Ryan Van Haren, Ph.D.
Conference Presentations
Surface state mediated ferromagnetism in Mn doped Bi2Te3 topological insulator thin films
APS March Meeting 2024
March 3-8, 2024; Minneapolis, MN
Topological insulators with spontaneous ferromagnetic order are of scientific interest due to this moment breaking time reversal symmetry in the topological surface states and opening a gap at the Dirac point. These ferromagnetic topological insulators can exhibit quantum anomalous Hall effects and topological Hall effects associated with chiral magnetic phases. While ferromagnetism can be induced in Bi2Te3 by doping it with transition metal ions, the mechanism of long range order in these materials is not well understood. In this work, we present magnetic and electronic characterization of ferromagnetic Mn doped Bi2Te3 thin films. We present evidence that the magnetic moment of the n-type doped films increases as the carrier density decreases. We argue that this is due to a Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in the conducting surface state electrons. This is contrary to the expected behavior for ferromagnetism in metals where the magnetic moment tends to increase with increasing carrier density. This deviation from the expected behavior is indicative of the unique electronic band structure of the 3D topological insulators.
*This work was supported in part by the Air Force MURI program, grant # FA9550-19-1-0307
Competing Magnetic Anisotropies in the Antiferromagnet Thin Film Alloy Ni1-xMnxF2
APS March Meeting 2023
March 5–10, 2023; Las Vegas Nevada
NiF2 and MnF2 both have a rutile crystal structure and antiferromagnetic (AF) order at low temperature, but NiF2 has an easy-plane plane anisotropy with an effective Dzyaloshinskii-Moriya interaction, while MnF2 is an easy axis uniaxial anisotropy. Bulk NiF2 orders at 73 K and prefers to orient the AF spins along the [100] or [010] directions, while bulk MnF2 orders at 67 K with an easy axis anisotropy that orients the spins along the [001] direction. The close structural similarity and relatively small lattice mismatch between the NiF2 and MnF2 crystals makes them good candidates for a mixed thin film alloy system with competing AF anisotropies. I will present our work growing and characterizing thin film Ni1-xMnxF2 alloys, a system which to date has not been studied. These alloys represent a system with competing single-ion AF anisotropies without frustration due to the exchange interaction. Thin films of varying stoichiometries were grown via MBE and characterized with x ray diffraction (XRD), which showed that the alloys grow epitaxially and single phase along the [110] direction. XRD measurements also show a smooth change of the lattice constants as the stoichiometry is changed and strain due to epitaxial growth on the MgF2 (110) substrate. The magnetic moment of these samples was measured and used to determine the magnetic anisotropy of the alloys, demonstrating that the easy axis flips near x = 0.7, and the effect of strain on transition temperature.
*This work was supported by the Air Force Office of Scientific Research under grat #FA9550-19-1-0307 and the National Science Foundation REU program under grant #1950907
Characterization of Novel Ferromagnetic Topological Insulator - Antiferromagnetic Insulator Thin Film Heterostructures
APS March Meeting 2022
March 14–18, 2022; Chicago Illinois
Magnetic topological insulators (TIs) in proximity with antiferromagnetic (AFM) insulators have the potential of exhibiting new emergent behavior as a result of the exchange interaction at the interface, including interface magnetic exchange effects, strong spin - orbit coupling bulk electronic transport, and novel topological surface state electronic transport. Here we report on the proximity effects of Mn doped Bi2Te3Â system, a ferromagnetic TI with a Curie temperature of ~ 16 K, with a series of MF2Â (M = transition metal) AFM ionic crystals. This work will demonstrate how these thin film heterostructures can be grown with high crystal quality via molecular beam epitaxy and discuss their magnetic and electronic properties, including possible meergent behaviors that arise from their interface interactions.Â
*This work was supported by the Air Force Office of Scientific Research under grat #FA9550-19-1-0307 and the National Science Foundation REU program under grant #1950907
Low Temperature Electronic Measurements in Novel Topological Insulator-Antiferromagnetic Insulator Thin Film Heterostructures
APS March Meeting 2021
March 15–19, 2021; Virtual
Topological insulators (TIs) are of great interest for their unique combination of insulating bulk and metallic edge states. In 3D TIs, these edge states are spin polarized 2D conducting surface states protected from backscattering by time reversal symmetry. These states have great applicational potential in spintronic and quantum computing devices. The Dirac cone that forms at the surface of a 3D TI is robust to non-magnetic perturbations, but a gap can be opened through proximity to an ordered magnetic material. The insulating antiferromagnet nickel fluoride NiF2Â makes an interesting candidate for this proximity effect because of its weak ferromagnetic moment resulting from a spontaneous canting, in addition to terahertz frequency magnon fluctuations arising from the antiferromagnetic ordering. Our work shows how a NiF2-3D TI interface can be epitaxially grown via molecular beam epitaxy and presents low temperature charge carrier measurements performed in these heterostructures. These experiments demonstrate these structures can be fabricated into thin film devices and presents a path forward for further study and manipulation of these topologically protected surface states.
*This work was supported by the Air Force Office of Scientific Research under grant FA9550-19-1-0307.
Tuning the Electronic Band Structure of Copper Selenide Cu2Se Thin Films Grown via Molecular Beam Epitaxy
2019 Annual Meeting of the APS Far West Section
November 1–2, 2019; Stanford, California
Copper chalcogenides, compounds consisting of copper and one or more of the chalcogen family of elements S, Se, and Te, have recently become of interest to materials scientists for their unique electronic band structures and predicted electronic topological behavior. Of particular interest among this class of materials is the copper selenide Cu2Se. This material has long been known to be an excellent thermoelectric material and has recently garnered interest for its electronic band structure that is tunable by introducing copper vacancies into the crystal structure. In this work, we will present our successful growths of high quality, single phase, copper deficient Cu2Se thin films in the (200) orientation via molecular beam epitaxy. Using reflection high energy electron diffraction (RHEED) and x-ray diffraction (XRD) measurements, we will show how we are able to quantify the copper concentration by analyzing the subtle shifts in our observed XRD spectra corresponding to small changes in the lattice spacing due to these copper vacancies. In this manner we will demonstrate how we are able to tune the copper vacancies and electronic band structure by precise control of the crystal's growth parameters.
*This work was supported by The University of California Multicampus Research Programs Initiative (UCOP-MRPI).