Thermal characterization of Permalloy nanostripes deposited on thick SiO2

Abstract

The concept of the Race-Track memory has encouraged an immense number of studies on the movement and dynamics of magnetic domain walls (DW) displaced by an electric current. Due to the large current densities required to move or transform the DW, a correct and precise knowledge of the temperature in the nanowire is mandatory, especially when the current is introduced in short pulses [1-4]. Here we present a complete characterization of the thermal behaviour of nanowires with pulsed current excitation based on an experimental calibration on Permalloy nanostripes on a SiO2 substrate. The resistance of the nanostripe is monitored as the current pulse flows through the stripe, with the help of a high frequency oscilloscope. These resistance values are compared to a Resistance versus Temperature calibration performed on a stripe of the same dimensions. The results are simulated with the help of COMSOL and the parameters required to match the experimental results are discussed within the work. The introduction of a thermal interface term has been crucial to achieve a good agreement between the experimental results and the simulation [5], and its impact on the final predictions is evaluated. All the parameters allow to extract valuable information of the temperature in the notch and, specifically, of the temperature gradient generated along the device. Thermal gradients along the device may be important on magnetic materials where the movement of DWs follows the Arrhenius law. In substrates with small thermal conductivity like amorphous SiO2, the temperature increases dramatically in standard working conditions. For instance, for 400nm-SiO2, the temperature can reach 1000 K on a 10nm thick Permalloy for J=5•107 A/cm2, and it can be 40 % hotter in a notch, with depth one third of the width of the wire. Also, for sharp notches, the current density can increase up to 2 orders of magnitude higher in the edge. This could be detrimental for some studies but allows large in plane thermal gradients that may be useful for other studies in spin caloritronics. Besides, the influence of the width and depth of the triangular notch has been discussed as well, which may show a path to an optimal notch design for obtaining more resilient devices in pulse current excitation DW experiments.