Thermal Spin Transfer Torque and Transverse Spin Relaxation in Spin Valves

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.

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