arXiv subjects

M. D. Stiles

Publications and source records attributed to M. D. Stiles.

At least 37 records · Page 2Linked to original sources

arXiv2014

Angular dependence of spin-orbit spin transfer torques

In ferromagnet/heavy metal bilayers, an in-plane current gives rise to spin-orbit spin transfer torque which is usually decomposed into field-like and damping-like torques. For two-dimensional free-electron and tight-binding models with Rashba spin-orbit coupling, the field-like torque acquires nontrivial dependence on the magnetization direction when the Rashba spin-orbit coupling becomes comparable to the exchange interaction. This nontrivial angular dependence of the field-like torque is related to the Fermi surface distortion, determined by the ratio of the Rashba spin-orbit coupling to the exchange interaction. On the other hand, the damping-like torque acquires nontrivial angular dependence when the Rashba spin-orbit coupling is comparable to or stronger than the exchange interaction. It is related to the combined effects of the Fermi surface distortion and the Fermi sea contribution. The angular dependence is consistent with experimental observations and can be important to understand magnetization dynamics induced by spin-orbit spin transfer torques

cond-mat.mtrl-sci↗

arXiv2013

Chirality from interfacial spin-orbit coupling effects in magnetic bilayers

As nanomagnetic devices scale to smaller sizes, spin-orbit coupling due to the broken structural inversion symmetry at interfaces becomes increasingly important. Here we study interfacial spin-orbit coupling effects in magnetic bilayers using a simple Rashba model. The spin-orbit coupling introduces chirality into the behavior of the electrons and through them into the energetics of the magnetization. In the derived form of the magnetization dynamics, all of the contributions that are linear in the spin-orbit coupling follow from this chirality, considerably simplifying the analysis. For these systems, an important consequence is a correlation between the Dzyaloshinskii-Moriya interaction and the spin-orbit torque. We use this correlation to analyze recent experiments.

cond-mat.mes-hall↗

arXiv2013

Current-induced torques and interfacial spin-orbit coupling

In bilayer systems consisting of an ultrathin ferromagnetic layer adjacent to a metal with strong spin-orbit coupling, an applied in-plane current induces torques on the magnetization. The torques that arise from spin-orbit coupling are of particular interest. Here, we calculate the current-induced torque in a Pt-Co bilayer to help determine the underlying mechanism using first principles methods. We focus exclusively on the analogue to the Rashba torque, and do not consider the spin Hall effect. The details of the torque depend strongly on the layer thicknesses and the interface structure, providing an explanation for the wide variation in results found by different groups. The torque depends on the magnetization direction in a way similar to that found for a simple Rashba model. Artificially turning off the exchange spin splitting and separately the spin-orbit coupling potential in the Pt shows that the primary source of the "field-like" torque is a proximate spin-orbit effect on the Co layer induced by the strong spin-orbit coupling in the Pt.

cond-mat.mes-hall↗

arXiv2013

Spin-Wave Propagation in the Presence of Interfacial Dzyaloshinskii-Moriya Interaction

In ferromagnetic thin films, broken inversion symmetry and spin-orbit coupling give rise to interfacial Dzyaloshinskii-Moriya interactions. Analytic expressions for spin-wave properties show that the interfacial Dzyaloshinskii-Moriya interaction leads to non-reciprocal spin-wave propagation, i.e. different properties for spin waves propagating in opposite directions. In favorable situations, it can increase the spin-wave attenuation length. Comparing measured spin wave properties in ferromagnet$|$normal metal bilayers and other artificial layered structures with these calculations can provide a useful characterization of the interfacial Dzyaloshinskii-Moriya interactions.

cond-mat.mtrl-sci↗

arXiv2013

Self-consistent calculation of spin transport and magnetization dynamics

A spin-polarized current transfers its spin-angular momentum to a local magnetization, exciting current-induced magnetization dynamics. So far, most studies in this field have focused on the direct effect of spin transport on magnetization dynamics, but ignored the feedback from the magnetization dynamics to the spin transport and back to the magnetization dynamics. Although the feedback is usually weak, there are situations when it can play an important role in the dynamics. In such situations, self-consistent calculations of the magnetization dynamics and the spin transport can accurately describe the feedback. This review describes in detail the feedback mechanisms, and presents recent progress in self-consistent calculations of the coupled dynamics. We pay special attention to three representative examples, where the feedback generates non-local effective interactions for the magnetization. Possibly the most dramatic feedback example is the dynamic instability in magnetic nanopillars with a single magnetic layer. This instability does not occur without non-local feedback. We demonstrate that full self-consistent calculations generate simulation results in much better agreement with experiments than previous calculations that addressed the feedback effect approximately. The next example is for more typical spin valve nanopillars. Although the effect of feedback is less dramatic because even without feedback the current can induce magnetization oscillation, the feedback can still have important consequences. For instance, we show that the feedback can reduce the linewidth of oscillations, in agreement with experimental observations. Finally, we consider nonadiabatic electron transport in narrow domain walls. The non-local feedback in these systems leads to a significant renormalization of the effective nonadiabatic spin transfer torque.

