Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F41: Novel Magnetic Structures and Excitations IIFocus

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Sponsoring Units: GMAG DMP Chair: Adam Ahmed, Ohio State University Room: BCEC 209 
Tuesday, March 5, 2019 11:15AM  11:51AM 
F41.00001: Observation of magnetic vortex pairs at room temperature in a planar αFe_{2}O_{3}/Co heterostructure Invited Speaker: Paolo G. Radaelli Vortices, occurring whenever a flow field ‘whirls’ around a onedimensional core, are among the simplest topological structures, ubiquitous to many branches of physics. In the crystalline state, vortex formation is rare, since it is generally hampered by longrange interactions: in ferroic materials (ferromagnetic and ferroelectric), vortices are observed only when the effects of the dipole–dipole interaction are modified by confinement at the nanoscale, or when the parameter associated with the vorticity does not couple directly with strain. We observed an unprecedented form of vortices in antiferromagnetic haematite (αFe_{2}O_{3}) epitaxial films, in which the primary whirling parameter is the staggered magnetization [1]. Remarkably, ferromagnetic topological objects with the same vorticity and winding number as the αFe_{2}O_{3} vortices are imprinted onto an ultrathin Co ferromagnetic overlayer by interfacial exchange. Our data, supported by mircomagnetic modelling, suggest that the ferromagnetic vortices may be merons (halfskyrmions, carrying an outof plane core magnetization), and indicate that the vortex/meron pairs can be manipulated by the application of an inplane magnetic field, giving rise to largescale vortex–antivortex annihilation. 
Tuesday, March 5, 2019 11:51AM  12:03PM 
F41.00002: Collinear Anomalous Hall Antiferromagnets Hua Chen, Allan MacDonald It is now well established both theoretically and experimentally that the anomalous Hall effect (AHE) can exist in certain noncollinear antiferromagnets with vanishing total magnetization. Using similar symmetry arguments we propose that the AHE and the related properties such as magnetooptical Kerr effect, anomalous Nernst effect, orbital magnetization, etc., can also exist in many collinear antiferromagnets with symmetryallowed spin canting. Similar to the noncollinear case, these AHErelated effects will still be finite when the net spin magnetization vanishes. We give two classic examples, NiF_{2} and αFe_{2}O_{3}, corresponding to different mechanisms for spin canting, i.e. singleion anisotropy and DzyaloshinskiiMoriya interaction. Although these two materials are good insulators and may not be easily doped in order to measure the AHE, the discussion is general and can be applied to many other canted antiferromagnets. We construct minimal models with spinorbit coupling terms compatible with these canting mechanisms, and discuss the similarities and differences between the collinear and the noncollinear cases. 
Tuesday, March 5, 2019 12:03PM  12:15PM 
F41.00003: Nonlocal Spin Transport Mediated by a Vortex Liquid in Superconductors Se Kwon Kim, Roberto Myers, Yaroslav Tserkovnyak Departing from the conventional view on superconducting vortices as a parasitic source of dissipation for charge transport, we propose to use mobile vortices as topologicallystable information carriers [1]. To this end, we start by constructing a phenomenological theory for the interconversion between spin and vorticity, a topological charge carried by vortices, at the interface between a magnetic insulator and a superconductor, by invoking the interfacial spin Hall effect therein. We then show that a vortex liquid in superconductors can serve as a spintransport channel between two magnetic insulators by encoding spin information in the vorticity. The vortexmediated nonlocal signal between the two magnetic insulators is shown to decay algebraically as a function of their separation, contrasting with the exponential decay of the quasiparticlemediated spin transport. 
Tuesday, March 5, 2019 12:15PM  12:27PM 
F41.00004: Detecting spin current fluctuations in quantum magnets via microwave resonators Joshua Aftergood, So Takei We theoretically examine spin transport across coupled quantum spin chains that are further coupled to a microwave cavity in series with a transmission line. Spins in the quantum magnet couple inductively to the microwave field and imprint spin transport signatures into the output photon field detected on the transmission line. We first show that in the noninvasive coupling limit the total output photon power is directly proportional to spin current noise at the cavity resonance frequency, and thus that spin current noise is detectable without recourse to destructive techniques. We also discuss the possibility of photon feedback effects on the spin conductivity and noise for the invasive coupling limit. 
