Thermal Relaxation Rates of Magnetic Nanoparticles in the Presence of Magnetic Fields and Spin-Transfer Effects

TL;DR

This study measures the thermal relaxation times of magnetic nanoparticles in tunnel junctions under various fields, voltages, and temperatures, revealing how spin torques influence energy barriers and fluctuations, with implications for magnetic memory design.

Contribution

It introduces a direct measurement method for relaxation dynamics in magnetic tunnel junctions, accounting for spin-transfer effects without complex heating models.

Findings

Effective temperature linearly depends on voltage due to in-plane torque.

Field-like torque contributes quadratically with voltage.

Method enables straightforward parameter determination in MTJ devices.

Abstract

We have measured the relaxation time of a thermally unstable ferromagnetic nanoparticle incorporated into a magnetic tunnel junction (MTJ) as a function of applied magnetic field, voltage V (-0.38 V < V < +0.26 V), and temperatures (283 K< T< 363 K) . By analyzing the results within the framework of a modified N\'eel-Brown formalism we determine the effective attempt time of the nanoparticle and also the bias dependences of the in-plane and out-of-plane spin torques. There is a significant linear modification of the effective temperature with voltage due to the in-plane torque and a significant contribution of a "field like" torque that is quadratic with voltage. The methods presented here do not require complicated models for device heating or calibration procedures, but instead directly measure how temperature, field, and voltage influence the energy landscape and thermal fluctuations of a two-state system. These results should have significant implications for designs of future nanometer-scale magnetic random access memory elements and provide a straightforward methodology to determine these parameters in other MTJ device structures.