Doctor of Philosophy (PhD), Ohio University, 2015, Electrical Engineering & Computer Science (Engineering and Technology)

The field of spintronics is considered as the next generation of spin-based electronics rather than the flow of charges utilized in electronics. It is expected that it will have some advantages in areas of information storage densities, switching speed, power consumption, manufacturing costs and others. One of the alternatives in developing a successful spintronics materials is the transition metal (TM)-doped III-V diluted magnetic semiconductors (DMSs) and GaMnAs is the proto-type ferromagnetic DMSs. Currently, the origin of ferromagnetism in GaMnAs is not fully clarified yet due to the complexity of an electronic band structure after doping of the Mn into GaAs. However, the magneto-optical characterization, especially, magnetic circular dichroism (MCD), is a very powerful technique to investigate DMS because one can obtain the information of the electronic band structure. Thus, we have performed systematic investigations of the MCD spectra and optical absorption spectra of the Ga1-xMnxAs with different concentrations of Mn. In this project, we have conducted the measurement using the transmission-mode MCD, the reflection-mode MCD and the magneto-optical Kerr effect (MOKE) for three different kinds of GaMnAs samples fabricated with the same growth conditions; GaMnAs on sapphire, GaMnAs on InP, and free-standing GaMnAs, respectively. We have successfully estimated the Zeeman splitting energy of both L (E1 and E1+delta1) and G (E0 and E0+delta0) critical points (CPs) for these materials. We utilized an energy derivative of the Gaussian function to decompose the MCD spectrum into the impurity band (IB) related background and two dispersion components around L-CPs which are expected in theory. Then, using the rigid band shift model we calculated the Zeeman splitting energy of E1 (L-CP). The Zeeman splitting energy at E1 (L-CP) was estimated to be larger than ~ 4 meV in Ga0.97Mn0.03As on sapphire, ~ 0.6 meV in Ga0.97Mn0.03As on InP, and ~ 6.5 meV in free-standing Ga0.9 (open full item for complete abstract)

Committee: Wojciech Jadwisienczak (Advisor); Savas Kaya (Committee Member); Avinash Kodi (Committee Member); Faiz Rahman (Committee Member); Martin Kordesch (Committee Member); Arthur Smith (Committee Member) Subjects: Chemical Engineering; Engineering; Materials Science; Optics; Physics

Doctor of Philosophy, The Ohio State University, 2015, Physics

Double perovskites have proven to be highly interesting materials, particularly in the past two decades, with many materials in this family exhibiting strong correlations. These materials are some of many novel complex oxides with potential spintronics application. Sr2CrReO6, in particular, is a double perovskite with one of the highest Curie temperatures of its class (> 620 K in bulk and ∼510-600 K in thin films), as well as high spin polarization, ferrimagnetic behavior, and semiconducting properties. This dissertation covers recent work in exploring and tuning physical properties in epitaxial films of Sr2CrReO6. It starts by providing a background for the field of spintronics and double perovskites, bulk and thin film synthesis of Sr2CrReO6, and standard and specialized characterization techniques utilized in both university and national laboratories, and then provides reports of work on Sr2CrReO6 epitaxial films. Examples of exploration and engineering of properties of Sr2CrReO6 include: (1) tuning of electrical resistivity, such as at T = 7 K by a factor of 18,000%, via control of oxygen partial pressure during film growth; (2) enhancement of interfacial double perovskite ordering, demonstrated with high-angle annular dark-field scanning transmission electron microscopy, via the use of double perovskite buffer layer substrates; (3) measurement of magnetization suppression near film/substrate interfaces via polarized neutron reflectometry, which reveals a reduction of thickness (from 5.6 nm to 3.6 nm) of the magnetically suppressed interface region due to buffer layer enhancement; (4) strain tunability of atomic spin and orbital moments of Cr, Re, and O atoms probed with x-ray magnetic circular dichroism, which demonstrates ferrimagnetic behavior and reveals important magnetic contributions of the oxygen sites (∼0.02 µB/site); (5) strain tunability of large magnetocrystalline anisotropy via applied epitaxial strain, revealing anisotropy fields of up to 10s of te (open full item for complete abstract)

Committee: Fengyuan Yang (Advisor); P. Chris Hammel (Committee Member); Ciriyam Jayaprakash (Committee Member); Richard Hughes (Committee Member) Subjects: Condensed Matter Physics

