Bachelor project: Back-hopping

Back-hopping in MgO-based magnetic tunnel junctions for spin-torque magnetic random access memory devices

A spin-polarized current flowing through a ferromagnet exerts a torque on the magnetization at the nanoscale, thereby providing means of manipulating it. In a nano-size magnet, spin-transfer torques can induce either magnetization reversal or steady-state precession. These phenomena have been proposed as write method for non-volatile magnetic memory devices and operating mechanism for tuneable radio-frequency nano-oscillators, respectively. Given their good scaling perspectives, spin-torque devices have recently been identified as one of the prime candidates for beyond Moore technologies. In particular, spin-torque devices based on MgO-based magnetic tunnel junctions show great promise for applications, although fundamental questions remain to be answered.

In particular, it has been demonstrated that over specific bias ranges, the applied voltage induces a telegraph-noise-like behavior known as ‘back-hopping’, whose origin remains to-date under debate. This phenomenon has not been reported in fully-metallic nanopillars, and represents a considerable draw-back for spin-torque-based magnetic memory devices, as it causes the switching to be unreliable.

The focus of this study will be to carry out a systematic study aiming at identifying and separating two types of contributions to the observed telegraph-noise-like behavior: thermally activated reversal (which is known to affect various types of nanostructures) versus the dependence of the spin-torque on the applied bias voltage (which is intrinsic to MgO-based magnetic tunnel junctions).

To this end, a sub-nanosecond voltage pulse generator and a 20 GHz single-shot oscilloscope will be utilized to characterize real-time switching in Fe/MgO/Fe devices with a lateral size of the order of 100 nm. Experiments will be performed at room temperature, as well as at 4.2 K.

Masters Physics students are invited to apply.

Publications:

1. “Phase diagrams of MgO magnetic tunnel junctions including the perpendicular spin-transfer torque in different geometries”, K Bernert, V Sluka, C Fowley., J Lindner, J Fassbender, AM Deac, Physical Review B, 89, 134415 (2014).