Session Q8: Focus Session: Spin Wave Phenomena and Spin Pumping and Damping

Spin wave dynamics and magnetization switching in exchange-coupled bilayers

Magnetic moments under the application of electric current or rf magnetic field show uniform or non-uniform dynamical motions, which are keys to develop novel spintronic devices such as nanometer-sized auto-oscillators and logic circuits. Spin waves are representative of spatially non-uniform magnetization dynamics. We demonstrated that spin waves could be utilized also to reduce the switching field in exchange-coupled bilayers consisting of hard magnetic L10-FePt and soft magnetic Permalloy (Py; Ni81Fe19). The switching field of L10-FePt was drastically reduced when the spin waves were excited. This ``spin wave-assisted magnetization switching'' is a route to balance competing goals for high coercive field, which is essential to maintain a good thermal stability of magnetization in a nanometer region, and low switching field, leading to the device operation with low power consumption. Those are important to realize high-performance spintronic and magnetic storage devices.In this talk, the concept and mechanism of spin wave-assisted magnetization switching are introduced. By comparing the experiments and the numerical simulation, it is found that perpendicular standing spin wave modes are mainly excited in Py of the exchange-coupled bilayers and those spin waves affect the dynamics of L10-FePt through the exchange coupling mechanism at the interface. The significant reduction of switching field is achieved by exciting the spin waves with large oscillation amplitude. In addition, the spin wave-assisted magnetization switching shows the characteristic magnetic field angular dependence, which is totally different from that of uniform magnetization dynamics. We also show the spin wave dynamics in perpendicularly magnetized exchange-coupled bilayers.   

Room-temperature extraction of spin lifetimes in metallic thin films via determination of the spin-pumping contribution to damping in ferromagnetic resonance experiments

Recent room-temperature measurements yield spin diffusion lengths for Pt and Pd that are smaller than the bulk electron mean free path at room temperature. One proposed explanation is the thickness-dependence of conductivity that results in shorter momentum lifetimes at small Pt/Pd thicknesses. We measured spin transport properties in Pd and Pt thin films at room temperature via fitting of ferromagnetic resonance (FMR) damping vs. NM thickness with the spin pumping model for ferromagnet (FM)/normal metal (NM) multilayers. We use a broadband, perpendicular FMR system to obtain high-precision values for the damping. The fits are based upon spin diffusion equations that include both momentum and spin scattering processes. By measuring thickness-dependent conductivity of the same films, we correlate the charge and spin transport parameters, permitting us to test multiple models for spin scattering. We explicitly show that the spin scattering time $\tau_{sf}$ must be shorter than the momentum scattering time tau over some range of NM thicknesses to adequately fit the data. Invocation of a simple monotonic proportionality between $\tau_{sf}$ and $\tau$ fails to fit the data. However, an inverse proportionality $\tau \sim 1/\tau_{sf}$ can fit the data, and $\tau < \tau_{sf}$ for sufficiently thin NM layer.   

Spin Pumping by Antiferromagnetic material

For a long time, spin-pumping is supposed to be impossible using antiferromagnetic materials (AFM) with compensated magnetization. We show that spin-pumping does not only exist in AFM with precessing staggered order, but is even stronger than its counterpart in ferromagnets. By calculating the scattering matrix of a normal metal/AFM interface based on a tight-binding model, we derive the pumped spin current in terms of the staggered order parameter and its time derivative. It is found that spin-pumping is of the same order of magnitude for both compensated and uncompensated interfaces. And the pumped spin current can be switched by changing the circular polarization of light exciting antiferromagnetic resonance, which introduces potential application. Besides spin current, we also define a new quantity--staggered spin current--and propose a novel pumping effect in AFM.   

