F. Hellman
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View article: Thermodynamic phase transitions of nematic order in magnetic helices
Thermodynamic phase transitions of nematic order in magnetic helices Open
A nematic phase lacks translation order but has orientational order. Nematic phases have been discovered in a variety of systems, including liquid crystals, correlated materials, and superconductors. Here, we report on a magnetic nematic p…
View article: Exploring Structural Anisotropy in Amorphous Tb-Co via Changes in Medium-Range Ordering
Exploring Structural Anisotropy in Amorphous Tb-Co via Changes in Medium-Range Ordering Open
Amorphous thin films grown by magnetron co-sputtering exhibit changes in atomic structure with varying growth and annealing temperatures. Structural variations influence the bulk properties of the films. Scanning nanodiffraction performed …
View article: Characterizing the nonequilibrium response of FeRh thin films using time-domain thermoreflectance
Characterizing the nonequilibrium response of FeRh thin films using time-domain thermoreflectance Open
Time-domain thermoreflectance (TDTR) characterization of FeRh throughout its first-order antiferromagnetic (AF) to ferromagnetic (FM) transition shows that the transient reflectance ΔR(t)/R strongly depends on the magnetic order of the sa…
View article: Nematicity of a Magnetic Helix
Nematicity of a Magnetic Helix Open
A system that possesses translational symmetry but breaks orientational symmetry is known as a nematic phase. While there are many examples of nematic phases in a wide range of contexts, such as in liquid crystals, complex oxides, and supe…
View article: Structural spillage: An efficient method to identify noncrystalline topological materials
Structural spillage: An efficient method to identify noncrystalline topological materials Open
While topological materials are not restricted to crystals, there is no\nefficient method to diagnose topology in non-crystalline solids such as\namorphous materials. Here we introduce the structural spillage, a new indicator\nthat predict…
View article: Strategies to reduce the thermoelastic loss of multimaterial coated finite substrates
Strategies to reduce the thermoelastic loss of multimaterial coated finite substrates Open
Thermoelastic loss is an important energy dissipation mechanisms in resonant systems. A careful analysis of the thermoelastic loss is critical to the design of low-noise devices for high-precision applications, such as the mirrors used for…
View article: Disorder-driven localization and electron interactions in Bi$_x$TeI thin films
Disorder-driven localization and electron interactions in Bi$_x$TeI thin films Open
Strong disorder has a crucial effect on the electronic structure in quantum materials by increasing localization, interactions, and modifying the density of states. Bi$_x$TeI films grown at room temperature and \SI{230}{K} exhibit dramatic…
View article: Exploring the origins of perpendicular magnetic anisotropy in amorphous Tb-Co via changes in medium-range ordering
Exploring the origins of perpendicular magnetic anisotropy in amorphous Tb-Co via changes in medium-range ordering Open
Amorphous thin films of Tb$_{17}$Co$_{83}$ (a-Tb-Co) grown by magnetron co-sputtering exhibit changes in magnetic anisotropy with varying growth and annealing temperatures. The magnetic anisotropy constant increases with increasing growth …
View article: Characterizing Temporal Heterogeneity by Quantifying Nanoscale Fluctuations in Amorphous Fe‐Ge Magnetic Films
Characterizing Temporal Heterogeneity by Quantifying Nanoscale Fluctuations in Amorphous Fe‐Ge Magnetic Films Open
Equilibrium phase transitions are influenced by fluctuations and often discussed within the framework of the Gibbs free energy, wherein the exchange of energy between system and thermal bath is stationary and all regions of the sample exhi…
View article: Establishing Coherent Momentum-Space Electronic States in Locally Ordered Materials
Establishing Coherent Momentum-Space Electronic States in Locally Ordered Materials Open
In our understanding of solids, the formation of highly spatially coherent electronic states, fundamental to command the quantum behavior of materials, relies on the existence of discrete translational symmetry of the crystalline lattice. …
View article: Observation of spin-momentum locked surface states in amorphous Bi2Se3
Observation of spin-momentum locked surface states in amorphous Bi2Se3 Open
Crystalline symmetries have played a central role in the identification and understanding of quantum materials. Here we investigate whether an amorphous analogue of a well known three-dimensional strong topological insulator has topologica…
View article: Characterizing the Nonequilibrium Response of FeRh Thin Films using Time-Domain Thermoreflectance (TDTR)
Characterizing the Nonequilibrium Response of FeRh Thin Films using Time-Domain Thermoreflectance (TDTR) Open
Time-Domain Thermoreflectance (TDTR) characterization of FeRh throughout its first-order antiferromagnetic (AF) to ferromagnetic (FM) transition shows that the transient reflectance, $Δ$R(t)/R, strongly depends on the magnetic order of the…
