Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H22: Spin Transport and Magnons in Magnetic InsulatorsFocus
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Sponsoring Units: GMAG DMP FIAP Chair: P Chris Hammel, Ohio State Univ - Columbus Room: LACC 402A |
Tuesday, March 6, 2018 2:30PM - 3:06PM |
H22.00001: Control and Local Measurement of the Spin Chemical Potential in a Magnetic Insulator Invited Speaker: Chunhui Du In recent decades, a large scientific effort has focused on harnessing spin transport for providing insights into novel materials and low-dissipation information processing. We introduce single spin magnetometry based on nitrogen-vacancy (NV) centers in diamond as a new and generic platform to locally probe spin chemical potentials which essentially determine the flow of spin currents. We use this platform to investigate magnons in a magnetic insulator yttrium iron garnet (YIG) on a 100 nanometer length scale. We demonstrate that the local magnon chemical potential can be systematically controlled through both ferromagnetic resonance and electrical spin excitation, which agrees well with the theoretical analysis of the underlying multi-magnon processes. Our results open up new possibilities for nanoscale imaging and manipulation of spin-related phenomena in condensed-matter systems. |
Tuesday, March 6, 2018 3:06PM - 3:18PM |
H22.00002: Magnon transfer torque in all insulating spin valve Yihong Cheng, Kai Chen, Shufeng Zhang Spin valves (SV), consisting of two metallic ferromagnetic layers separated by a non-magnetic layer, are essential building blocks for spin-based electronic devices. The information encoded in SV can be read and written by the giant magnetoresistance effect and the spin transfer torque. The physical mechanism underlying is the angular momentum exchange enabled by conduction electrons propagating across two magnetic layers. Here, we propose an all-insulating spin valve (ISV) structure made of an antiferromagnetic insulator sandwiched by ferromagnetic insulator (FI) layers. The incoherent magnons in the magnetic layers serve as angular momentum carriers and are responsible for the angular momentum transport. We predict two spin transport phenomena in the presence of temperature gradient: (1) a giant magneto spin-Seebeck effect in which the output spin current is controlled by the relative orientation of the two FI layers ,and (2) a magnon transfer torque that can be used for switching the magnetization of the FI layers with temperature gradient of the order of only 0.1 K/nm. The ISV provides a novel route to read and write magnetic information without the Joule heating compared to the conventional metal-based spin valve devices. |
Tuesday, March 6, 2018 3:18PM - 3:30PM |
H22.00003: Measuring YIG Magnonic Crystals at Millikelvin Temperatures Sandoko Kosen, Richard Morris, Arjan Van Loo, Alexy Karenowska Hybrid systems combining magnons and superconducting quantum circuits have attracted increasing interest in recent years [1-3]. Magnonic crystals (MCs) are one of the building blocks of room-temperature magnonics and are used to create devices with an engineered band structure. These devices, exhibiting tunable frequency selectivity and the ability to store travelling excitations in the microwave regime, may form the basis of new tools to be used in the context of quantum information processing. If this potential is to be realised, MCs must be demonstrated to work at the low temperatures required for microwave-frequency quantum experiments. We report the first measurements of the transmission of microwave signals through an MC at 20 mK and observe a magnonic bandgap in both continuous-wave and pulsed excitation experiments. The spin-wave damping at low temperatures in our yttrium iron garnet MC is higher than expected, indicating that further work is necessary before the full potential of quantum experiments using magnonic crystals can be realised. |
Tuesday, March 6, 2018 3:30PM - 3:42PM |
H22.00004: Thermographic Measurements of the Spin Peltier Effect in Metal/Yttrium-Iron-Garnet Junction Systems Shunsuke Daimon, Ken-ichi Uchida, Ryo Iguchi, Eiji Saitoh The spin Peltier effect (SPE) refers to heat-current generation in linear response to spin-current injection [1]. Recently, we established new measurement technique for investigating the SPE based on lock-in thermography (LIT) [2]. The LIT technique enables contact-free measurements of the spatial distribution of the SPE signals with high temperature resolution and realizes quantitative evaluation of the SPE signals. In this study, we report the thermal imaging of the SPE by means of LIT in various metal (Pt, W, Au, or Pt/Cu)/ferrimagnetic insulator [yttrium-iron-garnet (YIG)] junction systems. We successfully observed the clear temperature modulation induced by the spin-current injection not only in the Pt/YIG and W/YIG systems, but also in the Au/YIG and Pt/Cu/YIG systems. These results exclude the possible contamination of the SPE signal by anomalous Ettingshausen effects due to proximity-induced ferromagnetism near the metal/YIG interface. [1] J. Flipse et al., Phys. Rev. Lett. 113, 027601 (2014); [2] S. Daimon et al., Nat. Commun. 7, 13754 (2016), Phys. Rev. B 96, 024424 (2017). |
