Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K39: Spin PumpingFocus
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Sponsoring Units: GMAG DMP Chair: Chunhui Du, Harvard University Room: BCEC 207 |
Wednesday, March 6, 2019 8:00AM - 8:36AM |
K39.00001: Enhanced spin pumping into superconductors provides evidence for superconducting pure spin currents Invited Speaker: Chiara Ciccarelli Unlike conventional spin-singlet Cooper pairs, spin-triplet pairs can carry spin. Triplet supercurrents were discovered in Josephson junctions with metallic ferromagnet spacers, where spin transport can occur only within the ferromagnet and in conjunction with a charge current. Ferromagnetic resonance injects a pure spin current from a precessing ferromagnet into adjacent non-magnetic materials. For spin-singlet pairing, the ferromagnetic resonance spin pumping efficiency decreases below the critical temperature (Tc) of a coupled superconductor. Ciccarelli and collaborators present ferromagnetic resonance experiments in which spin sink layers with strong spin–orbit coupling are added to the superconductor. Their results show that the induced spin currents, rather than being suppressed, are substantially larger in the superconducting state compared with the normal state; although further work is required to establish the details of the spin transport process, they show that this cannot be mediated by quasiparticles and is most likely a triplet pure spin supercurrent. |
Wednesday, March 6, 2019 8:36AM - 8:48AM |
K39.00002: Probe of spin dynamics in superconducting NbN thin films via spin pumping Yunyan Yao, Qi Song, Yota Takamura, Juan Pedro Cascales, Wei Yuan, Yang Ma, Yu Yun, Xincheng Xie, Jagadeesh Moodera, Wei Han The emerging field of superconductor (SC) spintronics has attracted intensive attentions recently. Many fantastic spin-dependent properties in SCs have been discovered, including large magnetoresistance, long spin lifetimes, and the giant spin Hall effect, etc. Regarding the spin dynamics in superconducting thin films, few studies have been reported yet. Here, we report the investigation of the spin dynamics in an s-wave superconducting NbN film via spin pumping from an adjacent insulating ferromagnet GdN film. A profound coherence peak of the Gilbert damping of GdN is observed slightly below the superconducting critical temperature of the NbN, which agrees well with recent theoretical prediction fors-wave SCs in the presence of impurity spin-orbit scattering. This observation is also a manifestation of the dynamic spin injection into superconducting NbN thin film. Our results demonstrate that spin pumping could be used to probe the dynamic spin susceptibility of superconducting thin films, thus pave the way for future investigation of spin dynamics of interfacial and two-dimensional crystalline SCs. |
Wednesday, March 6, 2019 8:48AM - 9:00AM |
K39.00003: Efficient spin current generation from new, low-loss Mg(Al,Fe)2O4 films into heavy metals Lauren Riddiford, Jacob Wisser, Satoru Emori, Peng Li, Debangsu Roy, Egecan Cogulu, Olaf M Van T Erve, Yong Deng, Shan X Wang, Berend Jonker, Andrew D Kent, Yuri Suzuki Low-damping magnetic insulators are essential to the growing field of spintronics since they can generate and transfer spin currents without associated charge currents. We have recently developed a new ferrimagnetic insulator, MgAl0.5Fe1.5O4 (MAFO), which has Gilbert damping as low as 1x10-3. Through resonant excitation of the MAFO layer, we observed significant spin pumping from MAFO films into adjacent Pt and β-W layers by enhanced Gilbert damping and electrical voltage peaks that appear at ferromagnetic resonance (FMR). There is a reversal of the transverse voltage signal in MAFO/Pt and MAFO/β-W, which is consistent with the opposite signs of the spin Hall angle in these two materials and indicates the signal is due to the inverse spin Hall effect and not to proximity-induced anisotropic magnetoresistance (AMR). Further, angular dependent magnetoresistance (ADMR) measurements reveal that the spin Hall magnetoresistance (SHMR) is 7-10 times larger than the AMR contribution, so spin pumping effects dominate the magnetic dynamics of the bilayers. From FMR and ADMR measurements, we deduced a spin-mixing conductance of ~3x1014 Ω-1 m-2 in MAFO/Pt. Together, these measurements indicate that MAFO is an excellent candidate for efficient spin current-based heterostructures and devices. |
Wednesday, March 6, 2019 9:00AM - 9:12AM |