cond-mat.mtrl-sci↗

arXiv2013

Current induced torques and interfacial spin-orbit coupling: Semiclassical Modeling

In bilayer nanowires consisting of a ferromagnetic layer and a non-magnetic layer with strong spin-orbit coupling, currents create torques on the magnetization beyond those found in simple ferromagnetic nanowires. The resulting magnetic dynamics appear to require torques that can be separated into two terms, damping-like and field-like. The damping-like torque is typically derived from models describing the bulk spin Hall effect and the spin transfer torque, and the field-like torque is typically derived from a Rashba model describing interfacial spin-orbit coupling. We derive a model based on the Boltzmann equation that unifies these approaches. We also consider an approximation to the Boltzmann equation, the drift-diffusion model, that qualitatively reproduces the behavior, but quantitatively fails to reproduce the results. We show that the Boltzmann equation with physically reasonable parameters can match the torques for any particular sample, but in some cases, it fails to describe the experimentally observed thickness dependences.

cond-mat.mtrl-sci↗

arXiv2011

Mechanism for puddle formation in graphene

When graphene is close to charge neutrality, its energy landscape is highly inhomogeneous, forming a sea of electron-like and hole-like puddles, which determine the properties of graphene at low carrier density. However, the details of the puddle formation have remained elusive. We demonstrate numerically that in sharp contrast to monolayer graphene, the normalized autocorrelation function for the puddle landscape in bilayer graphene depends only on the distance between the graphene and the source of the long-ranged impurity potential. By comparing with available experimental data, we find quantitative evidence for the implied differences in scanning tunneling microscopy measurements of electron and hole puddles for monolayer and bilayer graphene in nominally the same disorder potential.

cond-mat.mes-hall↗

arXiv2011

Landau Levels and Band Bending in Few-Layer Epitaxial Graphene

The carrier density distributions in few-layer-graphene systems grown on the carbon face of silicon carbide can be altered by the presence of a scanning tunneling microscope (STM) tip used to probe top-layer electronic properties, and by a perpendicular magnetic field which induces well-defined Landau levels. Hartree approximation calculations in the perpendicular field case show that charge tends to rearrange between the layers so that the filling factors of most layers are pinned at integer values. We use our analysis to provide insight into the role of buried layers in recent few-layer-graphene STM studies and discuss the limitations of our model.

cond-mat.mes-hall↗

arXiv2011

Semiclassical Boltzmann transport theory for graphene multilayers

We calculate the conductivity of arbitrarily stacked multilayer graphene sheets within a relaxation time approximation, considering both short-range and long-range impurities. We theoretically investigate the feasibility of identifying the stacking order of these multilayers using transport measurements. For relatively clean samples, the conductivities of the various stacking configurations depend on the carrier density as a power-law for over two decades. This dependence arises from a low density decomposition of the multilayer band structure into a sum of chiral Hamiltonians. For dirty samples, the simple power-law relationship no longer holds. Nonetheless, identification of the number of layers and stacking sequence is still possible by careful comparison of experimental data to the results presented here.

cond-mat.mes-hall↗

arXiv2011

Effects of disorder on magnetic vortex gyration

A vortex gyrating in a magnetic disk has two regimes of motion in the presence of disorder. At large gyration amplitudes, the vortex core moves quasi-freely through the disorder potential. As the amplitude decreases, the core can become pinned at a particular point in the potential and precess with a significantly increased frequency. In the pinned regime, the amplitude of the gyration decreases more rapidly than it does at larger precession amplitudes in the quasi-free regime. In part, this decreased decay time is due to an increase in the effective damping constant and in part due to geometric distortion of the vortex. A simple model with a single pinning potential illustrates these two contributions.

cond-mat.mes-hall↗

arXiv2010

Temperature dependence of the diffusive conductivity of bilayer graphene

Assuming diffusive carrier transport and employing an effective medium theory, we calculate the temperature dependence of bilayer graphene conductivity due to Fermi-surface broadening as a function of carrier density. We find that the temperature dependence of the conductivity depends strongly on the amount of disorder. In the regime relevant to most experiments, the conductivity is a function of T/T*, where T* is the characteristic temperature set by disorder. We demonstrate that experimental data taken from various groups collapse onto a theoretically predicted scaling function.

cond-mat.mes-hall↗

arXiv2010

Current-induced torques in the presence of spin-orbit coupling

In systems with strong spin-orbit coupling, the relationship between spin-transfer torque and the divergence of the spin current is generalized to a relation between spin transfer torques, total angular momentum current, and mechanical torques. In ferromagnetic semiconductors, where the spin-orbit coupling is large, these considerations modify the behavior of the spin transfer torques. One example is a persistent spin transfer torque in a spin valve: the spin transfer torque does not decay away from the interface, but approaches a constant value. A second example is a mechanical torque at single ferromagnetic-nonmagnetic interface.

