic and spin current) which allows us to determine whether solitons can be moved and under what conditions. In our investigation we consider the examples of one dimensional domain walls and planar vortices focusing on their dynamics. It turns out that to effectively move the soliton either a finite ferromagnetic moment has to be induced through an asymmetric exchange, or there must be internal forces exerting a static friction on the defect. We also outline a procedure, using a crossed magnetic (out of plane) and spin current field, by which one can move antiferromagnet vortices in planar geometries without the requirement of an asymmetric exchange. [Preview Abstract] |
Friday, March 17, 2017 11:51AM - 12:03PM |
Y28.00004: Prediction of an intrinsic spin Hall effect without spin-orbit coupling in non-collinear antiferromagnets Yang Zhang, Jakub Zelezny, Jeroen van den Brink, Claudia Felser, Binghai Yan The spin Hall effect (SHE), which converts a charge current into a transverse spin current, has long been believed to be a phenomenon induced by the spin-orbit coupling. In this work, we have revealed the existence of an intrinsic SHE without the spin-orbit coupling by theoretical calculations. Such a SHE is realised in the chiral spin structure of non-collinear antiferromagnets, even when the scalar spin chirality is zero. We have obtained large intrinsic spin Hall conductivity in related compounds Mn$_3$Ge and Mn$_3$Sn, that are chiral antiferromagnetic above room temperature and also predicted to be Weyl semimetals recently. Our work provides further understanding on the spin Hall effect and paves a new way to design SHE materials based on the chiral magnetic materials. [Preview Abstract] |
Friday, March 17, 2017 12:03PM - 12:15PM |
Y28.00005: Orbital coupling of noncollinear antiferromagnets to magnetic fields Hua Chen, Qian Niu, Guang-Yu Guo, Allan H. MacDonald The same symmetry considerations that imply a non-zero anomalous Hall effect in certain noncollinear antiferromagnets also imply non-zero total spin magnetization due to canting and finite orbital magnetization. It has been understood recently that the orbital magnetization of periodic crystals has a so-called "itinerant" contribution related to the Berry curvature of Bloch bands, which is not necessarily small, in contrast to the canting-induced total spin magnetization. We have explicitly calculated the orbital magnetization of several noncollinear antiferromagnets. In all cases we find that they are orders of magnitude larger than the total spin magnetization. Coupling between orbital magnetization and external magnetic fields is thus expected to be dominant in switching the direction of the magnetic order, and hence the anomalous Hall effect. Our calculation points to the important role of the transverse spin-orbital susceptibility in noncollinear antiferromagnets, compared to spin-spin and orbital-orbital susceptibilities. We use simple models as well as first-principles calculations to demonstrate a number of unique behaviors associated with magnetic field-induced order parameter switching in noncollinear antiferromagnets, and discuss their experimental implications. [Preview Abstract] |
Friday, March 17, 2017 12:15PM - 12:27PM |
Y28.00006: Novel spin currents in non-collinear antiferromagnets Jakub Zelezny, Yang Zhang, Claudia Felser, Binghai Yan Many key spintronics phenomana are caused by spin currents. Here we study spin currents in non-collinear antiferromagnets Mn$_3$Sn and Mn$_3$Ir. It was recently demonstrated that a large spin Hall effect exists in these materials. We show by symmetry analysis and microscopic ab-initio calculations that in these antiferromagnets, also a different type of spin currents occur, which have an origin and symmetry distinct from the spin Hall effect. These spin currents are similar to spin-polarized currents in ferromagnets, however, unlike in ferromagnets, they also contain a transversal contribution (i.e., a spin current flowing in the direction transversal to the charge current). Our calculations reveal that both the spin-polarized currents and the transversal spin currents are large in the studied materials. These spin currents could have important applications since the effects that originate from the spin-polarized current in ferromagnets, like the tunneling magnetoresistance or the spin-transfer torque, will also be present in the non-collinear antiferromagnets. Furthermore, the transversal spin currents will contribute to the spin-orbit torque generated by the spin Hall effect. [Preview Abstract] |
