Session H33: Focus Session: Magnetization and Spin Dynamics II

Strong-field interactions between a nanomagnet and a photonic cavity

We analyze the interaction of a nanomagnet (ferromagnetic) with a single mode of a high-Q cavity in a fully quantum treatment and find that exceptionally strong magnetic coupling regime between a magnetic and a photonic system can be achieved. Coupling terms in several THz are predicted to be achievable in a spherical cavity of $\sim 1$~mm radius with a nanomagnet of $\sim 100$~nm radius and ferromagnetic resonance frequency of $\sim 200$~GHz. Since eigenstates of the magnet-photon system are entangled states of spin orientation and photon number over $10^5$ of each quanta, initial coherent states of spin and photon number evolve dynamically to produce large oscillations in the microwave output power and the nanomagnet spin orientation with exceptionally long dephasing times. Therefore, this distinguishable large total spin, long coherence times, and high power output of the nanomagnet-cavity system may serve as an efficient means of transferring information between a magnetic and a photonic system.   

Dissipative dynamics of magnetic solitons in metals

We develop the hydrodynamic theory of collinear spin currents coupled to magnetization dynamics in spin-textured, metallic ferromagnets. The equation of motion for the electronic current consist of a dissipative spin-motive force generated by magnetization dynamics and a magnetic texture-dependent resistivity tensor. The Onsager principle implies a reciprocal dissipative, adiabatic spin torque on the magnetic texture. Due to thermal fluctuations, electronic dynamics contribute to a non-local Gilbert damping tensor in the Landau-Lifshitz-Gilbert equation for the magnetization. Appling our hydrodynamic equations to soliton dynamics, we find that soliton motion generate electrical currents, which produce backaction through spin torques. We include such effects in a modified the Landau-Lifshitz-Gilbert equation and the corresponding solitonic equations of motion for collective coordinates. As an example, we consider the orbital motion of a vortex in a point-contact spin valve, and find modifications to orbit radius, frequency, and dissipation power.   

Dynamic Magnetization Reversal of Single Domain Magnetic Particle with Low Energy Barrier

Magnetization reversal process is one of the most fundamental topics for both basic and applied magnetism. From practical aspect, the reversal at short time, dominated by the precession of magnetization, determines the recording process for HDD and magnetic memory. The reversal at long time scale, dominated by thermal effect, limits the ultimate density for magnetic information storage devices. For thermally assisted reversal, the magnetization switches at short time scale, while the thermal energy is significant as compare to anisotropy energy. Thus the switching criterion and time dependence cannot be described using either formula along. Here micromagnetic simulation is utilized to study the magnetization reversal process under both high temperature and external applied field, with a low energy barrier. The results show how the switching criterion and the time dependent switching rate change for different applied field and temperature. The effect of applied field rise time and time dependent ambient temperature change are also included in this study.   

Field-driven domain wall propagation in magnetic nanowires

In this talk, I will present a global picture of magnetic-field induced domain wall (DW) propagation along a magnetic nanowire [1,2]: A static DW cannot exist in a homogeneous magnetic nanowire when an external magnetic field is applied. Thus, a DW must vary with time under a static magnetic field. A moving DW must dissipate energy due to the Gilbert damping. As a result, the wire has to release its Zeeman energy through the DW propagation along the field direction. The DW propagation speed is proportional to the energy dissipation rate that is determined by the DW structure. An oscillatory DW motion, either the precession around the wire axis or the breath of DW width, should lead to the speed oscillation. Based on this view, a relationship between the domain wall propagation velocity and the domain wall profile, regardless of the DW types, is found. A new velocity-field formula beyond the Walker breakdown field is derived. The formula is in excellent agreement with both experiments and numerical simulations. \\[4pt] [1] ``Magnetic field driven domain wall propagation in magnetic nanowires,'' X.R. Wang, P. Yan, J. Lu, and C. He, Annals of Physics 324, 1815 (2009).\\[0pt] [2]``High field domain wall propagation velocity in magnetic nanowires,'' X.R. Wang, P. Yan and J. Lu, Europhysiccs Letters 86, 67001 (2009).   

Magnetization Dynamics in the Cone Phase of Au/Co/Au thin films near the spin-reorientation transition

We report measurements of slow magnetization fluctuations in an ultrathin Au/Co/Au system that exhibits a spin reorientation phase transition as a function of temperature. The intermediate scattering function is well described by a stretched exponential that is indicative of cooperative motion throughout the temperature range of 150 - 300 K. The decay times were found to exhibit a maximum at the transition temperature. The slowdown has been explained as due to formation of a shallow double well in the energy landscape by the different competing interactions. Our results show that slow dynamics in the mesoscopic length scale can provide valuable insights into the nature of magnetic phase transitions. Work at LBNL is supported by DOE.   

Vortex dynamics in an equilateral triangular arrangement of three magnetic disks

Magnetic vortices in micron-sized ferromagnetic disks have been of great interest because of their potential applications in data storage. While the motion of a vortex in a single isolated magnetic disk has been studied extensively, vortex dynamics in multiple-disk planar geometries remains to be fully understood. We report direct time-resolved imaging and theoretical calculations of the vortex states in an equilateral triangular arrangement of three magnetic disks with varied center-to-center spacings. The free-motion trajectories of the vortex cores in the triangular arrangement of three permalloy disks of 2 micron radius were traced using time-resolved x-ray photoemission electron microscopy at beamline 4-ID-C of the Advanced Photon Source. The temporal resolution is 90 ps. The oscillation amplitude in the tri-disks with 4.5 micron center spacing was smaller than that with 5 micron center spacing. No significant frequency shift was observed. Theoretical calculation showed both frequency shift and trajectory change due to dipolar interaction of the disks at varied spacings.   

