Determination of the spin torque non-adiabaticity in perpendicularly magnetized nanowires

2012
Journal paper

Abstract

Novel nanofabrication methods and the discovery of an efficient manipulation of local magnetization based on spin polarized currents has generated a tremendous interest in the field of spintronics. The search for materials allowing for fast domain wall dynamics requires fundamental research into the effects involved (Oersted fields, adiabatic and non-adiabatic spin torque, Joule heating) and possibilities for a quantitative comparison. Theoretical descriptions reveal a material and geometry dependence of the non-adiabaticity factor beta, which governs the domain wall velocity. Here, we present two independent approaches for determining beta : (i) measuring the dependence of the dwell times for which a domain wall stays in a metastable pinning state on the injected current and (ii) the current-field equivalence approach. The comparison of the deduced beta values highlights the problems of using one-dimensional models to describe two-dimensional dynamics and allows us to ascertain the reliability, robustness and limits of the approaches used.

Official source

https://infoscience.epfl.ch/record/174511?ln=en

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Related publications

Current-induced domain wall motion in Co/Pt nanowires: Separating spin torque and Oersted-field effects

We report on low temperature current induced domain wall depinning experiments on (Co/Pt) multilayer nanowires with perpendicular magnetization. Using a special experimental scheme, we are able to extract the different contributions of the Oersted field and spin torque from the dependence of the depinning field on the injected current for selected magnetization configurations. The spin torque contribution is found to be dominant with a small contribution of the Oersted field leading to a nonadiabaticity factor beta in line with previous measurements. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3405712]

2010

The aim of this thesis work was to obtain a deeper understanding of spin-dependent transport by the study of mixed effects of heat and charge currents in magnetic nanostructures. Indeed, even thermodynamics establish a relationship between both currents, no previous works were performed in this sense up to now. In order to do this, an original experimental setup was developed which allows to carry out a novel transport measurement called the thermogalvanic voltage (TGV). It measures the AC voltage response of the sample driven by an oscillatory variation of the temperature induced by a chopped laser beam focused on the nanostructure under a direct current flow. The main advantage of this new experimental setup is that it can be used to perform three different transport measurements: resistance, thermoelectrical power and TGV measurements. Furthermore, the well-defined junctions of our magnetic nanowires, in addition to the lock-in detection of signals, allows to obtain a better quality of thermoelectrical measurements than those observed by other groups. Different kind of magnetic nanowires like ferromagnetic/non-magnetic multilayers and ferromagnetic homogeneous nanowires were studied by these three different transport measurements. All these nanowires were 6μm in length and 50nm in diameter. All TGV measurements reveal a linear dependence with currents. This behaviour can be interpreted in terms of adiabatic conductivity for homogeneous nanowires and in terms of Peltier effect for multilayer nanowires produced in ferromagnetic/non-magnetic interfaces. Furthermore, the magnetic field dependence of TGV (MTGV) cannot be accounted for by GMR and MTEP effects. Therefore, a novel spin-dependent mechanism is invoked in such measurements. A simple phenomenological model, ascribed in the framework of thermodynamics of irreversible processes, was developed to explain this effect. It allows to identify this phenomena to a novel spin-dependent variable: The asymmetry of spin-mixing mechanisms (ΔL). This variable will be related to the difference of transition rates between both spin channels (τ-1+- and τ-1-+) developed in ferromagnetic material due to electron-magnon interactions. A large magnetic field dependence of TGV signals were observed in magnetic granular systems. Moreover, MTGV shows a different dependence from GMR on magnetic field, grain size and temperature. This effect is identified as a spin-mixing mechanism produced by the electron spin precession about the magnetic grain moments.

EPFL2006

Spin caloritronics, i.e., the addition of thermal effects to the electrical and magnetic properties of nanostructures, has recently seen a rapid development. It has been predicted that a heat current can exert a spin torque on the magnetization in a nanostructure, analogous to the well-known spin-transfer torque induced by an electrical current. In this thesis, I provide the experimental evidence for this effect in spin valves, showing the switching field change with heat current. I present measurements of the second harmonic voltage response of Co-Cu-Co pseudo-spin-valves deposited in the middle of Cu nanowires. I exploit the quasi-1D nature of the nanostructures to generate a heat current by way of asymmetric Joule heating in the Co layers. Both the magnitude of the second harmonic response of the spin valve and the field value of the maximum response are found to be dependent on the heat current. Both effects show that the magnetization dynamics of the pseudo-spin-valves is influenced by the heat current. Thus, the data provide a quantitative estimate of the thermal spin torque exerted on the magnetization of the Co layers. In the last chapter, I measured the second harmonic response to an oscillating current crossing an exchange-biased spin valve as a function of the angle between the magnetization vectors of the fixed and free layers. This signal characterizes the linear response of the magnetization to a torque. A diffusive model is shown to predict that this angular dependence is quite sensitive to the value of the transverse spin diffusion length λJ , i.e. the decay length of the spin polarization which is perpendicular to the magnetization. Our observations imply a value of about 6 nm for CoFe.

EPFL2011

EPFL2006