PhD, University of Cincinnati, 2006, Arts and Sciences : Physics

We study the magnetic and optical properties of diluted magnetic semiconductor CdMnTe quantum dots (QDs) using various photoluminescence (PL) techniques. We use spatially resolved photoluminescence imaging to study single magnetic QDs. A solid immersion lens is combined with the confocal microscopy to achieve a high spatial resolution (~ 200nm) for PL imaging. We observe formation of exciton magnetic polarons (EMPs) in magnetic QDs for non-resonant excitation at B = 0T and T = 7K. However, the spin alignments of individual EMPs are distributed randomly resulting in zero global magnetization. Moreover, the spin alignment in the magnetic QD persists as long as the excitation is continued. This persistent magnetization is because of a long spin relaxation time of Mn ions. We estimate the relaxation time to be of the order of 1ms in CdMnTe QDs. For resonant excitations, CdMnTe QDs exhibit a strong PL polarization (40%) at B = 0T and T = 7K. The measurements show predominant &sigma +(&sigma -) –polarized PL emission for &sigma +(&sigma -) –polarized excitations that create spin polarized excitons. This suggests that one can control the spin alignment of magnetic impurities in magnetic CdMnTe QDs optically by using selectively polarized excitations. The magnetization created by such spin polarized excitons persists up to 170K. We attribute this robust persistent magnetization to strong three dimensional confinements of excitons in smaller CdMnTe QDs. In a low Manganese density QD sample, we observe a mixture of PL emission lines of magnetic and non-magnetic QDs. The results demonstrate that the Zeeman splitting of such non-magnetic QDs depends on the polarization of excitation. We believe that this excitation dependent Zeeman splitting is due to coupling between magnetic and non-magnetic QDs through the exchange interaction.

Committee: Dr. Leigh Smith (Advisor) Subjects: Physics, Condensed Matter

Doctor of Philosophy, The Ohio State University, 2024, Chemistry

The design of next generation electrical and thermal transport materials is of far-reaching importance for myriad applications from thermoelectrics to dynamic transport switching. To that end, axis-dependent conduction polarity and thermal-switching materials hold significant promise. Axis-dependent conduction polarity (ADCP) is a phenomenon in which the electrons (n-type carriers) and holes (p-type carriers) are preferentially conducted along orthogonal directions in a crystal. The driving force for this phenomenon is a large (> 10x) anisotropy in the electron and hole mobilities between orthogonal directions. Herein is discussed the development of the first air-stable, wide bandgap (> 0.4 eV) semiconductor that displays ADCP, orthorhombic PdSe2. The anisotropy in the hole mobilities between the cross-plane and in-plane directions is > 100x, with holes preferentially conducting along the cross-plane direction. Additionally, the onset temperature of ADCP can be controlled via extrinsic doping with Ir and Sb as p-type and n-type dopants, respectively. When the chemical potential is near the valance band (Ir doping), ADCP is not observed up to 400 K. When it is mid-gap the onset temperature is about 350 K. But when it is near the conduction band (Sb doping), the onset temperature can be as low as 100 K. The dopant dependent onset temperature indicates the necessity for both the conduction and valance bands to be populated sufficiently to observe ADCP. Studies in this model system pave the way for further ADCP studies in semiconductors. Solid-state thermal switching is the rapid and reversible control over the thermal conductivity of a material between some low and high value without the need for physical phase changes or moving parts. Topologically non-trivial materials are promising candidates for solid-state thermal switching on account of their anomalous transport properties. Therefore, EuCd2As2 and MnBi2Te4 were studied for their thermal switching potential. Eu (open full item for complete abstract)

Committee: Joshua Goldberger (Advisor); Christine Thomas (Committee Member); Patrick Woodward (Committee Member) Subjects: Chemistry

Doctor of Philosophy, The Ohio State University, 2022, Physics

This thesis uses scanning tunneling microscopy (STM) to study Fe dopants deposited on GaAs (110). Fe, a transition metal, can introduce magnetism into a nonmagnetic semiconductor host, GaAs, which could allow mass storage and processing of information simultaneously. An STM is a powerful tool to study individual dopants with atomic resolution. It can generate topographic images, giving structural information about the individual dopants in the semiconductor host; it can measure the electronic properties of the individual dopants, and with a spin-polarized tip, it can measure the local magnetic properties as well. The research presented here compares STM measurements taken with both a nonmagnetic and magnetic tip and shows the first magnetic contrast of individual magnetic dopants on a III-V semiconductor. We propose a preliminary model of the spin-polarized tunneling current which is affected by the tunneling rate from the sample bulk to the dopant, and the rate of tunneling from the dopant to the spin-polarized tip, as well as the exchange coupling to other spin defects.

