Session F14: Focus Session: Material Properties Important for Spin-torque Dynamics

Quantifying spin torque effects using a current-driven magnetic vortex

Spin transfer torques offer great potential for the development of spin-based devices for processing and storing information but there is still debate surrounding the relative contributions of the adiabatic and non-adiabatic spin torque effects. Magnetic vortices in patterned magnetic films provide a model system that can be used to quantify these effects. Micromagnetic calculations of the current-driven motion of a magnetic vortex in a patterned Permalloy element show that the two spin torque effects have distinguishable influences on the trajectories of the vortex core and, furthermore, that the effect of the current-generated magnetic fields (Oersted) that are often non-negligible when current flows through magnetic nanostructures can also be separated out. An analysis of a series of experimental images of vortex trajectories obtained using a recently developed dynamic Lorentz transmission electron microscopy technique provides a measure of the non-adiabatic spin torque parameter with greatly improved precision [1]. The work described here was carried out in collaboration with Shawn Pollard, L. Huang, Dario Arena, and Yimei Zhu (Brookhaven). \\[4pt] [1] S. Pollard, L. Huang, K. S. Buchanan, D. Arena, and Y. Zhu, {\it Nature Communications} {\bf 3}, 1028, 2012.   

Anisotropy of spin relaxation in metals and ultrathin metallic films

We predict a hitherto overlooked anisotropy of the spin relaxation time $T_1$ in non-magnetic metallic systems with respect to the orientation of the spin porlarization $\hat{s}$ of the injected electrons relative to the crystallographic directions. In the Elliott-Yafet mechanism, the spin relaxation time is related to the Elliott-Yafet parameter $b^2$ that quantifies the degree of spin-mixing of Bloch states due to spin-orbit interaction. It can be demonstrated that $b^2$ depends on $\hat{s}$ due to the directional dependence of the spin-orbit matrix-elements between Bloch states comprising directional orbitals. The directional dependence becomes very pronounced in the case of degeneracies or near-degeneracies leading to {\it spin-flip hot spots} or even extended {\it hot areas} on the Fermi surface. The calculated anisotropy can reach values as large as 830\% for hcp Hf or 87\% in W(110) 10-layer-films, as we find from first-principles calculations employing the Kohn-Korringa-Rostoker Green function method. The anisotropy offers interesting new functionalities in spintronics applications such as GMR, spin Hall effect as well as spin dynamics. [1] B. Zimmermann, P. Mavropoulos, S. Heers, N. H. Long, S. Blugel, and Y. Mokrousov, Phys. Rev. Lett., in press (arXiv:1210.1801).   

Extraction of spin-transport parameters from ferromagnetic resonance measurements of spin-pumping in metallic multilayers

We use ferromagnetic resonance to measure damping due to spin-pumping in symmetric multilayers of Ta(3)/Ni(x)/Pd(y)/CoFe(2)/Pd(y)/Ni(x)/Ta(3) (thicknesses in nm, 0$\le $x$\le $2nm, 0$\le $y$\le $10nm). The stack's symmetry ensures that spin-pumping on both sides of the ferromagnet is identical and allows us to unambiguously characterize the multilayer spin-transport properties. When x $=$ 0 (no Ni), the Pd-Ta interface is found to be strongly spin-impedant, due to the low spin conductivity of Ta, leading to greatly reduced damping for thin Pd. As the Pd thickness approaches the spin diffusion length, the damping increases. By inserting Ni, a strong spin absorber, at the Pd/Ta interface, the damping for thin Pd is maximized. Varying the thickness of the Ni layer can be used to tune the inter-layer spin current flow in the Pd/Ni/Ta heterostructure. Comparison of the data with the conventional model for diffusive spin-polarized transport in normal metals permits quantitative determination of the spin-diffusion length in the normal metals. The results have implications for the detection of spin-currents in lateral spin valves via the inverse spin Hall effect in high-resistivity materials such as Ta.   

Controlling the Gilbert damping using spin pumping and magnetic impurities

The ability to control the magnetic damping parameter of thin magnetic films is an important issue when designing for example giant magnetoresistance (GMR) devices. A well-known way to influence the damping of the ferromagnetic (F) layer is by using the spin pumping effect in which a spin current is emitted into an adjacent normal (N) layer by bringing the F-layer into ferromagnetic resonance (FMR). As N layer, we used the well studied strongly spin sinking material Pt and the bad spin sink Cu, but also a Cu layer with Co impurities. We find that by adding a small amount of Co impurities, the Cu layer becomes as effective in damping as a Pt layer. In the latter case, the damping is caused by the strong spin orbit coupling. Using magnetic impurities, we rather make use of the inelastic spin scattering. This opens up new ways to control the damping of a ferromagnetic thin layer, for example in current-in-plane (CIP) GMR sensors, where the extra damping can suppress the spin transfer torque which becomes dominant with the further decrease of the size of the sensor.   

