Field-free magnetization reversal by spin Hall effect and exchange bias

Abstract

Recently, is has been discovered that the magnetization of an ultra-thin out-of-plane ferromagnetic layer can be switched by the spin Hall effect (SHE). This spin Hall switching is a major contender for the write mechanism in novel magnetic memory devices such as MRAM (Magnetic Random Acces Memory). Unfortunately, an externally applied magnetic field is necessary to cause deterministic magnetization reversal by the SHE. Such a field is not suitable for implementation in actual devices.In this thesis, the external field is replaced by an effective magnetic field originating from the exchange bias effect between a ferromagnetic and an anti-ferromagnetic material. By creating an Pt/Co/IrMn trilayer in which the ferromagnetic Co layer is magnetized out-of-plane and the anti-ferromagnetic IrMn layer is magnetized in-plane, a novel exchange bias configuration, called orthogonal exchange bias, is successfully created. By investigating the thickness dependence of each layer, thermal stability and magnetic configurations, a thorough qualitative understanding of this system is achieved.By nanostructuring these orthogonal exchange biased samples, field-free deterministic magnetization reversal is observed via the SHE; a breakthrough in the field of MRAM research. The external field is successfully replaced by the intrinsic field from the exchange bias. The magnetization reversal showed characteristic features that appear to be intrinsic to the orthogonal exchange bias configuration. These features, most notably the gradual and partial switching of the magnetization, can be explained by the polycrystalline structure of the IrMn layer and current shunting related to the sample geometry. It is discovered that the effective exchange bias field can be compensated by applying a 5 mT external field. This compensation field is an order of magnitude smaller than the actual exchange bias that is found in full sheet samples. Whether this discrepancy is due to the nanostructuring of the sample or thermal effects on the exchange bias remains open for discussion. Still, this demonstration of field-free magnetization reversal by SHE and orthogonal exchange bias greatly increases the feasibility of future SHE-based MRAM devices.

Date of Award	29 Feb 2016
Original language	English
Supervisor	Henk J.M. Swagten (Supervisor 1) & A. van den Brink (Supervisor 2)