Competing Effect of Biquadratic and Heisenberg Coupling on Magnetic Tunnel Junction Molecular Spintronics Devices

TL;DR

This study investigates how biquadratic and Heisenberg exchange couplings influence magnetic behaviors in molecular spintronics devices, revealing that Heisenberg coupling dominates and stabilizes magnetic states.

Contribution

The paper provides a systematic analysis of the competing effects of BQC and HC in MTJMSDs using Monte Carlo simulations, highlighting HC's primary stabilizing role.

Findings

HC dominates device magnetization even with increased BQC

Devices with only BQC fail to reach magnetic stability

BQC explains complex magnetic phase orientations observed experimentally

Abstract

Heisenberg exchange coupling (HC) and biquadratic exchange coupling (BQC) are known to occur in magnetic tunnel junctions (MTJ) and nanoscale spintronics structures. MTJ-based molecular spintronics devices (MTJMSD) provide a platform to study these interactions and the correlated magnetic behavior they generate. Molecular spin channels formed along the exposed MTJ edge have been shown to produce strong exchange interactions that include HC and BQC, which can drive perpendicular alignment of spin vectors in adjacent ferromagnetic electrodes. Despite their importance, the competing roles of HC and BQC in MTJMSDs remain unclear. Monte Carlo simulations using a three-dimensional Heisenberg model were performed to systematically vary BQC strength under three conditions: no molecular HC, strong parallel HC, and strong antiparallel HC. The resulting magnetic and physical properties of the MTJMSDs were analyzed. Increasing BQC strength produced minimal changes in overall magnetization when strong HC was present, indicating that HC dominates device magnetization. Temporal evolution studies showed that devices with only BQC failed to reach magnetic stability, while devices with both HC and BQC achieved stable magnetic states due to the stabilizing influence of HC. These results show that BQC plays a secondary role in magnetization dynamics and cannot overcome the stronger stabilizing effect of HC. The presence of BQC offers a plausible explanation for experimentally observed magnetic phase orientations beyond simple parallel and antiparallel states in MTJMSDs.