Tuesday, March 6, 2018 3:42PM - 3:54PM |
H22.00005: Spin-current-less spin Hall effect in magnetic insulators Hua Chen, Qian Niu, Allan MacDonald The spin Hall effect (SHE) in a nonmagnetic conductor is conventionally described as a transverse spin current induced by a longitudinal electric field or voltage, which in turn leads to accumulations of opposite spin at opposite transverse boundaries. Despite its conceptual attractiveness, this definition of the SHE suffers from fundamental difficulties related to the lack of spin conservation, especially in the strongly spin-orbit-coupled systems where the SHE becomes more prominent. In this work we propose an alternative view of SHE phenomena by relating them to the polarization of spin density, in analogy to the charge polarization, induced by an electric field. The spin density polarization is well defined in insulating systems, and a counterpart of the conventional SHE can be present if there is a transverse response of it to an electric field. A constraint on this response from time reversal implies that in insulating systems it is non-zero only if the systems are magnetic. We use some toy models to illustrate the physical consequences of this magnetic spin Hall effect. |
Tuesday, March 6, 2018 3:54PM - 4:06PM |
H22.00006: Effect of magnons on interfacial thermopower of YIG/metal heterostructures Arati Prakash, Jack Brangham, Sarah Watzman, Fengyuan Yang, Joseph P Heremans We examine substrate-to-film interfacial phonon drag effects on typical spin Seebeck heterostructures, in particular studying the effect of magnons on the phonon-electron drag dynamics of nonmagnetic systems. To explore this concept, we compare the thermopower of Pt films grown on ferrimagnetic YIG to that grown on paramagnetic GGG. To isolate the hypothetical drag contribution from the magnons in YIG into the adjacent Pt film, we design a thermocouple device using a hybrid sample with half GGG/Pt and half GGG/YIG(250nm)/Pt. With a uniform applied temperature gradient, the Pt acts as a differential thermocouple. The effective voltage provides a direct measure of the difference in thermopower of the two systems, which we attribute to magnon dynamics in YIG and their interactions at the YIG/Pt interface. We conduct zero-field, longitudinal thermopower measurements and repeat the experiment using Ag and Al in place of Pt. We also investigate magneto-thermopower and YIG film thickness dependence. |
Tuesday, March 6, 2018 4:06PM - 4:18PM |
H22.00007: Temperature Dependence of YIG Magnon Polaritons from 5 Kelvin to Room Temperature Isabella Boventer, Marco Pfirrmann, Julius Krause, Yannick Schön, Mathias Kläui, Martin Weides In information technology research, the spin wave-based approach is promising for new applications such as data storage. A spin wave results from a collective spin excitation with the associated quanta called ‘magnon’. Experimentally, we interface magnons with microwave cavities to investigate dynamics within the magnetic system. Magnonic elements are strongly coupled to a photon resonator, resulting in hybridized magnon-resonator states, i.e. magnon polaritons. Our experimental apparatus allows for the resonant coupling of spin waves in a magnetic bulk or thin film to either a microwave cavity or a coplanar waveguide in the strong coupling regime [1, 2]. We present here a temperature dependent spectroscopic study of magnon polariton states from 5 K to 290 K. A millimetre sized YIG sphere is placed in the 6.5 GHz bright mode of a re-entrant cavity [3]. Specifically, temperature dependent features of the strongly coupled system such as the coupling strength g and linewidth, possibly governed by rare earth impurity scattering, of the Kittel mode are discussed [4]. |
Tuesday, March 6, 2018 4:18PM - 4:30PM |
H22.00008: Observation of phase transitions in cavity magnon-polaritons Dengke Zhang, Xiao-Qing Luo, Yi-Pu Wang, Tie-Fu Li, Jianqiang You Hybridizing collective spin excitations in ferromagnetic crystals and a cavity with high cooperativity provides a new research subject in the field of cavity quantum electrodynamics. Owing to the damping of both photons and magnons, the polaritons have limited lifetimes. However, stationary magnon-polariton states can be reached by a dynamical balance between pumping and losses, so the intrinsical nonequilibrium system may be described by a non-Hermitian Hamiltonian. We designed a tunable cavity magnon-polaritons system which has non-Hermitian spectral degeneracies. By tuning the magnon-photon coupling strength, we observed an exceptional point and spontaneous symmetry-breaking in the cavity magnon-polariton system, where coherent perfect absorption is achieved in the unbroken-symmetry regime but not in the broken-symmetry regime. Meanwhile, the experimental results clearly display a phase transition of the system from the magnetically induced transparency regime to the weak-coupling regime. |