Tuesday, March 5, 2019 12:27PM  12:39PM 
F41.00005: Development of a System for Low Temperature Optical Measurement of ThreeDimensional Magnon, Plasmon and Spin Torque Transfer Dynamics YuSheng Ou, Xinran Zhou, Harsha Kannan, Hang Chen, Rasoul Barri, Stephanie Ann Law, John Q Xiao, Matthew F Doty Spindependent phenomena in magnetic heterostructures and topological insulators (TIs) have attracted lots of attention from the perspective of both fundamental science and device development. For example, the spin orbital interaction in ferromagnet (FM)/heavy metal (HM) bilayers allows for allelectrical manipulation of magnetization. Moreover, the protected and lineardispersed surface states of TIs leads to the unique spinmomentum locking. Despite extensive studies of these novel phenomena, there remain important questions about their underlying mechanisms. For example, the dynamics of these phenomena, which are critical for device applications, remain poorly understood. To address these important questions in the field, we have developed an experimental apparatus allowing ultrafast and quasiDC optical study of magnon, plasmon, and spin orbit torque (SOT) in a wide variety of magnetic systems at low temperature and in the presence of a twodimensional magnetic field. To benchmark the capability of this instrument, we present data demonstrating timeresolved MagnetoOptical Kerr Effect (TRMOKE) study of the SOTdriven magnetization dynamics in both Py/Pt and CoFeB/Ta bilayers. 
Tuesday, March 5, 2019 12:39PM  12:51PM 
F41.00006: Timeresolved Magnetooptical Kerr Effect Studies on Permalloy/Ru/Permendur Trilayers Hengzhou Liu, Christopher E Stevens, Mojtaba Ranjbar, Johan Akerman, Yevgen Pogoryelov, Olof Karis, Dario A Arena, Denis Karaiskaj We investigate a series of magnetic trilayer samples consisting of Ni_{20}Fe_{80}(Py)/Ru [x]/Fe_{49}Co_{49}V_{2}(Pmd), where the Ru spacer thickness [x] is varied from 0.7 nm to 17 nm to change the indirect exchange coupling (IEC) between the two magnetic layers. We study the dynamics of these structures by using timeresolved magnetooptical Kerr effect (TRMOKE) to observe the initial demagnetization and subsequent precession of the magnetic moments as the system returns to equilibrium. Even though the TRMOKE signal is generated primarily from the top Pmd layer, we observe an unusual beating pattern in the TRMOKE signal indicative of two coupled oscillators with different precession frequencies. The coupling between the two oscillators depends on the thickness of the Ru spacer layer and hence the strength of the IEC. The results point to the usefulness of TRMOKE in investigating dynamic coupling effects, including spin pumping, in layered magnetic films. 
Tuesday, March 5, 2019 12:51PM  1:03PM 
F41.00007: Structural, Magnetic, and Transport Properties of Fe(1x)Rh(x)/MgO(001) Films Grown by MolecularBeam Epitaxy Antonio Mei, Darrell G. Schlom Fe(1x)Rh(x) layers are grown with varying rhodium fraction x on (001)oriented MgO substrates by molecularbeam epitaxy. Film structural, morphological, magnetic, and transport properties are investigated. At room temperature, layers are ferromagnetic (FM) for x ≤ 0.48 and antiferromagnetic (AF) for x > 0.48. Separating the two magnetically ordered phases at x = 0.48 is an abrupt change in the Fe(1x)Rh(x) lattice parameter of Δa = 0.0028 nm (Δa/a = −0.9%). For AF layers, the FM state is recovered by heating across a firstorder phase transition. The transition leads to a large resistivity modulation, Δρ/ρ = 80%, over a narrow temperature range, ΔT = 3 K, in stoichiometric Fe(0.50)Rh(0.50)/MgO(001). The resistivity change is explained using a model based on independent spin conduction channels. For samples with compositions deviating from x = 0.50, fluctuations broaden ΔT and defect scattering reduces Δρ/ρ. 
Tuesday, March 5, 2019 1:03PM  1:15PM 
F41.00008: Spin Transport Properties of FeRh Across its Magnetic Phase Transition Hilal Saglam, Changjiang Liu, Yi Li, Deshun Hong, Vedat Karakas, Ozhan Ozatay, Joseph Sklenar, Wei Zhang, Anand Bhattacharya, Axel F Hoffmann Recent discoveries in antiferromagnets (AFs) such as spinorbit torques, spin Seebeck effects and inverse spin Hall effects have opened up new possibilities for spintronics devices.[1] Of particular interest is the equiatomic FeRh, which undergoes a temperature driven antiferromagnettoferromagnet magnetic phase transition. This metallic AF is also promising for spintronics applications due to its relatively large spinorbit coupling arising from Rh. We have grown epitaxial FeRh films on MgO (100) and patterned them into measurement devices using photolithography and ion milling. We performed anomalous Hall effect (AHE) and anomalous Nernst effect (ANE) measurements on 20nmthick FeRh films at various temperatures. Our findings show a drastic suppression of both AHE and ANE signals in the AF phase. Interestingly, these nonvanishing signals are opposite in sign compared to their ferromagnetic counterparts, which may suggest changes of inherent symmetries in the electronic structure of FeRh across its magnetic phase transition. 
