Session A18: Focus Session: Spin-transfer Torque: Devices and Dynamics

Spin transfer torque in ferroelectric tunnel junctions

The worldwide interest for spintronics grows up every year, magnetic oscillators and resistance switchers became an important part of electronics with promising applications such as tunable microwave radiation, magnetic memory cells, magnetic field sensors, etc. A non-equilibrium spin-dependent transport in magnetic tunnel junctions comprising a ferroelectric barrier was studied. The exact solutions of the free electron Schr\"odinger equation for electron tunneling in the presence of interfacial screening are obtained by Bessel and Airy functions. As a result, bias-dependence of the tunneling magneto- and electro-resistance are obtained. The barrier asymmetry induced by the ferroelectric polarization produces strong modifications compared to regular tunnel junctions in the bias-dependence of the transport properties. Furthermore, manipulating the electric polarity of the barrier provides a way to control the magnitude and sign of the spin transfer torque.   

Spin-transfer torque at finite bias from density functional theory and non-equilibrium Green's functions

The spin-transfer torque (STT) exerted on a magnetic layer by a spin-polarized current represents a powerful handle to manipulate the magnetization. This can make magnetic random access memories a reality. We have now implemented STTs in the electron transport code Smeagol (www.smeagol.tcd.ie), which combines density functional theory with the non-equilibrium transport formalism. In particular we are able to compute the STT both in the linear response limit and at finite bias, and for magnets with an arbitrary complex electronic structure, including spin-orbit interaction. Examples will be provided for both magnetic tunnel junctions and spin-polarized scanning tunnel microscopy of magnetic ions on non-magnetic surfaces.   

Current-Induced Spin Wave Instability

Current in conducting ferromagnets imparts angular momentum to the magnetic texture. Above a critical current, an instability is reached wherein this angular momentum transfer is able to overcome intrinsic damping, and spin waves begin to grow exponentially in time. We examine the conditions required to observe this instability for bulk and surface spin waves in different dimensions, and investigate the subsequent spin wave turbulence engendered by nonlinear terms in the Hamiltonian that couple different modes.   

Magnetic droplets in nano-contact spin-torque oscillators with perpendicular magnetic anisotropy

The theoretical prediction, by Ivanov and Kosevich [1], of ``magnon drop'' solitons in thin films with perpendicular magnetic anisotropy (PMA) and zero damping, dates back to the 1970s. More recently, Hoefer, Silva and Keller [2], demonstrated analytically and numerically that related ``magnetic droplet'' solitons should be possible to excite in nano-contact spin-torque oscillators (NC-STOs) based on PMA materials, where spin transfer torque locally realizes the zero-damping condition required in [1]. In my talk, I will present the first experimental demonstration of such magnetic droplets, realized using 50-100 nm diameter nano-contacts (NCs) fabricated on top of orthogonal GMR stacks of Co8/Cu/Co0.3[Ni0.8/Co0.4]x4 (thicknesses in nm). The nucleation of a magnetic droplet manifests itself as a dramatic 10 GHz drop in microwave signal frequency at a drive-current dependent critical perpendicular field of the order of 0.5 - 1 T. The drop in frequency is accompanied by a simultaneous sharp resistance increase of the device and a sign change of its magnetoresistance, directly indicating the existence of a reversed magnetization in a region of the [Co/Ni] free layer underneath the NC. As predicted by numerical simulations the droplet exhibits rich magnetodynamic properties, experimentally observed as auto-modulation at approximately 1 GHz and sometimes sidebands at 1/2 and 3/2 of the fundamental droplet frequency. The 1 GHz modulation can be shown numerically to be related to the drift instability of the droplet [2], albeit with enough restoring force to make the droplet perform a periodic motion instead of leaving the NC region. The sidebands at 1/2 and 3/2 the droplet frequency are related to eigenmodes of the droplet perimeter. Magnetic droplet nucleation is found to be robust and reproducible over a wide number of NC-STOs with different NC sizes, making this new nanomagnetic object as fundamental and potentially useful to nanomagnetism as e.g. domain walls and vortices.\\[4pt] [1] B. A. Ivanov and A. M. Koseich, Zh. Eksp. Teor. Fiz. 72, 2000 (1977)\\[0pt] [2] M. A. Hoefer, T. J. Silva, and M. W. Keller, Phys. Rev. B 82, 054432 (2010)   

Precessional magnetization reversal in magnetic tunnel junctions with a perpendicular polarizer

