Midway evaluation in Physics - Kristbjörg Anna Þórarinsdóttir

Title: Tuning the magnetic interlayer coupling through proximity induced magnetism in amorphous heterostructures.

Doctoral candidate: Kristbjörg Anna Þórarinsdóttir

Doctoral committee: Friðrik Magnus, Research Scientist at the Science Institute, University of Iceland
Snorri Ingvarsson, Professor at the Faculty of Physical Sciences at the University of Iceland
Björgvin Hjörvarsson, Professor, University of Uppsala

Abstract: The magnetic proximity effect refers to an induced magnetic ordering in an intrinsically non-magnetic material due to proximity to a magnetically ordered material. Typically, it is observed in composite or layered materials where a ferromagnet induces a magnetization in a paramagnet. By changing the composition in amorphous Co(x)AlZr(x-1), we can tune the Curie temperature Tc to be far below room temperature so that the layer is on the verge of ferromagnetism. Here, we show that in Co60(AlZr)40 / Co85(AlZr)15 multilayers with alternating high and low Tc there is a proximity induced magnetization in the low-Tc layers, far above their intrinsic Tc. This induced magnetic state has a very large spacial extension and survives up to three times the intrinsic ordering temperature of the low Tc layer.

We also investigate the magnetic properties of crossed anisotropy films by layering Tb12Co88 and Co85(AlZr)15, with perpendicular magnetic anisotropy (PMA) and in-plane magnetic anisotropy (IMA), respectively. We find that due to the strong interlayer exchange coupling between these two layers, we get a change in spin-texture in the CoAlZr layer. Fitting the hysteresis loops to a hyperbolic function, we are able to distinguish a transition region where the magnetic moments rotate from the film plane. This region, where magnetization is tilted from the plane, essentially behaves like an extension of the TbCo layer and acts as a spring magnet. Magnetic materials with tilted magnetic anisotropy are of high interest for example in magnetic recording systems, as they allow for larger areal density due do a decrease in switching field dispersion.