Tuesday, March 5, 2019 1:15PM  1:27PM 
F41.00009: Dynamical spinreorientation transition in Fe/2ML Ni/W(110) ultrathin films David Venus, Gengming He, Randy KR Belanger, Peter Nguyen The spinreorientation transition occurring in perpendicularlymagnetized films as a function of film thickness is well known. Longrange dipole interactions act to form antiparallel domains in the perpendicular phase that precedes the reorientation to inplane magnetization via a canted phase. In equilibrium, the domains walls move freely to minimize the global energy and form a uniform striped pattern. This spatial averaging causes the reorientation to occur at a noninteger number of monolayers (ML). When the domain walls are pinned in thinner films, the system minimizes energy locally, and a metastable reorientation transition occurs on isolated islands that are 1 ML thicker than the surrounding film. These two versions of the same transition produce two separate peaks in the susceptibility χ_{⊥} (in a perpendicular field), if it is measured as the film is grown. We report here observation of a dynamical version of the reorientation transition as the domain walls depin and the system moves from locally to globally determined energetics. The measured susceptibility χ_{001} (in an inplane field) exhibits a divergence. These observations provide insight to the role of dipole interactions in the transitions from the perpendicular phase to the paramagnetic and canted phases. 
(Author Not Attending)

F41.00010: Comparative study of methodologies to compute the intrinsic Gilbert damping: interrelations, validity and physical consequences Filipe Guimaraes, Jens Renè Suckert, Jonathan Chico, Juba Bouaziz, Manuel dos Santos Dias, Samir Lounis Relaxation effects are of primary importance in the description of magnetic excitations, leading to a myriad of methods addressing the phenomenological damping parameter. In this work [1], we compare several wellestablished forms of calculating the intrinsic Gilbert damping within a unified theoretical framework, mapping out their connections and approximations required to derive each formula. Most methods lead to similar results for the bulk ferromagnets Fe, Co and Ni, due to the low spinorbit interaction strength and the absence of the spin pumping mechanism. Starting from simulated ferromagnetic resonance spectra based on the underlying electronic structure, we unambiguously demonstrate that the damping parameter obtained within the constant broadening approximation diverges for threedimensional bulk magnets in the clean limit, while it remains finite for monolayers. Our work puts into perspective the several methods to compute the Gilbert damping, building a solid foundation for future investigations of magnetic relaxation effects in any kind of material. 
Tuesday, March 5, 2019 1:39PM  1:51PM 
F41.00011: Quantum spintransfer torque induced nonclassical magnetization dynamics and electronmagnetization entanglement Priyanka Mondal, Marko Petrović, Petr Petr Plechac, Branislav Nikolic Spin transfer torque (STT) is usually known to appear from noncollinearity of two spins but recent experiments [A. Zholud et al., Phys. Rev. Lett. 119, 257201 (2017)] have shown change in magnetization in spin vavles at cryogenic temperature even when electron spin is collinear to magnetization, pointing at overlooked quantum effects in STT ( which can lead to highly nonclassical magnetization states ). Using fully quantum manybody treatment, we showed that change in magnetization below cryogenic temperature come from entanglement between injected electron subsystem and anisotropic quantum Heisenberg ferromagnetic chain, caused by STT coming from noncollinearity of the two subsystems ( which explains the experiment ). Furthermore, the same processes—entanglement and thereby induced decoherence—are present also in standard noncollinear geometry, together with the usual magnetization rotation. This is because STT in quantum manybody picture is caused only by electron spin↓ factor state, and the only difference between collinear and noncollinear geometries is in relative size of the contribution of the initial separable state containing such factor state to superpositions of separable manybody quantum states generated during time evolution. 
Tuesday, March 5, 2019 1:51PM  2:03PM 
F41.00012: Timeretarded damping and magnetic inertia in the LandauLifshitzGilbert equation selfconsistently coupled to electronic timedependent nonequilibrium Green functions Utkarsh Bajpai, Branislav Nikolic The conventional LLG equation is a widely used tool to describe dynamics of local magnetic moments, viewed as classical vectors. Here we demonstrate that recently developed [M. D. Petrovic et al., arXiv:1802.05682] selfconsistent coupling of the LLG equation to timedependent quantum electrons using timedependent nonequilibrium Green function (TDNEGF) microscopically generates timeretarded damping in the LLG equation described by a memory kernel. For sufficiently slow dynamics, the memory kernel can be expanded to extract a time dependent Gilbert damping and magnetic inertia terms. Using examples of precessing single or multiple magnetic moments, as well as fielddriven motion of a magnetic domain wall, we quantify the difference in their time evolution computed from conventional LLG equation vs. our TDNEGF+LLG approach. The faster DW motion predicted by TDNEGF+LLG approach reveals that important quantum effects, are missing from conventional classical micromagnetics simulations. We also demonstrate large discrepancy between TDNEGF+LLGcomputed nonperturbative result for charge current pumped by a moving DW and the same quantity computed by perturbative spin motive force formula combined with the conventional LLG equation. 
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