Research interests

We investigate the potential of structural and chemical short-range order, curvature, and strain to tailor magnetic and magneto-transport properties of nano structures, amorphous materials, and molecular magnets. This involves pushing boundaries of advanced synthesis, nanofabrication, characterization, and data analysis.

Structural and chemical order of atoms and electron orbitals define magnetic properties of condensed matter. A microscopic magnetization forms as a result of magnetic interactions in long-range-ordered systems. Spin-orbit coupling between adjacent atoms, originating from the hybridization of electron orbitals, causes ferro-, ferri- or antiferromagnetism. Systems with spin frustration or vector spin exchange, owing to an inversion symmetry breaking, can host chiral spin textures, including 3D topological magnetization vector fields. Dipole interactions in weak-exchange-coupled materials or between nano structures may lead to similar configurations at a larger length scale. This intriguing alternative allows for long-range ordering of chiral spin textures in frustrated systems and provides greater flexibility in materials synthesis of, i.e., amorphous quantum materials and ferromagnetic liquid droplets. In turn, magnetic properties may be employed to probe short-range order in otherwise non-magnetic materials.

Curious about the field of 3D nanomagnetism? Check out our recent reviews and perspectives on this topic.

Inversion symmetry breaking in magnetic materials manifests a vector spin exchange, known as Dzyaloshinskii-Moriya interaction (DMI), that constitutes an indirect exchange mediated by conduction electrons of adjacent atoms. The vector exchange is highly sensitive to structural and chemical order and both strength and sign oscillate locally similar to the RKKY coupling. While RKKY coupling distinguishes between parallel and antiparallel spin alignment, DMI discriminates left- and right-handed spin canting in form of a spin chirality selection. The latter may cause chiral magnetic domain walls, helical spins, and isotropic topological magnetic states. More sophisticated states, such as anisotropic solitons and topological knots, emerge in systems with spontaneous or local inversion symmetry breaking. The corresponding local DMI can be homogeneous, inhomogeneous or random on the microscale, and ideally requires a sub-atomically accurate placement of elements and atoms. However, this is experimentally impractical in view of both efforts and materials synthesis.

We investigate the potential of local inversion symmetry breaking caused by structural and chemical short-range order in form of random substitution/ intercalation of atoms, rough interfaces, and amorphicity to tailor magnetic properties and stabilize non-collinear, topological spin textures in solid-state materials. In particular, we synthesize amorphous films consisting of transition metals, metalloids, and rare-earth elements, and harness current, strain, and curvature to manipulate magnetic exchange, phase transitions, and spin excitations. Local variations in magnetic exchange interactions due to structural and chemical short-range order, including thermodynamically driven surface segregation, affect the magnetization vector field and require a joint investigation of microscopic properties and their macroscopic manifestation in form of, e.g., magnetization reversal, phase diagram and magneto-transport.

It is fascinating to see that amorphous materials can resemble inversion symmetry broken systems with similar magnetic properties, moments, and states. The disordered systems are distinguished from systems with global inversion symmetry breaking by their degenerate spin chirality that allows for forming isotropic and anisotropic topological spin textures at remanence while offering greater flexibility in materials synthesis, voltage, and strain manipulation.

Topic-related publications:

• Chiral ferrimagnetism in amorphous materials, Adv. Mater. 30, 1800199 (2018) 🔗 and highlight
• Itinerant ferromagnetism and intrinsic anomalous Hall effect in amorphous iron-germanium, Phys. Rev. B 101, 014402 (2020) 🔗
• Chiral spin textures in amorphous iron–germanium thick films, Adv. Mater. 33, 2004830 (2021) 🔗 and highlight
• Magnetism in curved geometries, J. Appl. Phys. 129, 210902 (2021) 🔗 and highlight
• Phase contrast imaging of non‐collinear spin textures with Lorentz microscopy, J. Mater. Res. 38, 4977 (2023) 🔗 and highlight

Funding:

• Nebraska EPSCoR FIRST Award #OIA-1557417
• UNL revision award

Magnetic properties of nano structures are distinct from bulk specimens due to spatial confinement and increased surface contributions. This may range from shifted transition temperatures to new or suppressed phases to a complete lack of magnetism at finite temperature. Magnetization configurations and spin excitations in nano structures separated by less than tens of nanometers are defined by the assembly's short-range order and magnetic dipole coupling provided the magnetization inside the nano structures behaves like a macrospin. Isotropic nano structures lacking a preferential orientation of the magnetization due to absent shape or negligible magneto-crystalline anisotropy resemble 2D XY macrospins, which are particularly sensitive to structural order.

Our group investigates 2D XY macrospin systems on planar and curved surfaces: planar nanodisk arrays and jammed nanoparticles at liquid-liquid interfaces constituting ferromagnetic liquid droplets. The primary goal is to understand and harness the mechanisms of magnetic order by disorder and particle jamming. The latter is essential to the field of reconfigurable self-assembly of nanoparticles at inversion symmetry-broken liquid-liquid interfaces and 3D printing of liquids with solid-state properties.

