Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session A46: Magnon and Spin Dynamics |
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Sponsoring Units: GMAG DMP Chair: Eric Montoya, University of California, Irvine Room: 708 |
Monday, March 2, 2020 8:00AM - 8:12AM |
A46.00001: Theoretical model for direct evidence of spatial stability of Bose-Einstein condensate of magnons Igor Borisenko, Boris Divinskiy, Vladislav Demidov, Gang Li, Thomas Nattermann, Valery L Pokrovsky, Sergej Demokritov Bose-Einstein condensation of quasi-equilibrium magnons is one of few macroscopic quantum phenomena observed at room temperature. However, for a long time it remained unclear, what physical mechanisms can be responsible for the spatial stability of the magnon condensate. Indeed, since magnons are believed to exhibit attractive interaction, it is generally expected that the magnon condensate should be unstable with respect to the real-space collapse, which contradicts all the experimental findings. Here, we provide direct experimental evidence that magnons in a condensate exhibit repulsive interaction resulting in the condensate stabilization and propose a mechanism, which is responsible for the interaction inversion. Our experimental conclusions are additionally supported by the theoretical model based on the Gross-Pitaevskii equation. Our findings solve a long-standing problem and provide a new insight into the physics of magnon Bose-Einstein condensates. |
Monday, March 2, 2020 8:12AM - 8:24AM |
A46.00002: Thermal Hall Effect in Collinear Antiferromagnets on a Square Lattice Nishchay Suri, Yinhan Zhang, Satoshi Okamoto, Di Xiao We show that the thermal Hall effect is possible in collinear antiferromagnets on a squre lattice once the magnon-phonon interaction due to the Dzyaloshinskii-Moriya interaction (DMI) is taken into account. We discuss the field- and DMI-dependence of the thermal Hall coefficient. We show that the thermal Hall conductivity is controlled by the resonant contribution from the anticrossing points between the magnon and phonon branches, and estimate its size in real materials. |
Monday, March 2, 2020 8:24AM - 8:36AM |
A46.00003: Magnon contributions to dielectric constant in spiral magnets Francesco Foggetti, Sergey Artyukhin
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Monday, March 2, 2020 8:36AM - 8:48AM |
A46.00004: Goos-Hänchen effect of spin waves at heterochiral interfaces zhenyu Wang, Yunshan Cao, Peng Yan <p class="MsoNormal">One of the most robust methods for measuring the Dzyaloshinskii-Moriya interaction (DMI) is the Brillouin light scattering spectroscopy (BLS) at present. But measuring the DMI parameter in a narrow magnetic strip is always a difficult problem because of the big laser spot size subjected to the diffraction limit of lights. In previous work, we proposed a nonlocal scheme to measure the DMI in a narrow magnetic strip by three-magnon processes, but it is only feasible for the magnetic strip with the width in the range of 50–100 nm. In this work, we observe a Goos-Hänchen shift of spin waves at the heterochiral interface when the spin-wave beam is totally reflected. We further explore the GH shift of spin waves by narrow DMI strips of different widths. It is found that the induced shift is independent of the strip width down to 10 nm, offering an approach to measure the DMI strength of ultranarrow magnetic strips of sub-50-nm scales. Our findings are helpful to understand the GH effect in chiral magnets and enable us to measure the DMI for ultranarrow magnetic strips via the magnonic GH shift, which fills in the gap of current technology.</p> |
Monday, March 2, 2020 8:48AM - 9:00AM |
A46.00005: Magnon spectrum in Ferromagnet-Superconductor heterostructure Bishal Parajuli, Shizeng Lin, Chih-Chun Chien Research on ferromagnet-superconductor heterostructures gives rise to novel quantum phases and excitations. Spin-waves (Magnons) can propagate in a ferromagnetic thin film while magnetic fields penetrate through a type-II superconducting thin film forming a vortex lattice. Here, we study the magnon spectrum in ferromagnetic thin films in the presence of a vortex lattice in a superconducting thin film separated by an insulating film. It was revealed that the spin-waves interact with the vortex lattice and the magnon spectrum is modified. Because of the periodic modulation of the magnetic field from the vortex lattice, the ferromagnetic film influenced by the vortex lattice acts as a magnonic crystal and results in the formation of bandgaps in the spin wave spectrum. The vortex lattice depends on the magnitude of the applied magnetic field, which can then be used to tune the spin wave band gaps for possible magnonic applications. |