K39.00004: Ferromagnetic Resonance and Spin Transport in Epitaxial Manganite/Ruthenate Bilayers Purnima Balakrishnan, Satoru Emori, Yuri Suzuki Spin current generation by spin-pumping from a ferromagnet into a non-magnetic metal has been of great interest recently. However the role of the interface in the efficient pumping of spin current into the non-magnetic metal has not been addressed extensively. We address this issue by studying coherent interfaces in epitaxial oxide bilayers. Here we report recent results on spin pumping in pulsed-laser deposited bilayers of the ferromagnetic metal La2/3Sr1/3MnO3 (LSMO) and the paramagnetic metals SrRuO3 (SRO) and CaRuO3 (CRO). X-ray diffraction indicates excellent crystallinity of the bilayers. CRO layers are under tensile strain on LSMO films grown on LSAT substrates, in contrast to SRO layers which are under compressive strain. Ferromagnetic resonance measurements of LSMO films indicate that the Gilbert damping parameter can be as low as 1×10-3. This damping increases with the addition of an epitaxial ruthenate overlayer of SRO or CRO consistent with spin pumping. From the ruthenate thickness dependence of the Gilbert damping, we estimate that the spin diffusion length is larger in CRO than in SRO. We will also discuss the role of proximity-induced magnetism on spin transport in bilayers with SRO versus CRO, which has no ferromagnetic transition. |
Wednesday, March 6, 2019 9:12AM - 9:24AM |
K39.00005: Atomic Spin-orbit Coupling-induced spin-flip scattering at Ferromagnet/Non-magnetic Metal Interface Mijin Lim, Hyun-Woo Lee Spin conservation at an interface influences several spin transport phenomena such as spin-to-charge conversion and spin-orbit torque. If spin flips at the interface, the spin information may be lost, which is called a spin-memory loss. [1] We claim that large atomic spin-orbit coupling(SOC) and interfacial inversion symmetry breaking(ISB) are important elements of spin-flip scattering in the heterostructure that consists of a nonmagnetic metal with strong SOC and a ferromagnet. To study those effects, we calculate the change of spin information during the reflection/transmission using the tight-binding method. Here, we report two main findings. The first is about the spin-flip resulting from the SOC-induced entanglement of spin and orbital degree of freedom. The other is that the inversion symmetry breaking at an interface, which allows specific inter-orbital hopping [2], generates an additional spin-flip. We argue that these processes contribute to the interfacial spin-memory loss and spin transparency in spintronics devices. [1, 3] |
Wednesday, March 6, 2019 9:24AM - 9:36AM |
K39.00006: Enhancement of spin-motive force in systems with broken inversion symmetry Yuta Yamane The exchange interaction between the conduction electron spin and the local magnetization is responsible for spin-transfer torque, which provides an promising way of manipulating the magnetization by charge current. The same interaction can also mediate an electric-voltage generation by dynamical magnetic textures, i.e., spin-motive force (SMF). A SMF reflects spatiotemporal variation of the magnetization, and thus offers a powerful method to probe and explore various dynamical magnetic textures. |
Wednesday, March 6, 2019 9:36AM - 9:48AM |
K39.00007: Optically generated spin currents in platinum/magnetic insulator bilayer structures Joseph Murphy, Subash Kattel, Jinjun Ding, Tao Liu, Mingzhong Wu, William Rice The generation and detection of pure spin current provides a pathway for solid-state devices to avoid Joule heating losses arising in electrical architectures. |
Wednesday, March 6, 2019 9:48AM - 10:00AM |
K39.00008: Nonlinear magnon study by microwave perpendicular pumping ferromagnetic resonance in thin films Tao Qu, Aneesh Venugopal, James Etheridge, Karthik Srinivasa, William Peria, Bethanie Stadler, Paul Crowell, Randall Victora To investigate the nonlinear effects arising from magnon coupling to microwaves and their application to the new functionality of monolithically integrated magnetic components on semiconductor substrates, it is important to exploit non-linear ferromagnetic resonance in magnetic insulator materials. We have studied the nonlinearity caused by the three magnon scattering mechanism in GHz ferromagnetic resonance in simulation and analytically. The thicknesses of the studied YIG films are varied in the range of nanometers to several micrometers. A realistic micromagnetic simulation is implemented to capture the physical mechanisms coupling magnons of sub-micrometer wavelengths and microwaves of centimeter wavelength using CUDA parallel high-performance computing. Using the Holstein-Primakoff transformation and equation of motion in the theoretical model, we obtain the analytical magnon dispersion and the power threshold function. The power threshold for the three-magnon process is sensitive to the material properties. We find that the non-linearity is due primarily to incoherent magnon modes propagating in the thin film plane, while the modes propagating out of the film plane exist, but with much smaller occupation numbers. |
Wednesday, March 6, 2019 10:00AM - 10:12AM |