Structural disorder in geometrically frustrated nanodisk arrays may lead to long-range-ordered chiral spin textures that undergo topological phase transitions with characteristic spatial and temporal correlations. We tailor thermal spin fluctuations to explore these topological phases by changing temperature and dipole coupling via lattice constant, film thickness, symmetry, disorder and strain.

Jamming of magnetic nanoparticles at liquid-liquid interfaces with virtually arbitrary shape enables numerous fundamental investigations and interesting applications. This includes realizing 2D XY macrospins in curved geometry, and chemically controlling spin frustration and magnetic properties. The short-range order of nanoparticles translates into a short-range-ordered, locked magnetization independent of droplet shape and allows for reshaping the liquid droplet while preserving its magnetic moment, and controlling its motion by an external magnetic field. To fully understand jamming of the nanoparticle ensemble, we study the interaction within the jammed layer(s) of nanoparticles, and between jammed and dispersed nanoparticles as a function of pH, droplet size, nanoparticle size and shape, and magnetic nanoparticle concentration. This multidisciplinary research is jointly carried out with Tom Russell (Berkeley Lab).

Topic-related publications:

• Spatial and temporal correlations of XY macrospins, Nano Lett. 18, 7428 (2018) 🔗 and highlight
• Reconfigurable ferromagnetic liquid droplets, Science 365, 264 (2019) 🔗 and highlight
• Perspective: ferromagnetic liquids, Materials 13, 2712 (2020) 🔗 and highlight
• Ferromagnetic liquid droplets with adjustable magnetic properties, Proc. Natl. Acad. Sci. USA 118, e2017355118 (2021) 🔗 and highlight
• Magnetism in curved geometries, J. Appl. Phys. 129, 210902 (2021) 🔗 and highlight
• Ballistic ejection of microdroplets from overpacked interfacial assemblies, Adv. Funct. Mater. 33, 2213844 (2023) 🔗 and highlight
• Angular dependence of the magnetization relaxation in Co/Pt multilayers, J. Phys.: Condens. Matter 36, 015802 (2024) 🔗
• Electronic transport properties of spin-crossover polymer plus polyaniline composites with Fe₃O₄ nanoparticles, J. Phys. Mater. 7, 015010 (2024) 🔗
• Self-Propulsion by Directed Explosive Emulsification, Adv. Mater. X, 2310435 (2024) 🔗 and highlight

Funding:

• NSF DMR #2203933 (magnetic order in disordered dipolar nano structures)

Local-moment molecular magnets, such as spin-crossover (valence tautomeric) molecules, are magneto-electric materials whose electron orbitals, orbital levels, and resistive states can be designed through ligand engineering, i.e., chemical synthesis. This includes shifting orbital levels and binding energy as well as completely altering orbital hybridization and magnetic and magneto-transport properties. The latter are essentially defined by the local inversion symmetry breaking by the ligand field. To this end, exchange, magnetic anisotropy, and conductivity can be modified through proximity to semiconducting seed layers, gate voltage, or chemistry (ligands). Intermolecular coupling in thin films with an inherent disorder is critical for both long-range order of magnetic states and electronic transport properties, including specifically magneto-resistance and magneto-impedance.

Our research interest concerns the magnetic and magneto-transport properties of local-moment molecular films. This multidisciplinary research is jointly carried out with Peter Dowben (UNL) and Talat Rahman (UCF).

Topic-related publications:

• Evidence of dynamical effects and critical field in a cobalt spin crossover complex, Chem. Commun. 58, 661 (2022) 🔗 and highlight
• Perturbing the spin state and conduction of Fe (II) spin crossover complexes with TCNQ, Mater. Chem. Phys. 296, 127276 (2023) 🔗
• Electronic structure of cobalt valence tautomeric molecules in different environments, Nanoscale 15, 2044 (2023) 🔗 and highlight
• Evidence of symmetry breaking in a Gd₂ di-nuclear molecular polymer, Phys. Chem. Chem. Phys. 25, 6416 (2023) 🔗
• Electronic transport properties of spin-crossover polymer plus polyaniline composites with Fe₃O₄ nanoparticles, J. Phys. Mater. 7, 015010 (2024) 🔗

Funding:

• NSF EPSCoR research grant #OIA-2044049 (RII Track-1: Emergent Quantum Materials and Technologies)

We are an experimental condensed matter physics group focusing on magnetic phenomena in nano structures and thin films with a microscopic magnetization.

We have a strong record of advancing science, i.e., 3D nanomagnetism, in unconventional ways, paving new avenues of research. We have harnessed strain and curvature to synthesize curved geometries and tailor magnetic exchange interactions; imprinted topological states via interlayer exchange coupling between soft- and hard-magnetic films; stabilized chiral, topological spin textures in amorphous materials with local inversion symmetry breaking; facilitated geometric spin frustration in XY macrospin systems and ferromagnetic nanoparticles at liquid-liquid interfaces to form shapable liquid magnets; and tuned magnetic and magneto-transport properties of molecular magnets by leveraging local inversion symmetry breaking.