Monday, March 2, 2020 9:00AM - 9:12AM |
A46.00006: Imaging spin-wave propagation and interference with electron spins in diamond Iacopo Bertelli, Joris Jip Carmiggelt, Brecht Simon, Coosje Pothoven, Jan Aarts, Toeno Van der Sar, Tao Yu, Gerrit Bauer, Yaroslav M. Blanter The coherent transport of spin information in magnetic insulators is not associated with the heat dissipation of electronic currents. Therefore, it is envisioned that the next generation of information-processing devices could be based on spin waves, the elementary excitations of magnets that can reach nanometer wavelengths and terahertz frequencies. Here we use electron spins in diamond to probe coherent spin-wave transport in the magnetic insulator yttrium-iron-garnet (YIG). We image propagating spin-waves and their interference, we extract the dispersion relation, demonstrate time-domain control and quantify the magnetization oscillations carried by these excitations. These results pave the way for fundamental studies of spin-wave transport and to harness spin-wave interference in magnonic devices. |
Monday, March 2, 2020 9:12AM - 9:24AM |
A46.00007: Experimental observation of exceptional surface in synthetic dimensions with magnon polaritons Xufeng Zhang, Kun Ding, Xianjing Zhou, Jing Xu, Dafei Jin Exceptional points (EPs) are singularities of eigen-energies in non-Hermitian systems. Intriguing phenomena have been previously observed around EPs. However, previous demonstrations are limited to 0-dimensional points and 1-dimensional lines. Here we report, to the best of our knowledge, the first experimental observation of an exceptional surface (ES) in a magnon polariton system. Magnon polaritons are hybrid excitations of electromagnetic waves and spin waves, which have recently emerged as a promising candidate for coherent information processing. We took advantage of the excellent tunability in magnon polaritons and introduced a 4-dimensional synthetic space, which enabled the observation of ES. In addition, we also observed an exceptional saddle point in the ES, which exhibits unique anisotropic behaviors in both the real and imaginary part of the eigenfrequencies. Our findings open up new opportunities for high-dimensional control of non-Hermitian systems and novel sensing applications. |
Monday, March 2, 2020 9:24AM - 9:36AM |
A46.00008: Magnon spectrum of the chiral ferrimagnet Cu2OSeO3: a coarse-grained approach Yi Luo, Guy G Marcus, Benjamin Trump, Jonas Kindervater, Matthew Stone, Tyrel McQueen, Oleg Tchernyshyov, Collin Leslie Broholm We report a comprehensive neutron scattering study of low energy magnetic excitations in the breathing pyrochlore helimagnetic Cu2OSeO3. Fully documenting the four lowest energy modes that leave the ferrimagnetic configuration of the "strong tetrahedra" intact (\hbar ω<13 meV), we find quadratic dispersion at the Γ point for energies above 0.2 meV with any gap less than 0.19 meV, two doublets separated by 1.6(2) meV at the R point, and a bounded continuum at the X point. Our constrained rigid spin cluster model relates these features to Dzyaloshinskii-Moriya(DM) interactions and the incommensurate helical ground state. Combining conventional spin wave theory with a spin cluster form-factor accurately reproduces the measured equal time structure factor through multiple Brillouin zones. An effective spin Hamiltonian describing the complex anisotropic inter-cluster interactions is obtained. |
Monday, March 2, 2020 9:36AM - 9:48AM |
