arXiv subjects

Brian J. Kirby

Publications and source records attributed to Brian J. Kirby.

At least 19 recordsLinked to original sources

arXiv2022

Interfacial Mixing Effect in a Promising Skyrmionic Material: Ferrimagnetic Mn$_4$N

Interfacial mixing of elements is a well-known phenomenon found in thin film deposition. For thin-film magnetic heterostructures, interfacial compositional inhomogeneities can have drastic effects on the resulting functionalities. As such, care must be taken to characterize the compositional and magnetic properties of thin films intended for device use. Recently, ferrimagnetic Mn$_4$N thin films have drawn considerable interest due to exhibiting perpendicular magnetic anisotropy, high domain-wall mobility, and good thermal stability. In this study, we employed X-ray photoelectron spectroscopy (XPS) and polarized neutron reflectometry (PNR) measurements to investigate the interfaces of an epitaxially-grown MgO/Mn$_4$N/Pt trilayer deposited at 450 $^{\circ}$C. XPS revealed the thickness of elemental mixing regions of near 5 nm at both interfaces. Using PNR, we found that these interfaces exhibit essentially zero net magnetization at room temperature. Despite the high-temperature deposition at 450 $^{\circ}$C, the thickness of mixing regions is comparable to those observed in magnetic films deposited at room temperature. Micromagnetic simulations show that this interfacial mixing should not deter the robust formation of small skyrmions, consistent with a recent experiment. The results obtained are encouraging in terms of the potential of integrating thermally stable Mn$_4$N into future spintronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Room-Temperature Valence Transition in a Strain-Tuned Perovskite Oxide

Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr1-xCaxCoO3). Here, we show that in thin films of the complex perovskite (Pr1-yYy)1-xCaxCoO3-δ, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. The technological implications of this result are accompanied by fundamental prospects, as complete strain control of the electronic ground state is demonstrated, from ferromagnetic metal under tension to nonmagnetic insulator under compression, thereby exposing a potential novel quantum critical point.

cond-mat.mtrl-sci↗

arXiv2022

Viscosity Metamaterials

Metamaterials are composite structures whose properties arise from a mesoscale organization of their constituents. Provided this organization occurs on scales smaller than the characteristic lengths associated with their response, it is often possible to design such materials to have properties that are otherwise impossible to achieve with conventional materials -- including negative indexes of refraction, perfect absorption of electromagnetic radiation, and negative Poisson ratios. Here, we introduce and demonstrate a new material class: viscosity metamaterials. Specifically, we show that we are able to rapidly drive large viscosity oscillations in a shear-thickened fluid using acoustic perturbations with kHz to MHz frequencies. Because the time scale for these oscillations can be orders of magnitude smaller than the timescales associated with the global material flow, we can construct metamaterials whose resulting viscosity is a composite of the thickened, high-viscosity and dethickened, low viscosity states. Such viscosity metamaterials can be used to engineer a variety of surprising properties including negative viscosities, a response that is inconceivable with conventional fluids. The high degree of control over the resulting viscosity, the ease with which they can be accessed, and the variety of exotic properties achievable by viscosity metamaterials make them attractive for uses in technologies for which control over fluid flows and their instabilities are critical, ranging from coatings to cloaking to 3D printing.

cond-mat.soft↗

arXiv2022

Chiral Spin Bobbers in Exchange-Coupled Hard-Soft Magnetic Bilayers

The spin structure of exchange-coupled MnBi:Co-Fe bilayers is investigated by X-ray magnetic circular dichroism (XMCD), polarized neutron reflectometry (PNR), and micromagnetic simu-lations. The purpose of the present research is two-fold. First, the current search for new permanent-magnet materials includes hard-soft nanocomposites, and the analysis of coercivity mechanisms in these structures is an important aspect of this quest. Second, topological micro-magnetic structures such as skyrmions have recently become of intense fundamental and applied research, for example in the context of spin-based electronics. We find that the magnetization reversal of the MnBi:Co-Fe bilayer structure involves a curling-type twisting of the magnetization in the film plane. This curling in the exchange-coupled hard-soft magnetic bilayers is reminiscent of chiral spin structures known as bobbers and, in fact, establishes a new type of skyrmionic spin structure.

cond-mat.mtrl-sci↗

arXiv2021

Controlling magnetic configuration in soft-hard bilayers probed by polarized neutron reflectometry