Friday, March 17, 2017 12:27PM - 12:39PM |
Y28.00007: Currentless reversal of N\'{e}el vector in antiferromagnets Yuriy Semenov, Xilai Li, Ki Wook Kim The bias driven perpendicular magnetic anisotropy is a magneto-electric effect that can realize 90$^0$ magnetization rotation and even 180$^0$ flip along the easy axis in the ferromagnets with a minimal energy consumption. This study theoretically demonstrates a similar phenomenon of the N\'{e}el vector reversal via a short electrical pulse that can mediate perpendicular magnetic anisotropy in the antiferromagnets. The analysis based on the dynamical equations as well as the micromagnetic simulations reveals the important role of the inertial behavior in the antiferromagnets that facilitates the N\'{e}el vector to overcome the barrier between two free-energy minima of the bistable states along the easy axis. In contrast to the ferromagnets, this N\'{e}el vector reversal does not accompany angular moment transfer to the environment, leading to acceleration in the dynamical response by a few orders of magnitude. Further, a small switching energy requirement of a few attojoules illustrates an added advantage of the phenomenon in low-power spintronic applications. [Preview Abstract] |
Friday, March 17, 2017 12:39PM - 12:51PM |
Y28.00008: High-frequency effects in antiferromagnetic Sr$_{\mathrm{\mathbf{3}}}$\textbf{Ir}$_{\mathrm{\mathbf{2}}}$\textbf{O}$_{\mathrm{\mathbf{7}}}$ Morgan Williamson, Heidi Seinige, Shida Shen, Cheng Wang, Gang Cao, Jianshi Zhou, John Goodenough, Maxim Tsoi Antiferromagnetic (AFM) spintronics is one of many promising routes for `beyond the CMOS' technologies where unique properties of AFM materials are exploited to achieve new and improved functionalities. AFMs are especially interesting for high-speed memory applications thanks to their high natural frequencies. Here we report the effects of high-frequency (microwave) currents on transport properties of antiferromagnetic Mott insulator Sr$_{\mathrm{3}}$Ir$_{\mathrm{2}}$O$_{\mathrm{7}}$. The microwaves at 3-7 GHz were found to affect the material's current-voltage characteristic and produce resonance-like features that we tentatively associate with the dissipationless magnonics recently predicted to occur in antiferromagnetic insulators subject to ac electric fields [1]. Our observations support the potential of antiferromagnetic materials for high-speed/high-frequency spintronic applications. This work was supported in part by C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA, by NSF grants DMR-1207577, DMR-1265162, DMR-1600057, and DMR-1122603, and by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award No. OSR-2015-CRG4-2626. [1] W. Chen and M. Sigrist, Phys. Rev. Lett. 114, 157203 (2015). [Preview Abstract] |
Friday, March 17, 2017 12:51PM - 1:03PM |
Y28.00009: Spin dynamics in the antiferromagnetic phases of the Dirac metals AMnBi(A $=$ Sr, Ca) Marein Rahn, Andrew Princep, Andrea Piovano, Jiri Kulda, Yan Feng Guo, You Guo Shi, Andrew Boothroyd The square Bi layers in AMnBi(A $=$ Sr, Ca) host strongly spin-orbit coupled bands that form highly anisotropic Dirac points. We have mapped the dispersion of Mn spin fluctuations in these materials by triple-axis neutron spectroscopy. The spectra show pronounced spin gaps of 10.(2) meV (Sr) and 8.(8) meV (Ca) and extend to maximum energy transfers of 61-63 meV. For each material, reciprocal space maps of the magnon spectral weight were recorded for both in-plane and out-of-plane momentum transfer. The observed spectra can be accurately reproduced by a simple linear spin-wave model. Detailed global '?fits of the full magnon dispersion allow tight constraints on the magnitude of in-plane and inter-layer exchange parameters as well as on the magnetocrystalline anisotropy constant. We '?find no evidence that the magnetic ground state of these materials is coupled to the topology of the Bi 6p$_{\mathrm{x,y}}$bands. [Preview Abstract] |
