Graduation Year

2025

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Sarath Witanachchi, Ph.D.

Co-Major Professor

Manh-Huong Phan, Ph.D.

Committee Member

Dario Arena, Ph.D.

Committee Member

Ioannis Spanopoulos, Ph.D.

Keywords

Ferromagnetism, ferroelectricity, spintronics, neuromorphic computing, energy harvesting

Abstract

There is an ever-growing need for larger computing power, nonvolatile memory and energy efficient devices as the number of applications that draw upon the current base of computer resources increases. At the forefront of research aiming to meet this need is the field of multiferroics. If it is possible to leverage the fast and energy efficient application of an electric field with the data storage capabilities of magnetic materials, then it would be possible to not only increase the robustness and energy efficiency of current memory, but possibly present new types of device components for computation application as well. Currently, there is not yet a material that satisfies the necessary requirements that would make this a reality. However, there are multiple research avenues that are being explored to satiate growing energy needs with multiferroic devices. One path towards the effective utilization of multiferroic materials has been not the direct control of magnetization via polarization state, but by control of the chirality of the magnetization vector as seen in bismuth ferrite (BFO) magneto electric spin orbit devices (MESO) [1]. BFO however is difficult to use in application because of its low magnetization and insulating nature. Another path towards electric field control of magnetism is the control of topological magnetic states such as antivortices and skyrmions with an electric field [2]. However, these states are hard to produce and unstable [2-7].

One particularly exciting material under study today is that of ε-Fe2O3, a multiferroic material that exhibits both magnetism and ferroelectricity well above room temperature [8]. Although theoretically determined to be classified as a charge transfer insulator, it possesses a moderate band gap of 1.6-1.9eV, making it a candidate for integration into devices as a semiconducting layer. Traditionally, conduction on this scale would make a harmful impact on a ferroelectric materials ability to hold a polarization state, but ε-Fe2O3 has been shown to hold its polarization for long periods of time [8]. It is also characterized as a hard ferrimagnet, boasting coercive fields up to 2 T, with a moderate magnetic remanence of ~40 emu/cm3. It has been synthesized into various forms, such as nanoparticles, micron sized crystals, and thin films. In thin film format, it has a reduced coercivity that has been attributed to the triaxial alignment of the easy axes within the material. While exciting research has been done on this system in the thin film regime, there is still a need for a detailed analysis of the effect of the growth parameters on various aspects of the resulting thin film.

In this work, thin films of ε-Fe2O3 were grown under various conditions and were characterized structurally, magnetically, and ferroelectrically using a suite of measurement techniques. These include atomic force microscopy (AFM), magnetic force microscopy (MFM), piezo force microscopy (PFM), in and out of plane X-ray diffraction (XRD), vibrating sample magnetometry (VSM), and 4-point resistivity measurements. We identify several key observations on growth conditions that lead to enhanced surface and magnetic properties of resultant thin films, allowing detailed surface magnetic characterization. By tuning the energy of the growth process, one can balance strain, crystallinity, phase purity, and magnetic coercivity within desired amounts. Through detailed magnetic measurements, such as first-order reversal curves (FORC), we identify that this system possesses large interaction and coercive field distributions. We identify these features as resulting from intergrain interactions and a grain size distribution through the assistance of micromagnetic calculations.

Through extensive micromagnetic calculations, a predictive model was developed to explain the magnetic behavior of films grown on substrates promoting triaxial crystallite domain growth, such as STO. We show a dependence of magnetic domain size and coercivity on grain size and anisotropy constant and use this to explain the experimentally obtained MFM images. These calculations show the existence of topological magnetic states being stabilized on ε-Fe2O3, the first time shown in thin films of the material, and identify these features in the experimentally obtained MFM images.

Lastly, we apply the obtained model to our thin film system utilizing the measured anisotropy constant and grain size distribution, and obtain a reasonable match for the magnetic field dependent characteristics of our sample. This demonstrates the utility of our predictive model, which is able to account for changes in anisotropy constant as well, such as those resulting from doping and strain.

This work advances the fundamental understanding of multiferroic ε-Fe2O3 thin films and widens their possible uses towards technological applications. We also provided a predictive model for future studies. Additionally, we demonstrate the creation of stable topological magnetic states such as vortices and antivortices in this system, opening up the possibility for future study on this interesting phenomenon in this material.

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