Measuring Entropy Generated by Spin-Transfer

We present an experimental protocol that allows the entropy generated by spin-transfer to be measured. We investigate the effect of strong spin injection on a ferromagnetic nanostructure, focusing on the quasi-static equilibrium states of a single magnetic domain. We measure the response of the magnetoresistance to a temperature excitation as a function of the magnetic field for different values of the injected current. This quantity is related to the thermal susceptibility of the ferromagnetic wire through the anisotropic magnetoresistance. We deduce the ferromagnetic entropy generated by spin injection by a thermodynamic Maxwell relation. We applied this protocol to single contacted Ni nanowires obtained by electrodeposition in a nanoporous template. We observe that the entropy produced by the spin injection is a linear function of the current and reaches 60% of the initial entropy of the system for 5 x 10(7) A/cm(2). This study shows that the effect of the spin-transfer in these samples results in the generation of incoherent excitations instead of rotation of the magnetization.