Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session V47: Magnetization Dynamics II, Damping and Spin PumpingFocus
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Sponsoring Units: GMAG DMP FIAP Chair: Barry Zink, University of Denver Room: 394 |
Thursday, March 16, 2017 2:30PM - 3:06PM |
V47.00001: Ultra-low magnetic damping in metallic and half-metallic systems. Invited Speaker: Justin Shaw The phenomenology of magnetic damping is of critical importance to devices which seek to exploit the electronic spin degree of freedom since damping strongly affects the energy required and speed at which a device can operate. However, theory has struggled to quantitatively predict the damping, even in common ferromagnetic materials. This presents a challenge for a broad range of applications in magnonics, spintronics and spin-orbitronics that depend on the ability to precisely control the damping of a material. I will discuss our recent work to precisely measure the intrinsic damping in several metallic and half-metallic material systems and compare experiment with several theoretical models.[1-7] This investigation uncovered a metallic material composed of Co and Fe that exhibit ultra-low values of damping that approach values found in thin film YIG.[8] Such ultra-low damping is unexpected in a metal since magnon-electron scattering dominates the damping in conductors. However, this system possesses a distinctive feature in the bandstructure that minimizes the density of states at the Fermi energy n(EF). These findings provide the theoretical framework by which such ultra-low damping can be achieved in metallic ferromagnets and may enable a new class of experiments where ultra-low damping can be combined with a charge current. Half-metallic Heusler compounds by definition have a bandgap in one of the spin channels at the Fermi energy. This feature can also lead to exceptionally low values of the damping parameter. Our results show a strong correlation of the damping with the order parameter in Co2MnGe. Finally, I will provide an overview of the recent advances in achieving low damping in thin film Heusler compounds. [1] Mankovsky et al., Phys. Rev. B 87, 014430 (2013) [2] Turek et al., Phys. Rev. B 92, 214407 (2015). [3] Gilmore et al., Phys. Rev. Lett. 99, 027204 (2007) [4] Thonig et al., New J. Phys. 16, 013032 (2014) [5] Brataas et al., Phys. Rev. Lett. 101, 037207 (2008) [6] Starikov et al., Phys. Rev. Lett. 105, 236601 (2010) [7] Liu et al., Phys. Rev. B 84, 014412 (2011) [8] Schoen et al., Nature Physics 12, 839 (2016) [Preview Abstract] |
Thursday, March 16, 2017 3:06PM - 3:18PM |
V47.00002: Spin angular momentum absorption at the magnetic phase transition of Permalloy antiferromagnetic surface oxidized layers Lamprini Frangou, Guillaume Forestier, Stephane Auffret, Serge Gambarelli, Vincent Baltz We report an alternative mechanism for the physical origin of the temperature-dependent ferromagnetic relaxation of Permalloy (NiFe) thin films. By use of spin pumping experiments we demonstrate that the peak in the temperature-dependence of the NiFe damping can be understood in terms of enhanced spin angular momentum absorption at the magnetic phase transition of antiferromagnetic surface oxidized layers. These results suggest a new direction for the understanding of a least understood physical question, opening pathways for further investigations. [Preview Abstract] |
Thursday, March 16, 2017 3:18PM - 3:30PM |
V47.00003: Exceptionally low magnetic damping in Co$_{0.25}$Fe$_{0.75}$ epitaxial films Aidan Lee, Yang Cheng, Jack Brangham, Shane White, William Ruane, Sisheng Yu, P. Chris Hammel, Fengyuan Yang Ferromagnetic alloy Co$_{x}$Fe$_{1-x}$ has a wide range of applications in magnetic devices and spintronics due to its strong magnetization and relatively low damping. It was recently shown that polycrystalline Co$_{x}$Fe$_{1-x}$ films of various Co concentrations grown on a Cu seed layer on Si exhibit minimal magnetic damping at x$=$0.25 [1]. We grow both polycrystalline and epitaxial Co$_{0.25}$Fe$_{0.75}$ films using off-axis sputtering. The polycrystalline films show FMR linewidths of approximately 23 G at 10 GHz, comparable to the values reported in ref. 1. Remarkably, the epitaxial Co$_{0.25}$Fe$_{0.75}$ films grown on MgO (100) have much smaller gilbert damping, $\alpha =$8x10$^{-4}$, and much narrower FMR linewidths, less than 10 G at 10 GHz, which are both comparable to those of high quality Y$_{3}$Fe$_{5}$O$_{12}$ films. The metallic nature of this material combined with its very low damping offers the opportunity to explore low-loss, charge-based, dynamic spin transport that cannot be achieved with an insulating ferrimagnet such as Y$_{3}$Fe$_{5}$O$_{12}$. 1. M. A. W. Schoen et. al., Nature Phys. 12, 839 (2016). [Preview Abstract] |