Our group takes advantage of a large variety of experimental expertise, which we resourcefully harness to address scientific questions. These include evaporation and sputter deposition of metallic elements; nanofabrication on substrates and silicon nitride nanomembranes employing polymer-based lithography, lift-off, and etching; strain engineering in epitaxial, polycrystalline, and amorphous materials as well as in metal/piezoelectric heterostructures; several microscopy techniques, including atomic force, piezo-response force, magnetic force microscopy, scanning electron microscopy and, focus ion beam microscopy; magneto-optical Kerr effect microscopy and magnetometry; magneto-transport; and ferromagnetic resonance spectroscopy.

We leverage unique capabilities at national and international large-scale user facilities to conduct synchrotron-based experiments and aberration-corrected transmission electron microscopy, such as x-ray absorption spectroscopy, transmission x-ray microscopy, x-ray photoemission electron microscopy, magnetic x-ray tomography, resonant x-ray scattering, x-ray photon correlation spectroscopy, and phase contrast imaging with Lorentz microscopy. We are experienced in performing static, stroboscopic time-resolved, and pump-free time-resolved measurements.

Our experimental expertise is complemented by numerical modeling using, e.g., micromagnetic simulations (Nmag, Spirit, Vampire, MuMax3, Boris Spintronics), and advanced data analysis relying on self-coded Python scripts for quantitative absorption spectroscopy, phase contrast imaging, spatial and temporal correlation, coherent scattering, and electronic communication with scientific equipment.

Topic-related publications:

• Roundtable report: Research opportunities in the physical sciences enabled by cryogenic electron microscopy, U.S. Department of Energy, Office of Basic Energy Sciences (2021) 🔗 and highlight
• Phase contrast imaging of non‐collinear spin textures with Lorentz microscopy, J. Mater. Res. 38, 4977 (2023) 🔗 and highlight

Our lab has four major research instruments which enable us to contribute to a large variety of condensed matter research: a ultra high-vacuum evaporation chamber, a closed-cycle LHe cryostat, a full-field Kerr microscope, and a magneto-optical Kerr effect setup enabling magnetometry and electronic transport measurements.

The ultra high-vacuum evaporation chamber (MBE Komponenten) is designed to synthesize amorphous, nano granular, and epitaxial films consisting of transition metals, metalloids, and/or rare-earth elements at variable substrate temperatures down to 100 K. It is equipped with effusion cells and an electron beam evaporator ensuring stable deposition rates for homogeneous and heterogeneous synthesis. Epitaxial film growth can be directly monitored with a RHEED detector.

The magnetic films are characterized using our closed-cycle LHe cryostat (Quantum Design DynaCool Physical Properties Measurement System; 9 T, 1.7 K) to probe magnetic hysteresis loops (saturation magnetization, magnetic anisotropy, and transition temperatures), AC magnetic susceptibility (transition temperatures), magneto-transport properties (ordinary, anomalous and topological Hall effects, and AC and DC resistivity), and ferromagnetic resonance spectroscopy (saturation magnetization, magnetic anisotropy, Lande factor). Magnetometry and transport measurements can be performed at pressures up to a few GPa.

Additionally, we have a full-field Kerr microscope (Evico Magnetics) and two magneto-optical Kerr effect magnetometers (home-built) for local surface-sensitive measurements of soft- and hard-magnetic materials, including films, crystals, nano structures, and structured liquids. The home-built setups are fully controlled by PyVISA/SCPI using a PyQt5 graphical user interface on OpenSUSE and use either an optical chopper (2 kHz) and dual-phase lock-in amplification or a photoelastic modulator. Both red and blue laser light can be used for probing metals and insulators and two quadrupole electromagnets for room-temperature and cryogenic temperature measurements (hysteresis, first-order reversal curves, magnetization relaxation, temperature dependence). It provides a playground for students to advance both experimental and hardware programming skills.

A high-performance desktop computer (Ubuntu) powered by 48 CPUs (376 GB DDR4 RAM) and one 48 GB RTX 8000 GPU and one 24 GB GTX 3090 Ti GPU facilitates numerical modeling of magnetic systems using open-source micromagnetic simulation frameworks (Nmag, Boris Spintronics, Vampire, and Spirit), iterative reconstruction of electron phases, and advanced data analysis. Additional computing power is provided by UNL's Holland Computing Center.

The Lab located at JH 341 is part of the department of Physics and Astronomy's Condensed Matter and Materials Physics group and works in conjunction with the Nebraska Center for Materials and Nanoscience (NCMN). NCMN is a central user facility adjacent to the Physics building, which offers nanofabrication, synthesis, and characterization capabilities essential to our research. We are further frequent users of large-scale user facilities of the Department of Energy, including synchrotron radiation facilities, electron microscopy centers, and nanofabrication facilities.

Our resources are used for both fundamental and applied research, and outreach and educational purposes.