Committee: Jay Gupta (Advisor); Andrew Heckler (Committee Member); Fengyuan Yang (Committee Member); Nandini Trivedi (Committee Member) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy, The Ohio State University, 2017, Electrical and Computer Engineering

Over the last two decades, spin transistors that operate using both charge and spin properties of electrons have motivated extensive studies of injection, detection and manipulation of electronic spin current in various material systems. Dilute magnetic semiconductors, in which the spin polarized charge carriers are coupled to the magnetic moment, are of particular interest due to their compatible lattice structures and similar growth methods to current Si and GaAs technology. The first part of this thesis focuses on the structural, magnetic and magnetotransport properties of magnetically doped GaN and 2D MoS2. The Gd doped AlN/GaN heterostructures are grown by plasma assisted molecular beam epitaxy. The Gd atoms are δ-doped at the AlN/GaN heterointerface where the two dimensional electron gas (2DEG) forms. These samples exhibit defect-induced room temperature ferromagnetism with an easy axis along the c-axis. However, the nonlinear Hall resistivity does not track the magnetization in these Gd doped samples indicating the lack of coupling between the conduction electrons in the 2DEG and the Gd-induced ferromagnetism. This makes Gd doped GaN not useful as a dilute magnetic semiconductor. Mn doped few-layer MoS2 samples synthesized via sulfurization of Mn thin film on sapphire are fabricated in the aim of realizing a 2D dilute magnetic semiconductor. However, these samples mainly show paramagnetism implying the lack of ferromagnetic coupling between the Mn dopants. In addition to the electronic spin current, magnonic spin current has recently received growing research interest since it serves as a new route for achieving novel thermoelectric generators and magnon transistors. The second part of the thesis focuses on the study of the transport properties of the thermally induced magnonic spin current via spin Seebeck effect. A nonlocal opto-thermal spin Seebck configuration is proposed and implemented to measure the spin diffusion length in yttrium iron garnet (YIG). (open full item for complete abstract)

Committee: Roberto Myers (Advisor); Joseph Heremans (Committee Member); Tyler Grassman (Committee Member) Subjects: Electrical Engineering; Materials Science

PhD, University of Cincinnati, 2000, Arts and Sciences : Physics

We use time-resolved magneto-photoluminescence spectroscopy to study spin relaxation of excitons in a series of strained and unstrained ZnMnSe-based heterostructures. In an unstrained ZnMnSe epilayer, we find rapid spin relaxation, with a spin relaxation time of less than five picoseconds. We attribute this rapid spin relaxation to the complicated band structure: the various exciton spin bands intersect each other numerous times. The excitons can freely scatter between the spin bands, resulting in rapid spin relaxation. In contrast, we find extremely slow spin relaxation in a strained ZnMnSe/ZnFeSe multiple quantum well, with a spin relaxation time of greater than one nanosecond. Once excitons cool to the bottom of the band, very little spin relaxation occurs, and an extremely non-thermal exciton spin distribution persists throughout the lifetime of the exciton. In addition, we show that the dominant spin relaxation mechanism in this structure is LO-phonon emission during the momentum relaxation process, which occurs within 1 ps of the exciting laser pulse. We find similar results in two additional strained structures. For a strained ZnMnSe epilayer and a strained ZnMnSe/ZnSe multiple quantum well, we also see very slow spin relaxation, with spin relaxation times of greater than 1 ns. We conclude that this effect is due to the removal of the light hole - heavy hole valence band degeneracy by the lattice strain. This eliminates the band-mixing effects that lead to rapid spin relaxation in unstrained ZnMnSe-based heterostructures, thus resulting in extremely slow spin relaxation. We also find that the addition of a small fraction of cadmium to a strained quantum well strongly increases the spin relaxation rate. In a strained ZnCdMnSe/ZnSe quantum well, we see rapid spin relaxation, with a spin relaxation time of less than 5 ps, similar to that of the unstrained ZnMnSe epilayer. However, as in the other strained structures, the excitons spins never fully thermalize wit (open full item for complete abstract)

Committee: Leigh Smith (Advisor) Subjects: Physics, Condensed Matter

Master of Science, The Ohio State University, 2012, Physics

As the power requirements of computation for large scale and mobile applications increase and the fundamental limits of current technology are being approached, the development of novel computational schema is required to continue the meteoric improvements in processing. One such schema, spintronics, relies on manipulation of the magnetic state of electrons in computation. Such approaches to computation would be facilitated by the development of high-quality ferromagnetic semiconductors capable of room temperature operation and by controlled injection of magnetically polarized electrons into conventional semiconductors. The first part of this work explores the utility of gadolinium doped GaN/AlGaN heterojunctions as a magnetic semiconductor system. The extremely large magnetization per magnetic dopant atom observed in gadolinium doped gallium nitride is also observed in some, but not all, of these heterojunctions. The second part of this work involves the fabrication of spin valve devices on undoped GaN/AlGaN heterojunctions. Such devices present one potential avenue for the control of magnetically polarized current within conventional semiconductor systems.

