Spin-wave thermal population as temperature probe in Magnetic Tunnel Junctions

TL;DR

This paper presents a method to measure the absolute temperature of Magnetic Tunnel Junctions using high-frequency electrical noise spectra, enabling precise thermal monitoring during spin-torque operations.

Contribution

The study introduces an analytical approach leveraging spin wave noise spectra from reference layers to accurately determine MTJ temperature under bias.

Findings

Temperature can be measured with ±60 K accuracy.

No significant heating occurs at typical spin-torque switching voltages.

The method isolates reference layer fluctuations unaffected by spin-torque.

Abstract

We study whether a direct measurement of the absolute temperature of a Magnetic Tunnel Junction (MTJ) can be performed using the high frequency electrical noise that it delivers under a finite voltage bias. Our method includes quasi-static hysteresis loop measurements of the MTJ, together with the field-dependence of its spin wave noise spectra. We rely on an analytical modeling of the spectra by assuming independent fluctuations of the different sub-systems of the tunnel junction that are described as macrospin fluctuators. We illustrate our method on perpendicularly magnetized MgO-based MTJs patterned in 50*100 nm2 nanopillars. We apply hard axis (in-plane) fields to let the magnetic thermal fluctuations yield finite conductance fluctuations of the MTJ. Instead of the free layer fluctuations that are observed to be affected by both spin-torque and temperature, we use the magnetization fluctuations of the sole reference layers. Their much stronger anisotropy and their much heavier damping render them essentially immune to spin-torque. We illustrate our method by determining current-induced heating of the perpendicularly magnetized tunnel junction at voltages similar to those used in spin-torque memory applications. The absolute temperature can be deduced with a precision of +/- 60 K and we can exclude any substantial heating at the spin-torque switching voltage.