A46.00009: Modeling Hall viscosity in magnetic Skyrmions Bom Kim Magnetic Skyrmions are topologically stable objects that are made with a bunch of spins tightly arranged in a smooth fashion. Their topological nature provides unusual and complex transport properties, such as Skyrmion Hall effect. Extensive Hall data have further revealed asymmetry between Skyrmion and Anti-Skyrmion Hall angles, which cannot be accounted by known mechanisms. Here, we explain this asymmetry by utilizing another universal transport coefficient called `Hall viscosity,' extensively studied in quantum Hall systems. Hall viscosity is modeled in steady-state Skyrmions motion by generalizing the Thiele equation with a transverse velocity component and is independent of the Skyrmion charge. Our analyses, based on available asymmetric Hall angle data, reveal this transverse force amounts 3% - 5.4% of the force due to Skyrmion Hall effect. Further clarification of Hall viscosity will be essential for designing next generation storage devices properly, not to mention for our deeper understanding of fundamental properties of nature. |
Monday, March 2, 2020 9:48AM - 10:00AM |
A46.00010: Spin dynamics in the skyrmion host lacunar spinel GaV4S8 Ganesh Pokharel, Hasitha Suriya Arachchige, Georg Ehlers, Seunghwan Do, Matthew Stone, Mark D Lumsden, Hao Zhang, Cristian Batista, Yiming Qiu, Randy Fishman, David Mandrus, Andrew D Christianson In the lacunar spinel GaV4S8, the interplay of spin, charge, and orbital degrees of freedom results in a complex phase diagram that includes ferroelectric, orbitally ordered and Néel type skyrmion phases. Below 12.7 K, GaV4S8 exhibits a cycloidal state at zero field and a Néel type skyrmion spin structure with the application of field. To understand the physics driving the formation of these novel phases, we have carried out inelastic neutron scattering measurements on GaV4S8 above and below the ordering temperature of 12.7 K. Dispersive spin excitations with a zone boundary energy 5.65 meV are observed along the [100], and [110] directions within the magnetically ordered phase. The excitation spectrum softens along the third high symmetry direction, [111]. Using a Heisenberg model with near-neighbor exchange couplings and Dzyaloshinskii-Moria (DM) interactions, the excitation spectra are simulated. Simulation shows ferromagnetic inter-tetrahedral couplings with J= -0.69(3) meV. It is also observed that the DM interactions are around an order of magnitude weaker than the near-neighbor exchange interactions. A small and finite value of the DM interaction at 2 K provides evidence that the ground state of GaV4S8 is a cycloid below the ordering temperature in zero applied field. |
Monday, March 2, 2020 10:00AM - 10:12AM |
A46.00011: Dynamics of a vortex near the edge of a thin-film ferromagnet Michael Bjerngaard, Derek Reitz, Oleg Tchernyshyov The low-frequency dynamics of magnetization in a thin-film ferromagnet with an easy-plane anisotropy can be efficiently described by a mapping to electrodynamics in 2+1 dimensions. Magnons turn into photons, whereas vortices become massless particles with an electric charge equal to the vortex number. In a previous work [1], we showed that a vortex-antivortex pair with equal skyrmion numbers revolve around the geometrical center and gradually spiral down towards annihilation. Their motion was accurately modeled by considering the balance of forces acting on these topological defects: the Coulomb attraction, the gyroscopic forces, and the viscous force from Gilbert damping. A vortex-antivortex pair with opposite skyrmion numbers will move mostly in the same direction, gradually approaching each other. A similar motion is expected of a vortex near a straight edge, which creates the image of an antivortex attracting the vortex toward the edge. We study this motion numerically and model it analytically. |
Monday, March 2, 2020 10:12AM - 10:24AM |
A46.00012: Soliton Bound States in Large-Spin Anisotropic Antiferromagnets Harry Lane, Frank Krüger, Elliot Christou, Chris Stock, Russell Ewings It has long been known that the low-energy dynamics of spin systems are well-described by classical linear spin-wave theory. This is particularly true in the case of systems with a large spin moment, where one can expand in powers of 1/S. For large-spin antiferromagnets, linear spin wave theory predicts dispersive magnon modes which are gapped in the presence of anisotropy. Such spectra are found time and again in the literature, but can the quantum nature of spin give rise to exotic excitations that are not well described by linear spin-wave theory? |
