The order in disorder: Magnetism in amorphous cobalt-based thin films and heterostructures

Abstract

The work in this thesis is focused on understanding the emergent properties of amorphous magnetic materials. Amorphous materials lack long-range structural ordering, characteristic of crystalline materials. Instead, these materials exhibit medium-to-short range order. In elemental form, metals always form crystals but can be amorphized by alloying with other metals of different atomic radii.

To study the magnetic properties of amorphous magnetic metals, we use CoAlZr and TbCo of various compositions, both in single layers and nanolaminates. The samples were fabricated using direct current magnetron sputtering in an ultra-high vacuum system. Structural characterization was done using x-ray reflectivity and grazing incidence x-ray diffraction. The magnetic properties were measured using magneto-optic Kerr effect, vibrating sample magnetometry, and polarized neutron reflectometry.

First, we show how the magnetic properties of the ferromagnetic Co are affected by diluting it with non-magnetic AlZr. We find that the critical temperature and magnetization of CoAlZr depends on composition, and decreases linearly with increasing AlZr content. Due to the disordered structure, there is a local distribution in Co concentration with regions of high and low Co density. We find a surprising manifestation of this chemical modulation, which emerges as competing anisotropies. By analyzing the anisotropy as a function of composition, we can define quantitatively a cobalt composition distribution on the nanoscale (Paper III). Similar to the CoAlZr, we also observe competing anisotropy in the TbCo alloy. We find, by analyzing the effective anisotropy as a function of composition and thickness, that there are two competing anisotropy terms in the TbCo layer: interface and bulk anisotropy. The interface favors in-plane magnetization while the bulk favors perpendicular magnetization (Paper IV).

In the second part of this study, we focus on hybrid structures composed of multilayers of alternating high- and low- \(T_\mathrm{c}\) CoAlZr and bilayers of TbCo and CoAlZr. In the CoAlZr multilayers, we find that within the low-\(T_\mathrm{c}\) layer there is a non-zero magnetization at three times its intrinsic ordering temperature which extends through at least 10~nm. This is due to the proximity to the ferromagnetic (high $T_\mathrm{c}$) layer (Paper I). In the TbCo/CoAlZr bilayers, we investigate the exchange coupling between two layers with crossed magnetic anisotropies. We find that a 7.5~nm interface layer of the CoAlZr is strongly exchange coupled to the TbCo with a magnetization perpendicular to the plane and switches in unison with the TbCo layer (Paper II). To resolve the depth dependence of the magnetization, these hybrid structures are then investigated further using polarized neutron reflectivity (Paper V).