Disorder- and strain-mediated magnetic phase modifications in FeRh thin films

Abstract

Systems that undergo metamagnetic phase transitions are of great academic interest, both from a fundamental science perspective and for their potential to be implemented in devices across a broad spectrum of technologies, such as in spintronics, data storage, logic, and magnetocalorics. One such system is equiatomic FeRh, which features an antiferromagnetic to ferromagnetic phase transition at ~100 degrees Celcius. The characteristics of this phase transition are extremely sensitive to a range of conditions such as strain state, composition, and applied magnetic field. Thus, there are multiple avenues through which the spatial distribution of magnetic phases can be tailored to suit a particular function. This thesis describes work to fabricate and characterise thin film structures incorporating FeRh in which the magnetic behaviour is controllably modulated, either through atomic disordering, exchange coupling, or magnetoelectric coupling. For each system studied, polarised neutron reflectometry (PNR) was used to determine the evolution of the magnetisation as a function of depth through the thin films. This work demonstrates that by irradiating FeRh thin films with Ne+ ions of varying fluence, it is possible to generate interfaces between regions of different magnetic phase distributions, allowing simple magnetic bilayers or a uniform magnetisation profile to be engineered. Here, PNR data are combined with positron annihilation lifetime spectroscopy and conversion electron Mossbauer spectroscopy to elucidate a correlation between the magnetic modifications and the induced disorder. The interlayer exchange coupling in an FePt/FeRh bilayer was probed using PNR over a range of temperatures and applied magnetic fields. In this system, the FePt coercivity is reduced when the FeRh is heated through the magnetic phase transition via an exchange spring mechanism. A characteristic length scale over which this mechanism is effective is determined. Finally, strain-mediated magnetoelectric coupling in a PMN-PT/FeRh artificial multiferroic structure was investigated. The relationship between the complex ferroelectric properties of the PMN-PT substrate and the resulting magnetoelectric effect is explored using a combination of magnetometry, X-ray scattering, and PNR measurements with in situ applied electric fields. These results demonstrate that magnetoelectric coupling can be observed, and highlight the challenges inherent to optimising the interfacial coupling in this system.

Date of Award	31 Dec 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Thomas Thomson (Supervisor)