The interaction between the spins of itinerant electrons and the magnetization of ferromagnetic materials is of great interest both for fundamental physics and applications. While a ferromagnetic layer can polarize the spin of electrons passing through it, a spin-polarized current also changes the magnetization of the ferromagnet via a spin-transfer torque (STT). Here we present an orthogonal spin transfer device [1] with an in-plane magnetized free layer (FL) and a perpendicularly magnetized spin polarizing layer, separated by a thin copper spacer. The initial STT acting on the in-plane FL is perpendicular to the plane due to the spin polarization from the polarizer. For large torques, the FL magnetization will be tilted out of its easy plane, which creates a demagnetization field on the order of tens to hundreds of millitesla. The FL magnetization will then precess about the demagnetization field. The FL in our device forms a magnetic tunnel junction with an in-plane magnetized reference layer (RL), which is used to read out the state of the free layer. The resistance of the device then depends on the relative orientation between the magnetizations of the FL and the RL. We experimentally demonstrated fast switching of the FL magnetization, switching for pulses less than 500 ps in duration [2]. We also conducted subthreshold single-shot time-resolved resistance measurements that probe the FL magnetization reversal mechanisms on time scales in which thermal fluctuations can play an important role. We identify the antiparallel (AP) and parallel (P) states and the transition between these two states during a pulse from single-shot oscilloscope traces. We find that there is a strong asymmetry between the AP to P and P to AP transitions under the same pulse conditions$^{\mathrm{\thinspace }}$[3]. The different switching processes can be explained by the strength of the perpendicular spin torque, which depends on the pulse current through the device and is initially larger in the P state than in the AP state. Spin torques from the RL also influence both the switching process and the switching probability. Our results illustrate new ways to control the magnetization of a nanomagnet on short time scales and optimize device operation.\\[4pt] [1] A. D. Kent, B. Ozyilmaz, and E. del Barco, Appl. Phys. Lett. \textbf{84}, 3897 (2004).\\[0pt] [2] H. Liu, D. Bedau, D. Backes, J. A. Katine, J. Langer, and A. D. Kent, Appl. Phys. Lett. \textbf{97}, 242510 (2010).\\[0pt] [3] H. Liu, D. Bedau, D. Backes, J. A. Katine, and A. D. Kent, Appl. Phys. Lett. \textbf{101}, 032403 (2012).   

Energy landscape for switching in spin-valve nanopillars with perpendicular magnetic anisotropy

Recent experiments have established that thermally activated switching in perpendicularly magnetized spin-valve (SV) nanopillars larger than about 40 nm in diameter is dominated by sub-volume nucleation and domain wall propagation. Despite this complex behavior, room temperature measurements of the switching field distributions indicate thermal activation over a single energy barrier [1]. To better understand the magnetization reversal process, we conducted temperature dependent studies of the switching statistics in nanopillars in which we stabilize non-uniform magnetization states formed by a sub-volume nucleation event. We present results on Co|Ni free layers in SV nanopillars, which include a perpendicularly magnetized fixed layer. Here we measure the distribution of switching events as a function of temperature from 20 K to 300 K. The temperature dependence of both nucleation and propagation distributions is consistent with a thermal activation model, with distinct field-dependent barrier heights for each stage in the reversal process. This is evidence of an energy landscape for switching, which should be relevant for understanding the switching of SV devices even at temperatures that no longer show metastable non-uniform states. [1] Appl. Phys. Lett. 100, 062404 (2012)   

Thermally-assisted magnetization reversal in nanomagnets with spin-transfer torque: diffusive energy space dynamics

A direct current applied to a nanomagnet produces a spin-transfer torque that drives the magnetization out of equilibrium.\footnote{J. C. Slonczewski, JMMM. 159, L1 (1996); L. Berger, Phys. Rev. B 54, 9353(1996).} In this talk, scalings between switching time and current for a macrospin under the effects of both spin-torque and thermal noise are explored analytically by focusing on its diffusive energy space dynamics. The procedure allows us to characterize the full dynamics with a one dimensional stochastic differential equation.\footnote{D. Pinna, D. L. Stein, A. D. Kent, arXiv:1210.7675, arXiv:1205.6509 (2012).} We establish the limits of this reduction and elucidate the nature of the limit cycle stabilities observed in nanomagnet reversal experiments. We further proceed to show that the thermally activated dynamics in the presence of a tilt between easy and spin-polarization axes differ only by a rescaling of the threshold current as long as easy, hard and spin-polarization axes all lie in the same plane. Our analytics are verified by employing modern GPU computational techniques to massively parallelize the Langevin equations and probing the long time switching behavior.   