FMR spin pumping in YIG/ferromagnet bilayers (ferromagnet $=$ Fe, Co, Ni, Py)

Generation of pure spin currents from ferromagnets (FM) to normal metals (NM) has been extensively studied by thermal and ferromagnetic resonance (FMR) spin pumping. Recently, Miao et al. demonstrated thermal injection of spin currents from Y3Fe5O12 (YIG) into Py detected by inverse spin Hall effect (ISHE) in the FM [1]. The ISHE in FM is in fact the inverse anomalous Hall effect (SHE), but with all the signatures of ISHE in NMs. Here we report robust FMR spin pumping in YIG/FM bilayers with FM $=$ Fe, Co, Ni and Py using cavity FMR. The resonance fields of the FMs and YIG are clearly separated, which allows distinction of spin pumping induced ISHE voltages at the YIG resonance field and the voltage signals at the FM resonance fields. The ISHE voltages reaches 220 uV for YIG/Py(2nm) bilayer and tens of uV for all YIG/FM bilayers with 10-nm FM at an rf power of 200 mW. The sign of the ISHE voltages for Py and Ni are opposite to those for Fe and Co, which agrees with the opposite signs of AHE in Ni as compared to Fe and Co. \\[4pt] [1] B. F. Miao, S.Y. Huang, D. Qu, and C. L. Chien, ``Inverse Spin Hall Effect in a Ferromagnetic Metal,'' Phys. Rev. Lett. 111, 066602 (2013).   

Spin current generation in graphene by dynamical spin pumping

Graphene is a promising material for spintronics applications given its unique properties. However, an efficient method to generate pure spin currents into this two-dimensional material is required to understand the spin dynamics and mechanisms associated to spin transport in graphene. Recently, we reported the first evidence of spin pumping in ferromagnet/graphene interfaces by studying the damping of the ferromagnet due to presence of graphene. We have extended the original studies towards different device configurations. Here we discuss the effect of the interface on the dynamical damping by studying different stacking orders of graphene and Permalloy layers. Our results confirm that the observed damping is indeed a signature of dynamical spin pumping wherein spin polarized currents are pumped into the graphene from the precessing magnetization of the ferromagnet. In addition, we performed comparative FMR studies of ferromagnet/Graphene strips buried underneath the central line of a coplanar waveguide. A larger FMR linewidth broadening is observed when the graphene layer protrudes away from the ferromagnet strip, indicating that the spin relaxation in graphene occurs away from the area directly underneath the ferromagnet being excited.   

Enhancement of Pure Spin Currents in Spin Pumping Y$_3$Fe$_5$O$_{12}$/Cu/metal Trilayers Through Spin Impedance Matching

Spin pumping, driven thermally as well as by Ferromagnetic Resonance (FMR), is being widely used to generate pure spin currents from ferromagnets (FM) into normal metals (NM). Typically, the NM is chosen to be a spin-sink-Pt, W or Ta, while lighter metals such as Cu are rarely used, except to decouple the FM and spin sink. The efficiency of spin pumping is largely determined by the spin mixing conductance of the FM/NM interface. Here, we report a comparative study of spin pumping in $\rm Y_3 Fe_5 O_{12}$/Cu/Pt and $\rm Y_3 Fe_5 O_{12}$/Cu/W trilayers with varying Cu thicknesses. Remarkably, we find that insertion of a Cu interlayer between YIG and W substantially improves (over a factor of 4) the spin current injection into W while similar insertion between YIG and Pt degrades the spin current. This is a consequence of a much improved YIG/Cu spin mixing conductance relative to that for YIG/W. This result shows that high quality multilayer FM/NM heterostructures can enable spin mixing conductances to be engineered to enable optimal spin pumping efficiency.   