Friday, March 17, 2017 1:03PM - 1:15PM |
Y28.00010: Magnetic and Thermal Properties of FeMn2O4 Single Crystals Roshan Nepal, Qiang Zhang, Wei Tian, Stephen Nagler, Rongying Jin We have investigated the magnetic and thermal properties of single-crystalline FeMn$_{\mathrm{2}}$O$_{\mathrm{4}}$, which forms an inverse spinel structure. Specific heat and thermal conductivity reveal smooth but unusual temperature dependence below 400 K. Quantitative data analysis suggests that these quantities contain large magnetic contributions, reflected by the T$^{\mathrm{3/2}}$ dependence of specific heat at low temperatures and the speedy drop of thermal conductivity below 75 K. Correspondingly, there is a sharp decrease of magnetization at T$_{\mathrm{N1}}\approx $ 75 K. Further investigation of neutron powder diffraction indicates that the system undergoes two magnetic transitions on cooling: one from paramagnetic to a collinear ferrimagnetic (FI) order at T$_{\mathrm{N2}}\approx $ 420 K, followed by another transition to a non-collinear FI order at T$_{\mathrm{N1}}\approx $ 75 K. The magnetic structure-property relationship will be discussed. [Preview Abstract] |
Friday, March 17, 2017 1:15PM - 1:27PM |
Y28.00011: Magnon-skyrmion interactions in collinear antiferromagnets Matthew Daniels, Ran Cheng, Jiang Xiao, Di Xiao Due to their favorable material parameters and intruiging magnonic properties, AFM spin waves have recently become the subject of intense research. We investigate the interaction between AFM spin waves and skyrmion configurations of the AFM staggered order. By expressing the spin wave forces on the skyrmion in terms of semiclassical wavepacket theory, we find that the primary interaction takes the form of a Lorentz force, with the role of electronic charge played by the magnon chirality. We also find that, while magnons penetrate the skrymion profile, they incur a reduction in the skyrmion’s inertial mass. Neglecting magnon-magnon interactions, we then integrate these equations of motion over wavepacket distributions to derive a spin wave current response formalism. [Preview Abstract] |
Friday, March 17, 2017 1:27PM - 1:39PM |
Y28.00012: Effects of interfacial frustration in ferromagnet/antiferromagnet bilayers Sergei Urazhdin, Tianyu Ma While the ferromagnet (F)/antiferromagnet (AF) bilayers have been extensively studied in the context of exchange bias, and more recently in the context of antiferromagnetic spintronics, the fundamental understanding of the nature of the magnetic state in this system is still a subject a debate. We will present measurements of magnetization aging in several F/AF systems based on AF=FeMn, CoO, and NiO, universally observed in all of these systems when AF layers are sufficiently thin. Quite generally, the aging curves are well-described by the power law with a small exponent. We show that the aging characteristics such as the dependence on temperature and the magnetic history are inconsistent with the Arrhenius activation, disproving the granular models of exchange bias. Furthermore, we show that the aging characteristics qualitatively change across the exchange bias blocking temperature, demonstrating that the latter is similar to the glass transition temperature, and is not simply of a characteristic activation temperature of the AF domains. We discuss the our findings in the context of frustration due to the random effective exchange field at the F/AF interface. [Preview Abstract] |
Friday, March 17, 2017 1:39PM - 1:51PM |
