Role of stress/strain in tailoring the magnetic and transport properties of magnetic thin films and multilayers

TL;DR

This paper investigates how stress influences magnetic anisotropy in polycrystalline thin films and multilayers, demonstrating real-time stress dependence and methods to tune magnetic properties for device applications.

Contribution

It provides direct in-situ evidence of stress effects on magnetic anisotropy and introduces a new multilayer fabrication technique to enhance and control magnetic properties.

Findings

Stress affects magnetic anisotropy in thin films.

External stress can tune the strength and direction of magnetic anisotropy.

A new multilayer deposition method enhances magnetic anisotropy.

Abstract

Magnetic anisotropy is a fundamental property of magnetic materials that determines the alignment of the spins along the preferential direction, called the easy axis of magnetization. Magnetic polycrystalline thin films offer several advantages over magnetic epitaxial thin films because of fabrication flexibility, higher coercivity and improved magnetic stability, higher magnetoresistance (useful in magneto-resistive devices such as magnetic field sensors and MRAM cells), cost-effectiveness and thermal stability, etc. In the case of polycrystalline thin films or multilayers, Magneto-crystalline anisotropy (MCA) is not expected due to the random orientation of grains. Therefore, understanding the origin of uniaxial magnetic anisotropy (UMA) is generally difficult and can't be understood in terms of crystal orientation. The origin of UMA in polycrystalline films is often related to the preparation conditions and substrate properties. In the present thesis, we have provided direct in-situ real-time evidence of the stress dependence of magnetic anisotropy through the multibeam optical stress sensor (MOSS) technique. Also, we have tuned the magnetic anisotropy in strength and direction using externally applied stress. To further increase the strength of the magnetic anisotropy, we have developed a new technique that creates a multilayer using a single magnetic material through sequential oblique and normal depositions. This oblique angle deposition technique also helps reduce the penetration of the top ferromagnetic layer inside the organic semiconductor layer in organic spin valve structures. We confirm our results through various in-situ (in UHV and HV) and ex-situ temperature-dependent conventional and unconventional structural, morphological, and magnetic measurements (both lab-based and synchrotron-based) that include MOKE, KERR, GIXRD, AFM, RHEED, and GISAXS, etc. measurements.