Giant spin pumping effect in microwave-driven ferromagnet-topological insulator systems

The spin pumping from microwave-driven precessing magnetization has recently emerged (together with spin transfer torque as its inverse effect) as one of the key phenomena of the second generation metal spintronics involving coherent spin states. The well understood pumping by ferromagnet-normal metal (FN) interfaces can also be exploited for spin batteries that generate pure spin current. However, typical output of FN systems is rather small due to spin accumulation driving the backflow of spins. Here we demonstrate that surprisingly large current can be pumped if the precessing ferromagnetic layer is surrounded by a topological insulator (based on graphene or HgTe), where vastly different dependence on the precession cone angle than in the FN-based devices is obtained. Thus, experimental realization of this proposal would make possible efficient conversion of microwave power into pure spin current, as well as understanding of how chiral spin-filtered edge state within finite-size topological insulators can be exploited in realistic inhomogeneous nanodevices.   

Pure spin transport in metallic nanostructures

While spin transport in most cases refers to spin-polarized charge currents, spin and charge currents can be decoupled using nonlocal geometries. Investigation of these pure spin currents can provide new insight into spin-dependent physics. This presentation will focus on two different aspects of pure spin transport in metallic nanostructures: spin Hall effects and spin relaxation. Spin Hall effects occur due to spin-dependent electron scattering, and they have been suggested as a pathway to pure spin currents without using ferromagnets. We studied the possibility of observing spin Hall effects in purely paramagnetic structures by investigating non-local transport in mesoscopic Hall bars fabricated from gold. However, our experiments did not show large spin Hall effects in gold, despite the strong spin-orbit coupling in this metal [1]. We will also present our results on pure spin transport in mesoscopic silver wires where spin current is generated via spin injection from permalloy. Using the nonlocal spin valve geometry and Hanle effect measurements we directly probed the spin relaxation in silver. By studying the temperature dependence of the spin relaxation rate, we were able to distinguish between different physical mechanisms leading to spin relaxation. We found that a diffusive electron transport in the wire produces a temperature dependent spin relaxation rate for electron scattering from the surfaces, with surface spin-flip probability $\sim$ 5 times higher than for the bulk scattering [2]. $^{\ast}$Work done in collaboration with A. Hoffmann, J. E. Pearson, S. D. Bader, and M. A. Garcia, and supported by UChicago Argonne, LLC, operator of Argonne National Laboratory, a U.S. Department of Energy Office of Science laboratory, operated under contract No. DE{\-}AC02-06CH11357. \\[4pt] [1] G. Mihajlovic, J. E. Pearson, M. A. Garcia, S. D. Bader, A. Hoffmann, Phys. Rev. Lett. 103, 166601 (2009) \\[0pt] [2] G. Mihajlovic, J. E. Pearson, S. D. Bader, A. Hoffmann, arXiv:0910.2744v2.   

Theory of spin wave driven spin Seebeck effect

We propose an explanation for the recently discovered spin Seebeck effect in terms of a spin-pumping-current driven through a ferromagnet/normal metal interface by a difference between the magnon temperature in the ferromagnet and the electron temperature in the normal metal. This spin current is proportional to the temperature difference, which is excited by an applied heat current through the ferromagnet, the spin-mixing conductance of the interface, and the inverse of a temperature-dependent magnetic coherence volume, and can generate an inverse spin Hall voltage (spin Seebeck signal) in a normal metal contact attached to the ferromagnet. A simple diffusion theory for the magnon thermalization is consistent with the spatial variation of the spin Seebeck effect measured in the insulator yttrium iron garnet (YIG) but not in Permalloy. The estimated magnitude of the spin Seebeck effect agrees with the experiments on Permalloy, but is too small for YIG.   

Creating Spin Switches and Junctions on Surfaces

Inspired by the work of Hirjibehedin \textit{et al}, (\textit{Science} \textbf{317} 1199) creating Heisenberg spin chains on an insulating surface, we examine geometries in which excitations down a spin chain are either blocked or transmitted depending on the state of a central junction, made from a spin dimer. The dimer state can be controlled by excitations down an additional chain, creating a spin switch. In addition to the technological applications of such a switch, the theoretical language developed has application to certain quantum computation schemes.   

Energetic analysis of fast magnetic switching

The speed of magnetic switching is an important parameter for many devices, like magnetic random access memory (MRAM) cells, or hard drive disk/heads. Traditionally switching processes have been evaluated by solving the Landau-Lifshitz (LLG) equation numerically. Criteria of successful switching were defined in phase space (instantaneous magnetic moments), or required full system relaxation. This presentation introduces another look at the switching processes, based on an energetic approach. The main idea is to split the total transient energy of the system into two parts: $E_{s}$, which includes switching stimulus only (e.g. external pulse field), and $E_{0}$ - covering effects important for final state. Monitoring a single value $E_{0}$, as LLG integration progresses, allows for detection of switched states. The method shows its full power in the case of magnetic nanostructures, which have a relatively simple energetic landscape of relaxed states (no magnetic domains), but may show very complex dynamical configurations. Another advantage is a possibility to account for finite temperature in switching criteria. The concept is illustrated by simulations of single spin and MRAM cell switching.