Monday, March 2, 2020 10:24AM - 10:36AM |
A46.00013: Tuning high-Q nonlinear dynamics in a disordered quantum magnet Daniel Silevitch, Christopher Tang, Gabriel Aeppli, Thomas F Rosenbaum Quantum states cohere and interfere. Atoms arranged imperfectly in a solid rarely display these properties. Here we demonstrate an exception in a disordered quantum magnet that divides itself into nearly isolated subsystems. We probe these coherent spin clusters by driving the system nonlinearly and measuring the resulting hole in the linear spectral response. The Fano shape of the hole encodes the incoherent lifetime as well as coherent mixing of the localized excitations. For the Ising magnet LiHo0.045Y0.955F4, the quality factor Q for spectral holes can be as high as 100,000. We tune the dynamics by sweeping the Fano mixing parameter q through zero via the ac pump amplitude as well as a dc transverse field. The zero crossing of q is associated with a dissipationless response at the drive frequency. We then explore the dynamics of this dissipationless state, focusing on the decoherence of the extended spin clusters. Identifying localized two-level systems in a dense and disordered magnet advances the search for qubit platforms emerging from strongly interacting, many-body systems. |
Monday, March 2, 2020 10:36AM - 10:48AM |
A46.00014: Enhanced self-focusing effect of spin-wave by a pulsed flat-top excitation field in a multi-domain state Kim Hyo Seok, Jongseok Lee, In Hyeok Choi Spin-wave (SW) or its quantum, magnon, is studied with renewed interest as a basis for wave-based classic information processing. Along with other physical waves, SWs also have wave-like properties, such as radiation, propagation, reflection, and refraction, which have been extensively investigated in order to manipulate SWs. Especially, there have been many methods reported to focus the SW via a nonlinearity, a graded refractive index, and a phase-controlled SW sources. In this work, we explore another efficient way of the SW focusing by using the spatially and temporally tailored magnetic field excitations. Using a micro-magnetic simulation, we explain how the SW propagates and is focused in a ferromagnetic thin film with a perpendicularly magnetized anisotropy after its excitation by a pulsed magnetic field having a flat-top amplitude distribution. In particular, we observe that the focusing appears more efficiently in the multi-domain state divided by Bloch walls compared to that in the single-domain state. Based on these results, we suggest the flat-top excitation of the magneto-static wave as an efficient way to create magnetic droplets in a multi-domain state of a perpendicularly magnetized system. |
Monday, March 2, 2020 10:48AM - 11:00AM |
A46.00015: Ferromagnetic resonance in few-layered CrI3 single crystals using magneto-Raman spectroscopy Rolando Valdes Aguilar, Amber McCreary, Thuc Mai, Franz G Utermohlen, Jeffrey Simpson, Kevin Garrity, Xiaozhou Feng, Dmitry Shcherbakov, Yanglin Zhu, Jin Hu, Daniel Weber, Kenji Watanabe, Takashi Taniguchi, Joshua Goldberger, Zhiqiang Mao, Chun Ning Lau, Yuan-Ming Lu, Nandini Trivedi, Angela Hight Walker We study in detail the structural and magnetic excitations of CrI3 in single crystalline thin flakes using inelastic light scattering in a magnetic field. We find that for fields above 6 T we observe the scattering from the zone center magnon, the ferromagnetic resonance, with a linear dependence on the magnetic field in this range. This behavior is well explained by the classical Kittel formula that predicts a value of the zero-field resonance at approximately 45 GHz with an effective g-factor of ~ 2. We also find that as a function of the scattered polarization angle with respect to the incoming polarization, the intensity has a two-fold symmetric pattern without nodes (i.e. the signal does not vanish). We will discuss the possible reasons for this behavior and compare it to that expected for the lattice excitations for the expected magnetic point group symmetry. |
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