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On top of the MBE system in the Lederman Lab during a repair.

About
 

I earned my PhD in Physics in 2023 from the University of California Santa Cruz where I studied under Professor David Lederman. My PhD work involved the growth of thin film topological insulators, ferromagnets, and antiferromagnets, and the crystallographic, electronic, and magnetic characterization of these materials.

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In 2024 I began work as a postdoctoral associate at the Laboratory for Physical Sciences in College Park Maryland, building on my work with novel topological insulator - magnetic material bilayers.

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Publications

Suppressed weak anti-localization in topological insulator - antiferromagnetic
insulator (BiSb)2Te3 - MnF2 thin film bilayers

Ryan Van Haren and David Lederman
Phys. Rev. B 110, 205409 – Published 5 November 2024

Thin films of the topological insulator (BiSb)2⁢Te3 oriented along the [0001] direction were grown via molecular beam epitaxy on substrates of Al2⁢O3 (0001) and MgF2 (110) single crystals, as well as on an epitaxial thin film of the antiferromagnetic insulator and predicted altermagnet MnF2 (110). Magnetoconductivity measurements of these samples showed close proximity of the Fermi level to the Dirac point and weak antilocalization at low temperature that was partially suppressed in the sample grown on the MnF2 layer. The magnetoconductivity data were fit to a model that describes the quantum corrections to the conductivity for the Dirac surface state of a three-dimensional topological insulator, from which values of the Fermi velocity and the phase coherence length of the surface state charge carriers were derived. The magnetoconductivity of the (BiSb)2⁢Te3−MnF2 bilayer samples were fit to a model describing the crossover from weak antilocalization to weak localization due to magnetic doping. The results are consistent with the opening of an energy gap at the Dirac point in (BiSb)2⁢Te3 due to magnetic proximity interactions of the topological surface states with the antiferromagnetic MnF2 insulator.

Electronic and magnetic properties of thin film transition metal fluorides, topological insulators, and their bilayers 

Ryan Van Haren
PhD Dissertation - Accepted December 2023

    Materials with long range magnetic order and strong spin-orbit coupling can exhibit unique physical phenomena when the materials are structured in novel configurations. Thin film growth via molecular beam epitaxy enables precise engineering of these materials into novel configurations by elemental doping and construction of bilayer structures.
The effect of random competing single-ion anisotropies in antiferromagnets was studied using epitaxial MnxNi1−xF2 antiferromagnetic thin film alloys. Both MnF2 and NiF2 have the tetragonal rutile crystal structure, but MnF2 has an easy axis magnetic anisotropy along the c-axis of the unit cell while NiF2 has an easy plane magnetic anisotropy perpendicular to the c-axis. Crystallographic and magnetization measurements demonstrated that the thin film alloys exhibit epitaxial strain from the MgF2 (110) substrates, and that pure MnF2 thin films exhibit piezomagnetic effects due to the epitaxial strain. Mean field theory is used to calculate the exchange energies of the alloy system and predict the existence of an oblique antiferromagnetic phase. Magnetization measurements show evidence of this oblique antiferromagnetic phase in addition to an emergent magnetic phase that is believed to be either a magnetic glassy phase or a helical phase.
Thin films of the topological insulator Bi2Te3 doped with Mn ions exhibit a spontaneous ferromagnetic moment below T ≈ 16 K. These Mn doped Bi2Te3 thin films are grown on several different substrates, hexagonal Al2O3 (0003), tetragonal MgF2 (110), and the tetragonal antiferromagnet NiF2 (110), with crystallographic characterization indicating single phase growth of the Mn doped Bi2Te3 film regardless of substrate. Electronic transport and magnetic moment measurements show that the ferromagnetic moment of the Mn doped Bi2Te3 thin films is enhanced as the Fermi level moves from the bulk conduction band and towards the bulk band gap, suggesting that electronic surface states play an important role in mediating the ferromagnetic order. Mn doped Bi2Te3 grown on antiferromagnetic NiF2 show evidence that the ferromagnetic moment of the Mn doped Bi2Te3 film is suppressed, suggesting the existence of an interface effect between the two magnetic layers.
The Fermi level of the co-doped topological insulator (BiSb)2Te3 can be tuned to lie in the bulk band gap by careful control of the (BiSb) stoichiometric ratio. Thin films of (BiSb)2Te3 are grown on both Al2O3 and antiferromagnetic MnF2. Perpendicular and parallel magnetoconductance measurements are performed and fit to several models of the magnetoconductance, including comparisons of the quasi-2D Hikami-Larkin-Nagaoka model to a model derived for 2D Dirac states. The fits of experimental data to theory suggest at improved conduction through the 2D topological surface states due to the tuned Fermi level. (BiSb)2Te3-MnF2 bilayers show evidence of enhanced magnetic scattering, suggesting the presence of magnetoelectric coupling effects at the interface.