Committee: Ezekiel Johnston-Halperin (Advisor); Roberto Myers (Advisor) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy, The Ohio State University, 2007, Chemistry

Vanadium tetracyanoethylene (V[TCNE]~2) is a magnetic semiconductor with an ordering temperature well above room temperature. Its highly disordered structure has hampered comprehensive descriptions of interactions between the V and TCNE sub-lattices that give rise to its magnetic and electrical properties. We report results of high-resolution x-ray absorption (XAS) and magnetic circular dichrosim (MCD) studies probing the electronic structure of V[TCNE]~2to elucidate the nature of these interactions. Included in this study are first reports of gas phase neutral TCNE XAS spectra as well as first reports of MCD spectra of the carbon and nitrogen absorption edges for V[TCNE]~2. The vanadium spectrum reveals a spin split L3and L2spectrum that is qualitatively modeled for V(II) using crystal field multiplet (CFM) theory except for a region of excess intensity on the high energy side of both the L3and L2absorption edges. We speculate the origin of this excess intensity is vanadium present in valence states higher than V(II) and antibonding states from hybridization of V and TCNE. Despite the localized nature of x-ray absorption, the C and N spectra of the TCNE suggest we are probing molecular final states of TCNE from different sites rather than atomically isolated states. In addition, the carbon and nitrogen absorption spectra reveal this molecular orbital structure remains largely intact in going from the gas phase to the condensed phase V[TCNE]~2. The value in high-resolution experimental data is exposed as new features are detected unveiling effects of magnetic exchange leading to spin splitting of the singly occupied molecular orbital of [TCNE]•−as revealed by carbon and nitrogen XAS and MCD spectra. Remarkable alignment of low photon energy features in vanadium, carbon, and nitrogen spectra reveal a strong hybridized structure between valence molecular orbitals of TCNE and vanadium 3d orbitals. Associated MCD spectra elucidate polarization associated with particular (open full item for complete abstract)

Committee: Arthur Epstein (Advisor) Subjects:

Doctor of Philosophy (PhD), Ohio University, 2011, Physics and Astronomy (Arts and Sciences)

We not only perform growth and scanning tunneling microscopy studies of magnetic films on semiconductors using molecular beam epitaxy / scanning tunneling microscopy system in an existing lab, but we also carry out development of a novel molecular beam epitaxy/pulsed laser deposition and superconducting magnet cryogenic spin-polarized scanning tunneling microscopy system in a completely new lab. We study the growth of iron nitride on gallium nitride using molecular beam epitaxy with Fe e-beam evaporation and radio frequency N-plasma growth. Thin iron nitride layers of thickness about 16 nm are grown and monitored in situ using reflection high energy electron diffraction. The samples following growth are analyzed ex situ using a variety of techniques including X-ray diffraction, Rutherford backscattering, and atomic force microscopy. The crystal phase and orientation with respect to the GaN substrate are deduced by monitoring the structure, morphology, and lattice constant evolution of the iron nitride film. The growth is discussed in terms of a 2-dimensional to 3-dimensional growth mode transition. The investigation of the initial phase of sub-monolayer iron deposition on GaN(0001) pseudo-1×1-1+1/12 surface is carried out. To begin with, we verify an atomically smooth GaN growth surface with in situ reflection high energy electron diffraction. Scanning tunneling microscopy shows smooth terraces separated by single and double height bilayer atomic steps. About 0.42 ML iron is deposited on a smooth GaN surface, and the subsequent scanning tunneling microscopy images reveal waffle-like 2-dimensional islands with a height of ~ 1.8-2.0 A, growing in a 2-dimensional mode outward from the GaN step edges of the pseudo-1×1-1 + 1/12 surface. A clear 6×6 structure is observed for the islands. The waffle-like islands also grow in the GaN spiral growth regions. Studies of iron/Ga-rich N-polar GaN(000-1) reveal the formation of quantum spintronic nanostructures on N-polar GaN(000 (open full item for complete abstract)

