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

Heisenberg Exchange Coupling (HC) and Biquadratic Exchange Coupling (BQC) are present in magnetic tunnel junctions(MTJ) and nanoscale elements-based spintronics¹. MTJ-based molecular spintronics (MTJMSD) offer an unprecedented new way to create devices and highly correlated physics phenomena. Molecular spin channels on the exposed edge of MTJ were found to produce strong exchange coupling experimentally and theoretically, and the nature of coupling was believed to be HC². BQC – which leads to the perpendicular alignment of the spin vectors of the adjacent FM electrodes – also occurs via the insulator or molecular nanostructures. Little is known about the competing effects between these two types of interlayer exchange couplings on MTJMSDs. A systematic study was performed using Monte Carlo simulations (MCS) based on a 3D Heisenberg model. The BQC strength was varied for a device with no molecular HC as well as strong parallel and antiparallel molecular HC coupling. The physical and magnetic properties of the MTJMSD were examined. It was found that increasing BQC strength in an MTJMSD with strong parallel and antiparallel molecular HC had little effect on overall device magnetization as HC still ultimately dominated the device magnetization. The effect of BQC on the device’s magnetic equilibrium state was also examined through its temporal evolution. Results suggest that when only molecular BQC is present within the device, the device is unable to reach magnetic stability; on the other hand, when HC and BQC are present within the device, HC encourages the device to achieve greater stability. The results ultimately indicate BQC plays a lesser role in overall device magnetization dynamics as it cannot overcome the more substantial magnetization effect produced by the HC. The presence of BQC provides a plausible explanation for the experimentally observed difference in magnetic phase orientations other than parallel and antiparallel states around MTJMSD.