Tuesday, March 6, 2018 4:30PM - 4:42PM |
H22.00009: Bistability of Cavity Magnon-Polaritons Yi-Pu Wang, Guo-Qiang Zhang, Dengke Zhang, Tiefu Li, Can-Ming Hu, Jianqiang You We experimentally demonstrate the magnon-polariton bistability in a cavity magnonics system consisting of cavity photons strongly interacting with the Kittel-mode magnons in a small yttrium iron garnet (YIG) sphere. The bistable behaviors are emerged as sharp frequency switchings of the cavity magnon-polaritons and related to the transition between states with large and small number of polaritons. In our experiment, we align, respectively, the [100] and [110] crystallographic axes of the YIG sphere parallel to the static magnetic field and find very different bistable behaviors (blue- and red-shift of the polariton frequency) in these two cases. The experimental results are well fitted and explained as being due to the Kerr nonlinearity with either positive or negative coefficient. Moreover, in the large dispersive regime, we observe simultaneous bistability of both lower branch (more magnons) and upper branch (more photons) polaritons by applying a drive field on the lower branch. |
Tuesday, March 6, 2018 4:42PM - 4:54PM |
H22.00010: The Boltzmann Transport theory of Magnon with N-process Normal Scattering Yuheng Li, Tao Liu, Jianwei Zhang Ferromagnetic insulators (FMIs) was most promising material for magnon devices to generate, process, and transport spin information over long distances. While, a precise magnon transport picture in FMIs thin film was still demanded. A novel complete picture of magnon transport in FMIs was builded with a new Boltzmann equation which combined magnon, phonon and spin-polarized electron system together. In three magnons scattering, we found the N-process scattering which due to dipole-dipole interaction, dominated the decay dynamics of magnon group. To conduct N-process scattering, a new spatial dependent magnon interaction field λ was introduced, which was discribing collective local field in magnons. We also found N-process scattering is the physical origin that why coherence magnons can transfer to thermal magnons in FI relaxation process. With those effects, we proposed a new method try to modify generation, amplification and controlling of magnon transport and the conversion to other particle(phonon). Furthermore, governing N-process scattering magnons which engendered by temperature gradient, could extend the global decay length of magnetic information. |
Tuesday, March 6, 2018 4:54PM - 5:06PM |
H22.00011: Long-wave magnons in a ferromagnetic film Gang Li, Chen Sun, Thomas Nattermann, Valery Pokrovsky An asymptotically exact theory of spectrum and transverse distribution of magnetization in long-wave magnons is presented. It is based on exact analytical solution of linearized Landau-Lifshitz equation in a film. The quantization of the transverse wave parameters and wave vectors is studied. |
Tuesday, March 6, 2018 5:06PM - 5:18PM |
H22.00012: Simulation and Optimization of PSSW Induced Spin Pumping Jiahao Zhang, Cai Chen, Aryan Navabi, Kang Wang, Greg Carman, Abdon Sepulveda Perpendicular standing spin waves (PSSWs) have been intensively studied to realize high frequency magnetic oscillations with low bias field. Our previous work reported larger than 20 GHz PSSW with only 100 Oe bias field in the undulating CoFeB thin film. In this work, a finite difference model was developed to simulate the time evolvement of magnetization by solving Landau–Lifshitz–Gilbert equation. In the model, the magnetization oscillation was studied with an applied AC magnetic field to the film. Simulation results show good agreement with the PSSW predicted resonance frequency vs. bias field as compared to experimental data. Furthermore, the PSSW is caused by the non-uniform magnetization through the thickness. The results also suggest the appropriate trend between the pumping current (proportional to the magnetization oscillation energy) and the bias field. Based on this model, we used both gradient descent and parametric sweep methods to optimize the geometry of the undulating structure. The resultant optimized design can achieve more than twice the spin pumping current compared to the pre-optimized design. This provides a better understanding of the design methodology necessary for high-frequency spintronic applications. |
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