Gilbert damping parameter characterization in perpendicular magnetized Co$_{2}$FeAl films

Materials with perpendicular magnetic anisotropy(PMA) have gotten extensive recent attention because of their potential application in spintronic devices such as spin transfer torque random access memory (STT-RAM). It was shown that a much lower switching current density(J$_{\mathrm{C}})$ is required to write STT-RAM tunnel junctions with perpendicular magnetic anisotropy ferromagnetic electrodes (p-MTJ). Additionally Heusler alloy Co$_{2}$FeAl is expected to further reduce J$_{\mathrm{C}}$ due to its ultra low Gilbert damping parameter. In our study, Heusler alloy Co$_{2}$FeAl films were prepared using a Biased Target Ion Beam Deposition (BTIBD) technique. We demonstrated a low Gilbert damping parameter achieved in thick B2-Co$_{2}$FeAl films. Besides, we achieved an interfacial PMA in ultra thin Co$_{2}$FeAl films by rapid thermal annealing (RTA) with no external field presented. Annealing conditions were carefully adjusted to maximize the interfacial PMA. However it was noticed that a higher annealing temperature was required for a low damping parameter which to some extent sacrificed the interfacial PMA. We also deposited ultra thin CoFeB films and characterized their damping parameters for comparison.   

Bonding, magnetic properties and stability of the half-Heusler alloys LiMnZ (Z=N, P, Si)

We examined the bonding and magnetic properties, as well as the stability, of three magnetic half-Heusler alloys, namely LiMnZ, with Z=N, P or Si, in the three different atomic ordering of the C1$_b$ crystal structure (i.e. $\alpha$, $\beta$, and $\gamma$ phases). Using LiMnP as a prototype, we examined the bonding properties of the three phases and found, at the optimized lattice constant, each phase is a ferromagnetic metal. Assuming a proper matching substrate could be found, we found that $\alpha$-LiMnN, LiMnSi in the $\beta$ and $\gamma$ phases, and LiMnP in the $\alpha$ and $\beta$ phases can be ferromagnetic half-metals at lattice constants larger than the optimized values. Volume stability studies showed that the $\beta$ phase is the most stable for these materials. In our search for more half-metals, we found that $\beta$-Li$_{0.75}$MnSi, $\beta$-Li$_{0.75}$MnP and $\gamma$-Li$_{0.75}$MnN can be half-metals at the respective LiMnSi half-metallic lattice constants. By comparing $\beta$-LiMnP to the metastable zinc blende phase of MnP, the role of Li in the structure, with respect to the elastic stability, electronic properties, and magnetic properties, was studied. Finally we examined the possibility of a compensated half-metallic phase in these materials.   

Dirac Half-Metal in a Triangular Ferrimagnet

Massless Dirac fermions show substantially different nature from ordinary electrons due to the peculiar cone-like dispersion with the point node. While it was originally introduced in the relativistic quantum theory, recent discovery of graphene, a single layer sheet of graphite, has carved out a new direction of their study in condensed matter systems. From the viewpoint of potential application to electronics, it is of great interest to control the electronic spin degree of freedom. However, there is not so much flexibility in graphene, as the relativistic spin-orbit interaction is very weak. Here, we present an alternative idea for realizing massless Dirac fermions in itinerant electrons coupled to a well-known ferrimagnet on a triangular lattice [1]. The Dirac fermions are spin-polarized, and stable in a wide range of the spin-charge coupling including typical values in solids. We demonstrate that, by an unbiased Monte Carlo simulation, such Dirac half-metal with ferrimagnetic order spontaneously emerges in a minimal Kondo-lattice type model. The realization will be beneficial for spintronics as a candidate for spin-current generator.\\[4pt] [1] H. Ishizuka and Y. Motome, preprint (arXiv:1210.6700), PRL in press.   