Thursday, March 16, 2017 3:30PM - 3:42PM |
V47.00004: Current Induced Damping Rate of Quantum Magnetic Moments in the Presence of the Spin-Orbit Interaction Farzad Mahfouzi, Nicholas Kioussis We have developed a novel formalism based on the framework of Keldysh Green function approach to investigate the effect of bias voltage on the damping rate of the metallic ferromagnets (FMs) in the presence of the spin-orbit interaction (SOI). In contrast to previous classical ferromagnetic model approaches this formalism takes into account the quantum mechanical nature of the magnetic moments at the atomic scale and allows us to consider the magnetization dynamics in different regimes, namely; (i) precessional and (ii) local spin flip dynamics. The former is associated with the rotation of the magnetic order parameter while the latter one describes the damping rate of the amplitude of the magnetic order parameter. We show that due to the SOI the bias voltage can lead to large changes of the damping rate of the magnetization and even sign reversal due to the energy transfer between excited itinerant electrons and the local moments. Unlike classical FMs coupled to itinerant electrons, this approach leads to a finite amplitude for the damping rate in ballistic regime of the electronic transport. The results suggest that the anti-damping rate efficiency is peaked with opposite signs when the transport is driven by only particle-like or hole-like carriers. [Preview Abstract] |
Thursday, March 16, 2017 3:42PM - 3:54PM |
V47.00005: Low Damping Spinel Ferrites for Spin Pumping Matthew Gray, Satoru Emori, Benjamin Gray, Hyung-Min Jeon, Brandon Howe, Yuri Suzuki Spin pumping phenomena at ferromagnet/metal interfaces have extensive applications ranging from electronic control of magnetic orientation to generation and manipulation of pure spin currents. Few spin pumping experiments have utilized spinel ferrites for the magnetic layer due to their typically large damping. Here we report on the spin pumping in low damping spinel ferrite based bilayers. (Ni,Zn)AlFeO$_{\mathrm{4}}$ thin films exhibit a damping factor as low as 3x10$^{\mathrm{-3}}$. These films also exhibit large negative perpendicular magnetic anisotropy of \textgreater 1 T, leading to low resonance fields three times smaller at 10 GHz than the prototypical spin pumping ferrimagnet Y$_{\mathrm{3}}$Fe$_{\mathrm{5}}$O$_{\mathrm{12}}$ (YIG). Upon addition of only 1.5 nm of a Pt film, Gilbert damping more than doubles, and we observe significant in-plane DC voltage during ferromagnetic resonance. These two effects indicate a large amount of spin pumping from the ferrite to Pt with an estimated effective spin mixing conductance of 3x10$^{\mathrm{18\thinspace }}$m$^{\mathrm{-2}}$, comparable to that reported for YIG/Pt. This system demonstrates the promise of spin pumping phenomena in the spinel ferrite family of materials. [Preview Abstract] |
Thursday, March 16, 2017 3:54PM - 4:06PM |
V47.00006: Systematic temperature and thickness dependence of magnetic properties in nanometer-thick garnet films Colin Jermain, Sriharsha Aradhya, Hanjong Paik, Jack Brangham, Michael Page, Neal Reynolds, Chris Hammel, Fengyuan Yang, Darrell Schlom, Robert Buhrman, Dan Ralph Insulating ferrimagnets are of interest for spintronic applications because they possess very small damping parameter, as low as 10$^{-5}$ in the bulk. Making practical devices from ferrimagnetic insulators will require techniques capable of growing very thin films (few tens of nm and below) while maintaining low damping. We report systematic ferromagnetic resonance (FMR) studies as a function of temperature, frequency and film thickness in two closely related garnet films. We demonstrate molecular-beam epitaxy growth of (111)-oriented lutetium iron garnet (Lu3Fe5O12) films as thin as 2.8 nm. In these films we measure damping that is among the lowest reported for films of comparable thickness grown by other techniques [1]. We also report temperature dependent FMR linewidths in off-axis sputtered (111)-oriented 15 nm thick yttrium iron garnet (Y3Fe5O12). Here we observe a 30-times increase in the damping as the film is cooled from room temperature to cryogenic temperatures, with a peak near 25 K clear peak in the linewidth at a characteristic temperature, indicatingcharacteristic of impurity-induced relaxation. Together these studies provide insight into damping mechanisms in nanometer thick garnet films that can guide the development of improved growth and device fabrication protocols. [1] APL 109, 192408 (2016). [Preview Abstract] |
