Exchange bias in patterned nanostructures

Abstract

Exchange bias between a ferromagnet (FM) and antiferromagnet (AFM), which is utilized to pin the magnetization of a FM into a fixed direction in space, is essential in commonly used electronic components such as magnetic recording heads and magnetic memory cells, as well as novel magnetic logic and memory devices. However, the exchange bias effect has been optimized in materials and used in devices for decades without a good scientific understanding, both due to lack of nanoscale research and conflicted results from differences in fabrication and feature size. In this thesis, we present a special fabrication method that produces exchange bias reliably and consistently. We also show the results of both experimental and simulated investigation of the properties of exchange biased nanostructures such as domain formation, magnetostatic interactions, and response to field-driven switching. -A fabrication method for creating locally exchange biased nanostructures is first developed. By etching back a predeposited FM film, and regrowing a thin FM layer and then the AFM film, this hybrid method combines the benefits of a clean interface produced using subtractive methods and the scalability produced using additive methods. Its consistency is analyzed through vibrating sample magnetometry (VSM) and scanning electron microscopy (SEM). Next, the fabrication method is applied to an array of nanodots with varying ion beam etch durations and dot diameters, demonstrating a reduced exchange bias for small diameters, and no significant change in exchange bias unless the ion beam etch duration exceeded 30s. Based on the consistency of this method, new device-like patterns were fabricated both experimentally and by modeling, in which a grating of AFM stripes was exchange biased with a continuous FM film. Competing magnetic interactions were found in the modeling, and produced extraordinary hysteresis loop shapes in the experimental samples. Next, a grating of AFM stripes was exchange biased with a 900 offset grating of FM stripes using the same fabrication method, which simulates an array of individual magnetic devices. A different set of competing magnetic interactions was found, and the feature sizes of the FM and AFM components were demonstrated to tune these interactions and thus the switching behavior of such devices. Exchange bias of materials with perpendicular magnetic anisotropy (PMA) was attempted by exchange coupling a PMA FM material with an in-plane FM material, which in turn exchange couples with the AFM material. However, the magnitude of the exchange bias was found to be negligible when compared to the coercivity of the PMA material.