GMAG PhD Dissertation Research Award Talk: Experimental characterization of non-isochronous properties of spin torque nano-oscillators

Spin Torque Oscillators (STO) are nano-sized auto- oscillators whose frequency can be tuned in a wide range. This tuning originates from the nonlinear properties of the underlying magnetization dynamics that is induced by spin transfer torque (STT) in magnetic nanostructures. Being highly tunable in frequency has the inconvenience of being very sensitive to noise. As a result the spectral purity of STOs is below the one required for applications. The magnetization dynamics induced by STT has been described theoretically in the frame of a non-isochronous auto-oscillator model [1]. Within this model the important information on the excitation mode is contained within two phenomenological parameters which are, the amplitude-phase coupling $\nu $, and the amplitude relaxation rate $\Gamma_{\mathrm{p}}$. They completely determine the non-autonomous oscillator response and their experimental determination is thus an important issue. This presentation describes several experimental methods to extract these parameters. The first involves time domain noise spectroscopy which permits the power spectral density of phase and amplitude noise to be extracted [2]. The analysis of such noise in light of theoretical models allows not only direct extraction of the nonlinear parameters, but also to quantify the phase noise - the characteristics important for applications. This is demonstrated for magnetic tunnel junction devices. A second method involves the analysis of higher harmonic linewidths $\Delta $f$_{\mathrm{n}}$ [3], where it is shown that due to the non-isochronous property of STOs, the relationship between $\Delta $f$_{\mathrm{n}}$ and $\Delta $f$_{\mathrm{1}}$ is nontrivial and allows one to extract $\nu $ and $\Gamma_{\mathrm{p}}$. We then apply the information gathered on the autonomous dynamics of STOs to understand their non-autonomous dynamics that are a prerequisite for the use of STOs in more complex devices (field sensors, frequency synthesizers, etc). It is shown experimentally how the nonlinear parameters $\nu $ and $\Gamma_{\mathrm{p}}$ determine this non-autonomous behavior of the STO. \\[4pt] [1] A. Slavinand and V. Tiberkevich, IEEE Trans on Mag 45, 1875 (2009)\\[0pt] [2] M. Quinsat et al. APL 97, 182507 (2010)\\[0pt] [3] M. Quinsat et al. PRB 86, 104418 (2012)   

Correlation effects in disordered conductors with spin accumulation

We consider the effect of electron-electron interaction on the density of states of disordered thin film paramagnetic conductor in the presence of spin accumulation and magnetic field. We assume a mechanism of electrical spin injection from a ferromagnet into a paramagnet as a particular realization of the spin accumulation. We show that interaction correction to the electron density of states of the paramagnet exhibits singularities at energies corresponding to the difference between chemical potentials of electrons with opposite spins. The correction to the conductivity of the paramagnet in the metallic region as well as in the hopping region in the presence of spin accumulation is calculated.   

Spin transport in the Neel and collinear antiferromagnetic phase of the two dimensional spatial and spin anisotropic Heisenberg model on a square lattice

We analyze and compare the effect of spatial and spin anisotropy on spin conductivity in a two dimensional S=1/2 Heisenberg quantum magnet on a square lattice. We explore the model in both the Neel antiferromagnetic (AF) phase and the collinear antiferromagnetic (CAF) phase. We find that in contrast to the effects of spin anisotropy in the Heisenberg model, spatial anisotropy in the AF phase does not suppress the zero temperature regular part of the spin conductivity in the zero frequency limit - rather it enhances it. In the CAF phase (within the non-interacting approximation) the zero frequency spin conductivity has a finite value which is suppressed as the spatial anisotropy parameter is increased. Furthermore, the CAF phase displays a spike in the spin conductivity not seen in the AF phase. We also explore the finite temperature effects on the Drude weight in the AF phase (within the collision less approximation). We find that enhancing spatial anisotropy increases the Drude weight value and increasing spin anisotropy decreases the Drude weight value. Based on these studies we conclude that antiferromagnets with spatial anisotropy are better spin conductors than those with spin anisotropy at both zero and finite temperatures.   

Study of the Switching Current Density Reduction in Spin Transfer Torque Magnetic Tunneling Junction by Using Micromagnetic Simulations

We investigate the reduction of switching current density of the spin transfer torque magnetic tunneling junction (STT-MTJ) with micromagnetic simulations for the various parameters and structures. We introduce a non-collinear magnetization polarizer layer and find noticeable switching current density reduction. The physical origin of the reduction ascribe to the enhanced coherent spin rotation due to the asymmetry breaking. Furthermore, when we cut the one edge of the MTJ ellipse structure, conspicuous reduction of switching current density is also found. In contrast to the normal MTJ structure where the switching process is accompanied with non-coherent spin dynamics, enhanced coherent spin rotations play an important role in both new structures. In addition, we find that the switching current density is sensitively varied with the exchange stiffness and junction size due to the weakly quantized spin wave vector. Based on our micromagnetic simulations, we open new path to reduce the switching current density with new MTJ structures.