Thursday, March 16, 2017 4:06PM - 4:18PM |
V47.00007: Thermal spin current control of magnetic damping in Py/Ag/CoFe2O4/Pt multilayers Chris Safranski, Ilya Krivorotov, Ching-Tzu Chen, Jonathan Sun With the development of devices based on spin transport such as magnetic tunnel junctions and spin torque oscillators, there is the ever increasing need for a more efficient way to generate high spin current densities. The spin Seebeck effect has been of interest recently since it converts thermal gradient from ohmic heating to spin current. In this talk, we investigate generation of spin current via a temperature gradient applied across an insulating ferromagnetic film of cobalt ferrite (CoFe$_{2}$O$_{4})$. In our experiment, a direct charge current applied to a Py/Ag/CoFe$_{2}$O$_{4}$/Pt multilayer stack creates a temperature gradient across the CoFe$_{2}$O$_{4}$ layer thickness via ohmic heating of the adjacent metallic layers. We report that a thermal spin current is then injected from the CoFe$_{2}$O$_{4}$ layer into the Py layer as confirmed by (i) spin torque ferromagnetic resonance (ST-FMR) measurements and (ii) direct inverse spin Hall voltage detection. ST-FMR measurements reveal that the thermal current applies an anti-damping torque to the Py layer and thereby reduces its FMR spectral linewidth. The presence of the thermal spin current is also confirmed via measurements of the inverse spin Hall voltage produced by this current in the multilayer stack. [Preview Abstract] |
Thursday, March 16, 2017 4:18PM - 4:30PM |
V47.00008: A microscopic formulation of dynamical spin injection in ferromagnetic heterostructures Amin Ahmadi, Eduardo Mucciolo We present a novel microscopic formulation of dynamical spin injection from a ferromagnet (FM) into a two-dimensional nonmagnet (NM) material. The formulation employs a tight-binding Hamiltonian in the presence of a time-dependent boundary condition, which plays the role of FM region and models the hybridization at the FM-NM interface. The spin current expression is written in terms of Green's functions of the NM portion, allowing one to apply efficient recursive numerical methods for the computation of spin currents. In addition, both the atomic structure of the materials involved and the particular geometry of the system can be taken into account. Another advantage of the formulation is the possibility to include accurate, microscopic models of spin relaxation in the NM portion. [Preview Abstract] |
Thursday, March 16, 2017 4:30PM - 4:42PM |
V47.00009: Magnetization dynamics enabling reliable nanosecond-timescale switching through the spin Hall effect Sriharsha Aradhya, Graham Rowlands, Shengjie Shi, Erin Yandel, Daniel Ralph, Robert Buhrman We have recently reported that spin Hall torque can drive magnetic switching that is simultaneously fast and reliable (\textless 10$^{\mathrm{-5}}$ write error rates with 2 ns pulses) in 3-terminal magnetic tunnel junctions (3T-MTJs) with in-plane magnetization [1]. This is in contrast to switching of in-plane 2-terminal MTJs by conventional spin transfer torque (STT), where there are 10's of ns latency times. Here we present a comprehensive elucidation of the switching dynamics in 3T-MTJs obtained through fast-pulse measurements in a variety of material stacks and detailed micromagnetic simulations. We demonstrate that the interaction between the self-generated Oersted field in 3T-MTJs and the micromagnetics of the free layer can lead to reliable sub-nanosecond reversal. We further show through simulations that an artificially reversed Oersted field, which corresponds to the field-like component of spin torque in 2-terminal STT-switched MTJs, leads to undesirable pre-switching dynamics that are highly reminiscent of the latency times in literature. These results establish the in-plane 3T-MTJ as an attractive memory element for applications that do not require ultra-high density, due to its high magnetoresistance read signal, low impedance write path, and fast reliable switching. [1] S. V. Aradhya et al., Nano Lett., 2016, 16 (10), 5987. [Preview Abstract] |
Thursday, March 16, 2017 4:42PM - 4:54PM |