K39.00009: Spin pumping in a material with metal-insulator transition Taqiyyah Safi, Luqiao Liu Ferromagnetic resonance driven spin pumping is a powerful tool to study the generation and detection of pure spin currents. Using spin pumping, people have characterized the spin orbit interaction induced charge—spin conversion in various materials, including paramagnetic, ferromagnetic and antiferromagnetic metals, semiconductors, as well as superconductors. It is interesting to ask whether the charge to spin conversion efficiency undergoes any significant change in a material with spontaneous phase transitions. In this talk we will present our work on spin pumping in VO2, a prototype metal-insulator transition (MIT) material. By exciting ferromagnetic resonance in an adjacent magnetic insulator, we are able to inject spin current into VO2 and quantify the variation in spin pumping voltage during the phase transition. We find that large spin pumping voltages exist in both metallic and insulating phase of VO2, despite of the huge resistance change across this transition. To this end, we will present our work on the variation of spin mixing conductance and the spin Hall angle across the phase transition region. |
Wednesday, March 6, 2019 10:12AM - 10:24AM |
K39.00010: Topological Transport of Vorticity in Heisenberg Magnets Ji Zou, Se Kwon Kim, Yaroslav Tserkovnyak We study a robust topological transport carried by vortices in a thin film of an easy-plane ferromagnetic insulator between two metal contacts. A vortex, which is a nonlocal topological spin texture in two-dimensional magnets, exhibits some beneficial features as compared to skyrmions, which are local topological defects. In particular, the total topological charge carried by vorticity is robust against local fluctuations of the spin order-parameter magnitude. We show that an electric current in one of the magnetized metal contacts can pump vortices into the insulating bulk. Diffusion and nonlocal Coulomb-like interaction between these vortices will establish a steady-state vortex flow. Vortices leaving the bulk produce an electromotive force at another contact, which is related to the current-induced vorticity pumping by the Onsager reciprocity. The voltage signal decays algebraically with the separation between two contacts, similarly to a superfluid spin transport. Finally, the vorticity and closely related skyrmion type topological hydrodynamics are generalized to arbitrary dimensions, in terms of nonsingular order-parameter vector fields. |
Wednesday, March 6, 2019 10:24AM - 10:36AM |
K39.00011: Observation of NMR spin pumping Yuki Shiomi, Jana Lustikova, Shingo Watanabe, Daichi Hirobe, Saburo Takahashi, Eiji Saitoh We report spin pumping from nuclear magnetic resonance (NMR), in which nuclear spin dynamics emits a spin current, a flow of spin angular momentum of electrons. By using the canted antiferromagnet MnCO3, in which typical nuclear spin-wave formation is established due to the reinforced hyperfine coupling, we find that a spin current is generated from an NMR. |
Wednesday, March 6, 2019 10:36AM - 10:48AM |
K39.00012: Tunable Quantum Dynamics in a Disordered Magnet Christopher Tang, Daniel Silevitch, Gabriel Aeppli, Thomas F Rosenbaum Quantum memories depend on maintaining coherence in minimally-interacting qubits. In an attempt to engineer such memories, we decouple coherent many-body excitations from their environment in the dilute Ising magnet LiHo0.045Y0.955F4 by driving the system into a non-linear regime with a Floquet drive. In this material, clusters of dozens to hundreds of spins bind together and can be excited resonantly by a strong ac magnetic field. The structure of these clusters can by studied via a Fano resonance technique using pump-probe magnetic susceptibility measurements, revealing lifetimes and coherent quantum interference between different excitation channels. In addition, we can tune the dynamics of the quantum degrees of freedom by tuning the strength of the pump ac magnetic field, or by introducing a static transverse field that serves to quantum-mechanically mix spin states. At special values of these two fields, zero-crossings of the Fano asymmetry parameter are observed, which are associated with a dissipationless response at the drive frequency. At these points, certain spin-clusters are minimally coupled with their environment, due to quantum interference between different excitation channels. The lifetimes of these excitations can be measured directly in the time-domain. |
Wednesday, March 6, 2019 10:48AM - 11:00AM |
K39.00013: Backflow Effect on Spin Diffusion at the Normal Metal-Superconductor Interface Muhammad Faiz, Raghava Panguluri, Benjamin Balke, Sabine Wurmehl, Claudia Felser, Andre Petukhov, Boris Nadgorny The behavior of spin propagation in metals in various measurement schemes is shown to be qualitatively different than a simple exponential decay - due to the backflow effect on spin diffusion in the presence of interfaces. We utilize the spin sensitivity of Nb superconducting point contacts to evaluate the spin current depolarization in a normal metal. The spin current was injected from bulk Co2Mn0:5Fe0:5Si Heusler alloys into gold films of variable thickness. While our phenomenological theoretical results are consistent with gradually decaying spin polarization as the film thickness increases, the spin diffusion length in Au is found to be 285 nm, more than two times larger than without taking the backflow effect into account. |
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