Y28.00013: Competing effect of ferromagnetic/antiferromagnetic interfacial couplings in epitaxial Ni/CoO/Fe trilayer grown on vicinal MgO(001) Mengmeng Yang, Qian Li, Alpha N'Diaye, Elke Arenholz, Andreas Scholl, Anthony Young, Qiaoyan Dong, Chanyong Hwang, Jia Li, Ziqiang Qiu It is well known that cooling of a ferromagnet (FM)/antiferromagnet (AFM) bilayer trains the AFM spin configuration which in turn affects the FM layer property. Cooling of a FM/AFM/FM trilayer therefore results in a competing effect between the two FM/AFM interfacial couplings. Here we report an experimental study on Ni/CoO/Fe single crystalline trilayer grown epitaxially on a 7° vicinal MgO(001) substrate. Utilizing element-resolved X-ray Magnetic Circular Dichroism (XMCD) and X-ray Magnetic Linear Dichroism (XMLD) measurements, we found that the Ni magnetization is canted towards out-of-plane direction after cooling the Ni/CoO/Fe trilayer from room temperature to 80K to establish the CoO AFM order. This canting disappears in Ni/CoO bilayer, showing that the canting is from the competing effect between the Ni/CoO and CoO/Fe interfacial couplings. Detailed analysis shows that the CoO spins have an out-of-plane component due to the presence of Fe in the Ni/CoO/Fe trilayer. Photoemission Electron Microscopy (PEEM) imaging shows an orthogonal interfacial coupling at both the Ni/CoO and the CoO/Fe interfaces. [Preview Abstract] |
Friday, March 17, 2017 1:51PM - 2:03PM |
Y28.00014: Detection of Pinned Uncompensated Magnetization in Antiferromagnet Using Magnetotransport and Polarized Neutron Reflectometry. Pavel N. Lapa, Igor V. Roshchin, Artur Glavic, Haile Ambaye, Valeria Lauter, K. D. Belashchenko, Tatiana Eggers, Casey W. Miller, Junjia Ding, John. E. Pearson, Valentine Novosad, J. S. Jiang, Axel Hoffmann Pinned uncompensated magnetization (PUM) in an antiferromagnet (AF) is a keystone of exchange bias and future AF-based spintronic devices. Using magnetotransport and polarized neutron reflectometry (PNR) measurements, PUM is studied in Cu/FeMn/Cu AF-only exchange bias system. For spin valves composed of ferromagnetic (permalloy) and AF (FeMn) layers separated by a conducting buffer (Cu), angular dependence of the resistance is measured in external magnetic field. Scattering on the PUM in the AF yields giant magnetoresistance (GMR) which is used for a quantitative estimate of the PUM in different magnetic fields and at different temperatures. We detected PUM only at the interface of FeMn with the bottom Cu layer. Amazingly, it survives in 110 kOe magnetic field. This is an evidence that the PUM is a part of the staggered AF spin structure of FeMn. The correlation between the results obtained using magnetometry, PNR, and magnetotransport techniques will be discussed. Work is supported by Texas A{\&}M Univ., US DOE MSE (ANL) and BES SUE (ORNL-SNS), NSF-CAREER (USF, RIT), NSF DMR MRSEC (UNL). [Preview Abstract] |
Friday, March 17, 2017 2:03PM - 2:15PM |
Y28.00015: Magnon spin texture in momentum space Nobuyuki Okuma A Magnon plays a central role in recent spintronics. In a ferromagnet, magnons are bosons with spin 1, and the current of the magnons can be interpreted as a spin current. More recently, magnons in other magnetic structures such as antiferromagnets have attracted much attention and been expected to have new properties. In this study, we consider the momentum-dependence of magnon spin moment. In some electron systems such as a topological insulator surface state, electron spin moment depends on the electron momentum (spin-momentum locking). We generalize the notion of the momentum-dependence of spin moment to the magnonic systems. We define the momentum-dependent spin for general magnon Hamiltonians. As an example, we consider the magnon spin texture in momentum space for a kagome lattice antiferromagnet. When the system has a rotational symmetry, spin moment has no momentum-dependence since spin is a good quantum number. To break the rotational symmetry, we consider the Dzyaloshinskii-Moriya term, which plays an important role in a realized kagome lattice antiferromagnet, adding to the Heisenberg Hamiltonian. Using the definition, we obtain three magnon bands: two dispersive bands with and without spin texture, and the flat band with spin texture. [Preview Abstract] |
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