Emergent magnetic phases and piezomagnetic effects in MnxNi1−xF2 thin film alloys

Ryan Van Haren, Nessa Hald, and David Lederman
Phys. Rev. B 108, 134437 – Published 30 October 2023

     The effect of random competing single-ion anisotropies in antiferromagnets was studied using epitaxial MnxNi1−xF2 antiferromagnetic thin film alloys grown via molecular beam epitaxy. The crystal structure of this material is tetragonal for all values of x, and the Mn sites have a magnetic easy axis single-ion anisotropy while the Ni sites have an easy plane anisotropy perpendicular to the Mn easy axis. Crystallographic and magnetization measurements demonstrated that the thin film alloys were homogeneously mixed and did not phase-separate into their constituent parts. Pure MnF2 thin films epitaxially grown on MgF2 exhibited compressive strain along all three crystallographic axes which resulted in piezomagnetic effects. The piezomagnetism disappeared if the film was grown on a (MnNi)F2 graded buffer layer. A mean-field theory fit to the transition temperature as a function of the Mn concentration x, which takes into account piezomagnetic effects, gave a magnetic exchange constant between Mn and Ni ions of JMnNi=0.305±0.003~meV. Mean-field theory calculations also predicted the existence of an oblique antiferromagnetic phase in the MnxNi1−xF2 alloy which agreed with the experimental data. A magnetic phase diagram for MnxNi1−xF2 thin film alloys was constructed and showed evidence for the existence of two unique magnetic phases, in addition to the ordinary antiferromagnetic and paramagnetic phases: an oblique antiferromagnetic phase, and an emergent magnetic phase proposed to be either a magnetic glassy phase or a helical phase. The phase diagram is quantitatively different from that of FexNi1−xF2 because of the much larger single-ion anisotropy of Fe2+ compared to Mn2+.

Surface state mediated ferromagnetism in Mn0.14Bi1.86Te3 thin films

Ryan Van Haren, Toyanath Joshi, and David Lederman
Phys. Rev. Materials 7, 034201 – Published 23 March 2023

     A spontaneous ferromagnetic moment can be induced in Bi2Te3 thin films below a temperature T ≈ 16 K by the introduction of Mn dopants. We demonstrate that films grown via molecular beam epitaxy with the stoichiometry Mn0.14Bi1.86Te3 maintain the crystal structure of pure Bi2Te3. The van der Waals nature of inter-layer forces in the Mn0.14Bi1.86Te3 crystal causes lattice mismatch with the underlayer to have a limited effect on the resulting crystal structure, as we demonstrate by thin film growth on tetragonal MgF2 (110) and NiF2 (110). Electronic transport and magnetic moment measurements show that the ferromagnetic moment of the Mn0.14Bi1.86Te3 thin films is enhanced as the Fermi level moves from the bulk conduction band and towards the bulk band gap, suggesting that electronic surface states play an important role in mediating the ferromagnetic order. Ferromagnetic Mn0.14Bi1.86Te3/antiferromagnetic NiF2 bilayers show evidence that the ferromagnetic moment of the Mn0.14Bi1.86Te3 film is suppressed, suggesting the existence of an interface effect between the two magnetic layers.

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 Bi2Te3 system, a ferromagnetic TI with a Curie temperature of ~ 16 K, with a series of MF2 (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. 
*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 NiF2 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).

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rtvanharen
Jan 152 min read

Chiral spin texture

In some magnetic materials under certain conditions, a stable twisted or whirling configuration of local spins can form. These magnetic configurations are known as chiral spin textures and are of scientific interest both as manifestations of unique topological properties and for their potential use in spintronic devices. When these twisted spin textures form at the surface of a magnetic material as a 2D structure, then they are called skyrmions, named after the theoretical physicist Tony Skyrme. These chiral spin textures can also form in the bulk of a material, where the 3D spin structures are known as Hopfions, named after the German physicist Heinz Hopf for his contributions to the field of topology.

Chiral spin textures do not seem to form in every magnetic material, but are

frequently observed in systems that exhibit the Dzyaloshinskii-Moriya inter-

action (DMI), an anisotropic superexchange interaction present in some an-

tiferromagnets that causes canting of the magnetic moments away from the

Néel vector and manifests as a weak ferromagnetic moment perpendicular to

the Néel vector [1, 2] . Phenomenologically the DMI can be expressed as an

⃗⃗⃗energy term in the spin Hamiltonian as D · (Si × Sj ) [3, 4]. The DMI, and

the accompanying chiral spin textures that often form, make materials that exhibit this interaction especially interesting for magnetic studies. One such material is the antiferromagnet NiF2 [5, 6].

Interestingly, while NiF2 exhibits the DMI, the other antiferromagnetic transi- tion metal fluorides with the same crystal structure as NiF2 (specifically MnF2, CoF2, and FeF2) do not exhibit the DMI. This is a result of the magnetic anisotropy of NiF2, which has a Néel vector that points along either the [100] or [010] crystallographic directions, while the other transition metal fluorides named have a Néel vector that points along the [001] [5]. The presence of the DMI in NiF2 makes it an especially interesting material to study, both alone and in alloys, for the potential to host chiral spin textures such as skyrmions or Hopfions.


[1] A. Fert, N. Reyren, V. Cros, Nature Reviews Materials 2017, 2, Number: 7 Publisher: Nature Publishing Group, 1–15.

[2] N. Kent, N. Reynolds, D. Raftrey, I. T. G. Campbell, S. Virasawmy, S. Dhuey, R. V. Chopdekar, A. Hierro-Rodriguez, A. Sorrentino, E. Pereiro, S. Ferrer, F. Hellman, P. Sutcliffe, P. Fischer, Nature Communications 2021, 12, Number: 1 Publisher: Nature Publishing Group, 1562.

[3] I. Dzyaloshinsky, Journal of Physics and Chemistry of Solids 1958, 4, 241–255.

[4] T. Moriya, Physical Review 1960, 120, Publisher: American Physical Society, 91–98.

[5] T. Moriya, Physical Review 1960, 117, Publisher: American Physical Society, 635–647.

[6] A. Borovik-Romanov, A. Bazhan, N. Kreines, Journal of Experimental and Theoretical Physics 1973, 37, 695–702.

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