This dissertation is focused on ferromagnetic, multiferroics, and two-dimensional (2D) perovskites, exploring different unique collective magnetic and ferroic characters: (1) ferromagnetic thin film Co/Pd multilayers, (2) BaTiO3-BiFeO3 (BTO-BFO) a magneto-electric materials system, and (3) CuCl4 halide organic-inorganic perovskites. Low-power all-optical memory offers a unique opportunity to achieve ultra-fast magnetic switching in which the switching dynamics are not thermally mediated and occur on the order of the laser pulse.
However, it is challenging to achieve a low-power optically excited magnetization precession angle above 90 degrees, which is required for magnetic switching. Co/Pd thin film multilayers were investigated for their potentially large perpendicular magnetic anisotropy (PMA) with three differing regimes of magnetic anisotropy: in-plane, weakly out-of-plane, and out-of-plane. Utilizing the time-resolved magneto-optical Kerr effect (TR-MOKE), we observed clear magnetic precession (on the order of a few GHz) with magnetic precession angle increasing (up to 4.5 degrees) for thinner Co samples which demonstrated stronger PMA. We observed a clear connection between PMA strength and precession amplitude as well as a large efficiency of energy transfer between spin and orbital subsystems for our strongest PMA sample. BTO-BFO is a strong room-temperature multiferroic with enhanced magneto-electric properties compared to BFO. We utilized time-resolved differential reflectivity (TR-DR) and TR-MOKE to observe strong coherent acoustic phonons in thin films as well as nanorods. Our nanorods showed additional modes (a new 20 GHz and 6 GHz mode) not observed in thin films including the fast 33 GHz mode which showed some weak tunability with high magnetic fields (up to 10 T). The observed tunability of these modes in an external magnetic field shows interesting coupling between magnetic moment and phononic modes which may be caused by the breaking of the spin-cycloid at the interface of the nanorods and the surface of the nanorods. We also observed second harmonic generation (SHG) emission which demonstrated a large enhancement in our nanorod structures with further observation of wavelength dependence. Finally, ferromagnetic resonance on our nanorod and thin film BTO-BFO structures indicated very weak Gilbert damping (on the order of 10−3), demonstrating the practicality of our structure for low-spin loss applications. Lastly, this dissertation focuses on a project around CuCl4 and CuCl2Br2 perovskites in which we observed time-dependent SHG. An increase in SHG as a function of infrared laser exposure is shown to coincide with changes in the crystal structure of the Cu perovskite materials. This increase in SHG was shown to last for a few days after hours of laser exposure indicating a slow hysteretic change to the crystal structure of the perovskites. / Doctor of Philosophy / Multifunctionality in materials is important for various applications including future mem- ory devices where ferromagnetism (collective magnetic order), ferroelectricity (collective electric polarization order), and piezoelectricity (collective strain order) can be implemented in a given device. This dissertation centers on three material systems for exploring ferroic orders: Co/Pd thin multilayers, BaTiO3-BiFeO3 (BTO-BFO) films and nano-rod arrays, and Cu halide organic-inorganic perovskite thin films and 2D structures. Co/Pd thin films demonstrate interesting ferromagnetic order with magnetic anisotropy in which the magnetization of the thin film has a preferred direction based on the thickness of the thin film.
BTO-BFO demonstrates coupling between ferroelectric and antiferromagnetic order. The magnetic information may be controlled by applying electric fields or strain and Cu halide perovskites demonstrate potentially created ferroelectric order under long-term laser expo- sure with high ferroelectric switching speeds. Dynamics and nonlinear optical responses in these materials systems were explored with Ti:Sapphire pulsed lasers (∼ 100 fs). Our techniques allowed us for a better understanding of fast carrier and spin dynamics after optical excitation. Furthermore, nonlinear optics, in which two or more photons can be used to emit higher energy photons, were employed to explore the ferroelectric properties within these material systems. The results presented in this dissertation provided information on collective orders and fundamental interactions in several less-explored material systems.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/116736 |
| Date | 04 December 2023 |
| Creators | Smith, Nicholas William |
| Contributors | Physics, Khodaparast, Giti, Nguyen, Vinh, Barnes, Edwin Fleming, Zhou, Wei |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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