V47.00010: Spin Pumping in Py/Ir and Py/Cu/Ir Thomas White, Tighe Bailey, Casey W. Miller We report on spin pumping in Py/Ir and Py/Cu/Ir as functions of Ir thickness. Samples were fabricated by magnetron sputtering. Py and Cu layers were held constant at 5nm and 4nm, respectively, while the Ir thickness ranged from 1nm to 500nm. Room temperature FMR from 3-40 GHz was conducted in the flip-chip geometry using the NanOsc PhaseFMR tool with the applied field in the plane of the sample. We find very strong enhancement of the Gilbert damping parameter, $\alpha$, when the Ir directly contacts the Py ($\alpha$ = 0.023), relative to samples in which Cu separates the Py and Ir ($\alpha$ = 0.013); the control Py had $\alpha$ = 0.0085. This indicates the presence of spin memory loss at the interface between Py and Ir. While the Py/Ir series had the inhomogeneous broadening increased about a factor of two relative to the Py/Cu/Ir series, we found no significant evidence for the Ir thickness affecting $\alpha$, the effective magnetization, or the g-factor. The lack of thickness dependence of $\alpha$ indicates the spin diffusion length in Ir is of the order 1nm. [Preview Abstract] |
Thursday, March 16, 2017 4:54PM - 5:06PM |
V47.00011: Spin pumping in oxide heterostructures Sam Crossley, Adrian G. Swartz, Kazunori Nishio, Yasuyuki Hikita, Harold Y. Hwang Resonant excitation of magnetic films is an emerging technique for injecting large spin currents into adjacent materials for spintronics applications. Heterostructures comprised of transition metal oxides, with tunable spin-charge-orbital degrees of freedom, are a promising platform to realize diverse functionalities for new operational regimes in spintronics devices. Moreover, it is now possible to realize atomically precise interfaces in oxide heterostructures, enabling a careful examination of the correlation between structure, composition, and spin pumping characteristics. Using pulsed laser deposition, we have fabricated epitaxial oxide heterostructures comprised of ferromagnetic manganites and candidate high-spin orbit materials in order to understand the potential phase space for spin-orbit coupling length scales in oxide materials. One promising class of materials are the Ruddlesden-Popper series of perovskite iridates which can now be thermodynamically stabilized in the ultrathin limit. We will report our results on the enhanced damping of ferromagnetic resonance due to spin pumping oxide heterostructures. [Preview Abstract] |
Thursday, March 16, 2017 5:06PM - 5:18PM |
V47.00012: Magnetization dynamics under electromagnetic fields in the wavepacket methods. Bangguo Xiong, Hua Chen, Xiao Li, Qian Niu In this work we try to understand the magnetization dynamics in magnetic materials with electrons described by the semiclasscial wavepacket methods. Using the Lagrangian of electron wavepackets under slowly varying magnetization, we can explicitly write down the dynamic equations for both electrons and magnetization order, where the mutual interplay between the two presents itself naturally. It turns out that, more general than LLG equation, the magnetization dynamics is written as a first order differential equation as for a general vector, which allows a detailed discussion on physical process studied before, such as spin transfer torque, spin orbital torque and damping mechanism, and also gives the vortex-like torques that can pump energy into the system. Since electrons are easy to control by electromagnetic fields, we expect a theory that electromagnetic fields through coupling to electrons can be used to manipulate the magnetization. It is interesting that this formalism on magnetization dynamics can be used to study the electromagnetic response of bulk electrons, from which the current and magnetization expressions are extracted that match well with previous studies. [Preview Abstract] |
Thursday, March 16, 2017 5:18PM - 5:30PM |
V47.00013: Ferromagnetic resonance in superconductor/ferromagnet bilayers David Sanchez, Jacobo Santamaria, Pierre Mergny, Hiroshi Naganuma, Abdelmadjid Anane, Javier E. Villegas We experimentally investigate broadband ferromagnetic resonance (FMR) in bilayers that combine the high-temperature superconductor YBa$_{\mathrm{2}}$Cu$_{\mathrm{3}}$O$_{\mathrm{7}}$ with different ferromagnets, either Permalloy or the half-metallic La$_{\mathrm{1-x}}$Ca$_{\mathrm{x}}$MnO$_{\mathrm{3}}$. The observed behavior is compared to that of reference ferromagnetic layers. In bilayers, the FMR signal contains contributions from both the superconducting and ferromagnetic layers. The FMR linewidth is studied as a function of temperature (2K-200K) and frequency (up to 20 GHz) to obtain the damping constant above and below the superconducting critical temperature. The results will be discussed in the frame of the spin-pumping theory considering the superconductor a spin sink where part of the FMR generated angular momentum relaxes in the superconductor martial trough spin-pumpin [1]. [1] Yokoyama, T. {\&} Tserkovnyak, Y. Tuning odd triplet superconductivity by spin-pumping. Phys. Rev. B 80, 104416 (2009) [Preview Abstract] |
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