cond-mat.mes-hall↗

arXiv2010

Effects of Disorder and Internal Dynamics on Vortex Wall Propagation

Experimental measurements of domain wall propagation are typically interpreted by comparison to reduced models that ignore both the effects of disorder and the internal dynamics of the domain wall structure. Using micromagnetic simulations, we study vortex wall propagation in magnetic nanowires induced by fields or currents in the presence of disorder. We show that the disorder leads to increases and decreases in the domain wall velocity depending on the conditions. These results can be understood in terms of an effective damping that increases as disorder increases. As a domain wall moves through disorder, internal degrees of freedom get excited, increasing the energy dissipation rate.

cond-mat.mes-hall↗

arXiv2009

Magnetic dynamics with spin transfer torques near the Curie temperature

We use atomistic stochastic Landau-Lifshitz-Slonczewski simulations to study the interaction between large thermal fluctuations and spin transfer torques in the magnetic layers of spin valves. At temperatures near the Curie temperature $T_{\rm C}$, spin currents measurably change the size of the magnetization (i.e. there is a {\it longitudinal} spin transfer effect). The change in magnetization of the free magnetic layer in a spin valve modifies the temperature dependence of the applied field-applied current phase diagram for temperatures near $T_{\rm C}$. These atomistic simulations can be accurately described by a Landau-Lifshitz-Bloch + Slonczewski equation, which is a thermally averaged mean field theory. Both the simulation and the mean field theory show that a longitudinal spin transfer effect can be a substantial fraction of the magnetization close to $T_{\rm C}$.

cond-mat.mes-hall↗

arXiv2009

Spin Transfer Torques

This tutorial article introduces the physics of spin transfer torques in magnetic devices. We provide an elementary discussion of the mechanism of spin transfer torque, and review the theoretical and experimental progress in this field. Our intention is to be accessible to beginning graduate students. This is the introductory paper for a cluster of "Current Perspectives" articles on spin transfer torques published in volume 320 of the Journal of Magnetism and Magnetic Materials. This article is meant to set the stage for the others which follow it in this cluster; they focus in more depth on particularly interesting aspects of spin-torque physics and highlight unanswered questions that might be productive topics for future research.

cond-mat.other↗

arXiv2008

Non-Adiabatic Spin Transfer Torque in Real Materials

The motion of simple domain walls and of more complex magnetic textures in the presence of a transport current is described by the Landau-Lifshitz-Slonczewski (LLS) equations. Predictions of the LLS equations depend sensitively on the ratio between the dimensionless material parameter $β$ which characterizes non-adiabatic spin-transfer torques and the Gilbert damping parameter $α$. This ratio has been variously estimated to be close to 0, close to 1, and large compared to 1. By identifying $β$ as the influence of a transport current on $α$, we derive a concise, explicit and relatively simple expression which relates $β$ to the band structure and Bloch state lifetimes of a magnetic metal. Using this expression we demonstrate that intrinsic spin-orbit interactions lead to intra-band contributions to $β$ which are often dominant and can be (i) estimated with some confidence and (ii) interpreted using the "breathing Fermi surface" model.

cond-mat.mtrl-sci↗

arXiv2007

Model for a collimated spin wave beam generated by a single layer, spin torque nanocontact

A model of spin torque induced magnetization dynamics based upon semi-classical spin diffusion theory for a single layer nanocontact is presented. The model incorporates effects due to the current induced Oersted field and predicts the generation of a variety of spatially dependent, coherent, precessional magnetic wave structures. Directionally controllable collimated spin wave beams, vortex spiral waves, and localized standing waves are found to be excited by the interplay of the Oersted field and the orientation of an applied field. These fields act as a spin wave ``corral'' around the nanocontact that controls the propagation of spin waves in certain directions.

cond-mat.mtrl-sci↗

arXiv2007

Ab initio studies of the spin-transfer torque in tunnel junctions

We calculate the spin-transfer torque in Fe/MgO/Fe tunnel junctions and compare the results to those for all-metallic junctions. We show that the spin-transfer torque is interfacial in the ferromagnetic layer to a greater degree than in all-metallic junctions. This result originates in the half metallic behavior of Fe for the $Δ_1$ states at the Brillouin zone center; in contrast to all-metallic structures, dephasing does not play an important role. We further show that it is possible to get a component of the torque that is out of the plane of the magnetizations and that is linear in the bias. However, observation of such a torque requires highly ideal samples. In samples with typical interfacial roughness, the torque is similar to that in all-metallic multilayers, although for different reasons.

cond-mat.mes-hall↗