Committee: Arthur Smith PhD (Advisor); Nancy Sandler PhD (Committee Member); Saw-Wai Hla PhD (Committee Member); Hugh Richardson PhD (Committee Member) Subjects: Condensed Matter Physics; Materials Science; Physics

Doctor of Philosophy (PhD), Ohio University, 2006, Physics (Arts and Sciences)

Magnetic nitrides and magnetic heterostructures have attracted considerable attention due to their potential importance to magnetic sensor technology. This project investigates the growth, electronic, magnetic properties of magnetic manganese nitride and manganese nitride based magnetic heterostructures. A series of magnetic materials have been studied. It is composed of three types of magnetic systems: magnetic nitride-manganese nitride, ferromagnetic metal on antiferromagnetic nitride - Fe/Mn3N2; and ferri-magnetic nitride on nitride semiconductor – Mn4N/GaN. These magnetic materials have been grown by molecular beam epitaxy and investigated with various analytical tools. Reflection high energy electron diffraction and X-ray diffraction are used to determine the structures of these materials and their epitaxial relationship with the substrates. Scanning tunneling microscopy is used to study the phase transformation of a thin manganese nitride film, the well-ordered array of MnN-bonded Mn-tetramer clusters on Mn3N2 (001) surface, the step structures and morphology of Mn3N2 (010) surface and Fe thin films on Mn3N2 (010). The row-wise antiferromagnetic Mn3N2 (010) surface has been investigated using spin-polarized scanning tunneling microscopy. It is shown that the magnetic contrast in atomic-scale images is a strong function of the bias voltage around the Fermi level. Atomic force microscopy is also used to investigate the morphology of Fe films on Mn3N2 (010) and Mn4N thin films on GaN(0001).

Committee: Arthur Smith (Advisor) Subjects:

Master of Science, Miami University, 2011, Physics

We present experimental research on magneto-transport at Copper and GaMnAs interface. We report the values of specific contact resistance (ARc) between GaMnAs/Cu interfaces using Circular Transmission Line Method (CTLM) for a wide range of temperature (15K to 290K) above and below Curie temperature (Tc) which is at 145K. Our values of specific contact resistance are very low and close to the order of ~10-8 Ωcm2 which agrees with literature and changes abruptly to close to double of the value at Curie temperature when GaMnAs has phase transition between nonmagnetic to ferromagnetic. We suggest that this arises due to suppression of one of the two spin conduction channels when the phase transition of GaMnAs takes place. We also found the Specific contact resistance has a peak shifted towards lower temperature which suggests the magnetization in the GaMnAs film is suppressed near the Copper interface.

Committee: Khalid Eid PhD (Advisor); Michael Pechan PhD (Committee Member); Jan Yarrison-Rice PhD (Other); Herbert Jaeger PhD (Other) Subjects: Physics

Doctor of Philosophy, Case Western Reserve University, 2007, Physics

The thesis presented here deal with calculations of electronic and magnetic properties of transition metal based materials. Electronic structure of Mn-doped ScN: a possible new magnetic semiconductor We performed fully relaxed full-potential linear muffin-tin orbital method calculations of Mn-doped ScN using a supercell approach. We found that a t2g like defect level exists in the gap and gives rise to a magnetic moment between 2 an 3 μB. Calculations for 64 atom cells with two Mn in 1st-4th neighbor positions indicated a preference for ferromagnetic coupling. By mapping the energy differences on a Heisenberg Hamiltonian and assuming interactions with distant atoms except those in the adjacent unit cells are zero, we extracted the exchange interactions, which were found to be rather large and indicate a Curie temperature above room temperature even for only 3% Mn. Calculations of the miscibility indicated only 1% equilibrium miscibility at typical growth temperatures. However, non-equilibrium growth techniques have shown that in practice mixed alloys up to 26% Mn can be grown. We also studied the effect of n-type doping. Unexpectedly, Mn defects in the negative charge state still have an even larger magnetic moment with an increase in the eg state contribution. Subsequently, we carried out further calculations of the exchange interactions using non-collinear magnetic configurations in which the spin is slowly rotated. it was found that the previous calculations overestimate the J0=∑i J0i, i.e. the sum of all interactions connected to a given site, by about 30%. Further studies using the Liechtenstein linear response approach show that the latter is a sum over many long range interactions extending significantly beyond the range of the cells we had used. In this approach the long range interactions are obtained by Fourier transform of the Jij(k) for a mesh of k-points in the supercell. The nearest neighbor interactions are found to be an order of magnitude smaller th (open full item for complete abstract)

Committee: Walter Lambrecht (Advisor) Subjects: Physics, Condensed Matter