Electron tunneling induced nonequilibrium magnetization noise in single Co nanoparticles

We have studied magnetic hysteresis loops of single Co nanoparticles in Al/Al$_2$O$_3$/(Co nanoparticles)/Al$_2$O$_3$/Al tunnel junctions using electron tunneling measurement at mK-temperatures. The magnetic switching field decreases and its distribution broadens versus tunneling current while the current does not heat the environment. The finding indicates that the magnetic switching field can be interpreted as a thermometer of the nonequilibrium magnetization noise. We present a phenomenological model that incorporates magnetic anisotropy fluctuations among discrete levels, to explain the noise properties.   

X-ray imaging of magnetic normal modes driven by spin transfer torque in magnetic nanopillar devices

We have used time-resolved x-ray microscopy to image the fundamental dynamical modes that are driven by spin transfer torque in magnetic devices. We apply a continuous microwave current to exert an oscillating spin torque in a nanopillar structure. By varying frequency and the applied magnetic field, this spin torque selectively excites different individual magnetic normal modes, which are then imaged by x-ray pulses synchronized to the microwave current. We obtain images with 70 ps time resolution and 25 nm spatial resolution. Our results identify modes having different spatial distributions of amplitude and phase, which can be explained by the combined effects of spin transfer torque and the Oersted field. We will discuss the implications of our results for understanding spin-torque-driven magnetic dynamics.   

Spin Dynamics and Resonant Inelastic X-ray Scattering in Chromium with Commensurate Spin-Density Wave Order

After the discovery of iron-pnictide superconductors, the spin dynamics of itinerant antiferromagnetic systems with multi-orbital has attracted much attention. In order to elucidate such spin dynamics, we focus on a similar system, chromium, which is known to show a spin density wave (SDW), and theoretically investigate dynamical spin susceptibilities and $L_3$-edge resonant inelastic X-ray scattering (RIXS) spectra [1]. We use multi-band Hubbard model composed of $3d$ and $4s$ orbitals. After the SDW mean-field approximation, we obtain the dynamical spin susceptibilities and RIXS spectra by employing random phase approximation. In our calculation, we assume the perfect commensurate SDW state. We find a collective spin-wave excitation undamped up to $\sim 0.6$~eV. Above the energy, excitation overlaps individual particle-hole excitations as expected. In RIXS spectra, particle-hole excitations with various orbital channels show a large spectral weight, masking the spectra of the spin collective mode. However, it may be possible to detect the spin-wave excitation in RIXS experiments in the future if resolution is high enough. [1] K. Sugimoto, Z. Li, E. Kaneshita, K. Tsutsui, and T. Tohyama, arXiv:1211.1598   

Spin Transfer torques in Antiferromagnets

Spin Transfer Torque (STT) has attracted tremendously growing interest in the past two decades. Consisting on the transfer of spin angular momentum of a spin polarized current to local magnetic moments, the STT gives rise to a complex dynamics of the magnetization. Depending on the the structure, the STT shows a dominated In plane component for spin valves [1], whereas both components coexist for magnetic tunneling junctions (MTJ) [2]. For latter case the symmetry of the structure is considered to be decisive in identifying the nature and behavior of the torque [3]. In the present study we are interested in magnetic structures where we substitute either one or both of the magnetic layers by antiferromagnets (AF). We use Non-equilibrium Green's function formalism applied on a tight-binding model to investigate the nature of the spin torque. We notice the presence of two types of torque exerted on (AF), a torque which tends to rotate the order parameter and another one that competes with the exchange interaction. We conclude by comparison with previous works [4-5].\\[4pt] [1] Xia, K., Kelly, P. J., Bauer, G. E. W., Brataas, A. {\&} Turek, Phys. Rev. B 65, 220401 (2002). [2] Sankey, J. C. et al. Nature Phys. 4, 67--71 (2008). [3] A. Kalitsov. et al. and W. H. Butler, Phys. Rev. B 79, 174416 (2009). [4] A. S. N\'u\~nez , R. A. Duine,~Paul Haney, and A. H. MacDonald, Phys. Rev. B 73, 214426 (2006). [5] R. A. Duine et al., Phys. Rev. B 75, 014433 (2007).