View article: Large spin-orbit torque generated by amorphous iron silicide
Large spin-orbit torque generated by amorphous iron silicide Open
Spin-orbit torque (SOT) shows great potential for next-generation memory technology. Conventionally, the underlying physics of SOT has been associated with heavy (high-Z) elements and/or intrinsic bandstructure. Here, we report large spin-…
View article: Structural tunability and origin of two-level systems in amorphous silicon
Structural tunability and origin of two-level systems in amorphous silicon Open
Amorphous silicon films prepared by electron-beam evaporation have systematically and substantially greater atomic density for higher thickness, higher growth temperature, and slower deposition rate, reaching the density of crystalline Si …
View article: Strategies to reduce the thermoelastic loss of multimaterial coated finite substrates
Strategies to reduce the thermoelastic loss of multimaterial coated finite substrates Open
Thermoelastic loss is an important energy dissipation mechanisms in resonant systems. A careful analysis of the thermoelastic loss is critical to the design of low-noise devices for high-precision applications, such as the mirrors used for…
View article: Coexistence of soft and hard magnetic phases in single layer amorphous Tb–Co thin films
Coexistence of soft and hard magnetic phases in single layer amorphous Tb–Co thin films Open
Amorphous, ferrimagnetic Tb–Co thin films prepared with a thin Ta underlayer and either a Ta or a Pt overlayer show evidence of both soft and hard magnetic phases. At room temperature, the films exhibit conventional ferromagnetism, but low…
View article: Decoupling between propagating acoustic waves and two-level systems in hydrogenated amorphous silicon
Decoupling between propagating acoustic waves and two-level systems in hydrogenated amorphous silicon Open
Specific heat measurements of hydrogenated amorphous silicon prepared by hot-wire chemical vapor deposition show a large density of two-level systems at low temperature. Annealing at 200 {\deg}C, well below the growth temperature, does not…
View article: Structural disorder-driven topological phase transition in noncentrosymmetric BiTeI
Structural disorder-driven topological phase transition in noncentrosymmetric BiTeI Open
In this work, we investigate using local structural disorder to induce a topologically nontrivial phase in a solid state system. Using first-principles calculations, we introduce structural disorder in the trivial insulator BiTeI and obser…
View article: Magnetic Materials: Chiral Spin Textures in Amorphous Iron–Germanium Thick Films (Adv. Mater. 8/2021)
Magnetic Materials: Chiral Spin Textures in Amorphous Iron–Germanium Thick Films (Adv. Mater. 8/2021) Open
Topological solitary fields, such as magnetic and polar skyrmions, are envisioned to revolutionize microelectronics. These configurations have been stabilized in solid-state materials with a global inversion symmetry breaking, which transl…
View article: Role of element-specific damping in ultrafast, helicity-independent, all-optical switching dynamics in amorphous (Gd,Tb)Co thin films
Role of element-specific damping in ultrafast, helicity-independent, all-optical switching dynamics in amorphous (Gd,Tb)Co thin films Open
Ultrafast control of the magnetization in ps timescales by fs laser pulses
\noffers an attractive avenue for applications such as fast magnetic devices for
\nlogic and memory. However, ultrafast helicity-independent all-optical switching
\…
View article: Chiral Spin Textures in Amorphous Iron–Germanium Thick Films
Chiral Spin Textures in Amorphous Iron–Germanium Thick Films Open
Topological solitary fields, such as magnetic and polar skyrmions, are envisioned to revolutionize microelectronics. These configurations have been stabilized in solid‐state materials with a global inversion symmetry breaking, which transl…
View article: Unexpected dependence of the anomalous Hall angle on the Hall conductivity in amorphous transition metal thin films
Unexpected dependence of the anomalous Hall angle on the Hall conductivity in amorphous transition metal thin films Open
The anomalous Hall effect (AHE), and magnetic and electronic transport properties were investigated in a series of amorphous transition metal thin films— FexSi –x, FexGe1-x, CoxGe1–x, CoxSi1–x, and Fe1–yCoySi. The experimental results are …
View article: Magnetotactic Bacteria Accumulate a Large Pool of Iron Distinct from Their Magnetite Crystals
Magnetotactic Bacteria Accumulate a Large Pool of Iron Distinct from Their Magnetite Crystals Open
Magnetotactic bacteria (MTB) produce iron-based intracellular magnetic crystals. They represent a model system for studying iron homeostasis and biomineralization in microorganisms. MTB sequester a large amount of iron in their crystals an…
View article: Tilted fluctuation electron microscopy
Tilted fluctuation electron microscopy Open
Fluctuation electron microscopy (FEM) is a scanning nanodiffraction-based method that offers a unique approach to characterizing nanometer-scale medium-range order (MRO) in disordered materials. In addition to determining the degree of MRO…