Spin pumping and Gilbert damping in atomically flat nanometric thick YIG\textbar NM system

Epitaxial nanometric thick ytrrium iron garnet (YIG) films grown on (111) and (110) gadolliun gallium garnet (GGG) substrates via PLD exhibit an atomically flat surface. This extremely flat surface with a roughness $\sim$ 0.1 {\AA} offers a more controlled study of the physical mechanism behind the newly observed damping in YIG\textbar NM bilayer systems. Our bilayer systems consist of a 30 nm thick YIG film, either (111) or (110), and a non-magnetic layer, either beta-phase Ta or Pd, with thickness ranging from 0 to 20 nm. We have performed ferromagnetic resonance (FMR) experiments and observed systematic thickness dependence of the FMR linewidth. As the thickness of NM increases, the FMR linewidth increases rapidly and then slowly approaches saturation. The effect of the YIG surface on the Gilbert damping due to the magnetic proximity effect and on spin pumping in such bilayer systems will be discussed.   

Investigation of Spin Pumping in YIG/Cu/Py using Ferromagnetic Resonance

Spin pumping in YIG/Au/Fe structures has been demonstrated where the YIG film serves as a spin battery, while the Fe film functions as a spin sink [1]. In principle, the insulating YIG film can also absorb spin currents through interfacial \emph{s-d} interactions and function as a spin sink for spin pumping. In this presentation we report on the coupling between the YIG and Permalloy (Py) films in YIG/Cu/Py systems from the viewpoint of the spin pumping effect, where both layers function as either a spin battery or a spin sink. We found an increased Gilbert damping for both the YIG and Py films by means of ferromagnetic resonance (FMR) measurements. We discuss the Gilbert damping constant ($\alpha$) of YIG(40nm), Cu(5nm)/Py(3nm), and YIG(40nm)/Cu(5, 20nm)/Py(3nm) and apply these values to spin diffusion model for the calculation of spin mixing conductance. These results show the spin pumping effect at both the ferrimagnetic/NM and ferromagnetic/NM interfaces in YIG/Cu/Py structures and the dual function of the YIG and Py films in terms of the generation and absorption of spin currents.\newline [1] B. Heinrich et. al., Phys. Rev. Lett. $\bf{107}$, 066604 (2011).   

Modifying magnetic switching in permalloy film nanostructures using the native oxide

Thin films of nickel-iron alloys of the nominal concentration near 80\% Ni, are very commonly used in applications and in fundamental studies of spin, charge and heat transport in nanomagnetic systems. These permalloy (Py) films are straightforward to grow by various techniques and typically produce predictable, controllable and repeatable magnetic properties, including small coercivity, low magnetocrystalline anisotropy, and low magnetostriction. We have found that greater complexity can be added to the switching behavior of thin films of permalloy by oxidation of thin ($\sim$4 nm) layers followed by subsequent growth of Py. Under some circumstances, this can cause apparent negative coercivity in the switching observed in anisotropic magnetoresistance (AMR) of micromachined strips with an expected shape anisotropy. Here we will present results on growth and AMR measurements of the effects in various oxidized Py-Py layered samples. It is not yet clear if the effects are reproducible enough to be used for intentional manipulation of switching behavior in Py nanostructures.   

Control of perpendicular magnetic anisotropy and intrinsic Gilbert damping in $L1_{0} $ordered FePt(Pd) thin films

The dependence of perpendicular magnetic anisotropy (PMA) and intrinsic Gilbert damping $\alpha_{0} $ on some leading parameters, such as spin-orbital coupling strength $\xi $, are investigated in $L1_{0} $ ordered FePt(Pd) thin films by time-resolved magneto-optical Kerr effect measurements and spin dependent ab initio calculations. Continuous tuning of PMA and $\alpha_{0} $ over a wide range of magnitude is realized by modulating the chemical substitution and ordering. Spin orbital coupling strength can be effectively adjusted by replacing Pt with Pd atoms, which keeps other leading parameters with negligible changes. Measured PMA and $\alpha_{0} $ from experiment are proportional to $\xi^{1.6}$ and $\xi ^{2}$ at 200K, while first principle calculations predict for both a quadratic dependence on $\xi $. The degree of chemical order in real samples can also significantly affect PMA and $\alpha_{0} $through leading parameters other than spin orbital coupling strength.