Hard/soft magnetic bilayer thin films have been widely used in data storage technologies and permanent magnet applications. The magnetic configuration and response to temperatures and magnetic fields in these heterostructures are considered to be highly dependent on the interfacial coupling. However, the intrinsic properties of each of the layers, such as the saturation magnetization and layer thickness, also strongly influence the magnetic configuration. Changing these parameters provides an effective method to tailor magnetic properties in composite magnets. Here, we use polarized neutron reflectometry (PNR) to experimentally probe the interfacial magnetic configurations in hard/soft bilayer thin films: L10-FePt/A1-FePt, [Co/Pd] /CoPd, [Co/Pt] /FeNi and L10-FePt/Fe, which all have a perpendicular magnetic anisotropy in the hard layer. These films were designed with different soft and hard layer thicknesses (t_soft and t_hard) and saturation magnetization (M_s^soft and M_s^hard), respectively. The influences of an in-plane magnetic field (H_ip) and temperature (T) are also studied using a L10 FePt/A1-FePt bilayer sample. Comparing the PNR results to micromagnetic simulations reveals that the interfacial magnetic configuration is highly dependent on t_soft, M_s^soft and the external factors (H_ip and T), and has a relatively weak dependence on t_hard and M_s^hard. Key among these results, for thin t_soft, the hard and soft layers are rigidly coupled in the out-of-plane direction, then undergo a transition to relax in-plane. This transition can be delayed to larger t_soft by decreasing M_s^soft. Understanding the influence of these parameters on the magnetic configuration is critical to designing functional composite magnets for applications.

cond-mat.mtrl-sci↗

arXiv2021

Electrically Enhanced Exchange Bias via Solid State Magneto-Ionics

Electrically induced ionic motion offers a new way to realize voltage-controlled magnetism, opening the door to a new generation of logic, sensor, and data storage technologies. Here, we demonstrate an effective approach to magneto-ionically and electrically tune exchange bias in Gd/Ni$_{1-x}$Co$_{x}$O thin films (x=0.50, 0.67), where neither of the layers alone is ferromagnetic at room temperature. The Gd capping layer deposited onto antiferromagnetic Ni$_{1-x}$Co$_{x}$O initiates a solid-state redox reaction that reduces an interfacial region of the oxide to ferromagnetic NiCo. Exchange bias is established after field cooling, which can be enhanced by up to 35% after a voltage conditioning and subsequently reset with a second field cooling. These effects are caused by the presence of an interfacial ferromagnetic NiCo layer, which further alloys with the Gd layer upon field cooling and voltage application, as confirmed by electron microscopy and polarized neutron reflectometry studies. These results highlight the viability of the solid-state magneto-ionic approach to achieve electric control of exchange bias, with potentials for energy-efficient magneto-ionic devices.

cond-mat.mtrl-sci↗

arXiv2021

Synergistic effect of shear and ADP on platelet growth on ZTA and Ti6Al4V surfaces

Continuous-flow ventricular assist devices (VADs) have been an increasingly common, life-saving therapy for advanced heart-failure patients, but elevate the risk of thrombosis due to a combination of non-physiological hemodynamics and synthetic biomaterials. Limited work has been done to address platelet adhesion and aggregation on artificial surfaces under flow with sub-threshold concentrations of weak agonists. We perfused a blood analog containing hemoglobin-depleted red blood cells and fluorescently labeled platelets across a titanium alloy (Ti6Al4V) and zirconia-toughened alumina (ZTA) surface at shear rates of 400 and 1000 s-1. Upstream of the specimen, sub-threshold concentrations of ADP were uniformly introduced at concentrations of 0, 5, and 10 nM. Time-lapse videos of depositing platelets were recorded, and the percentage of the surface covered was quantified. Surface coverage percentages at 400 s-1and 1000 s-1were compared for each concentration of ADP and material surface combination. We observed a threshold concentration of ADP that expedites platelet deposition that is dependent on both shear and material surface chemistry. Additionally, we observed embolization when thrombus areas exceeded 300μm2, which was dependent on the combination of shear, ADP concentration, and material surface. This work is the first to simultaneously examine the three key contributing factors leading to thrombotic events. Our findings assist in considering alternative material choices constituting VADs and the need to address material reactivity in assessing antiplatelet agent tests.

q-bio.TO↗

arXiv2020

Correlating magnetic structure and magnetotransport in semimetal thin films of Eu$_{1-x}$Sm$_x$TiO$_3$

We report on the evolution of the average and depth-dependent magnetic order in thin film samples of biaxially stressed and electron-doped EuTiO$_3$ for samples across a doping range $<$0.1 to 7.8 $\times 10^{20}$ cm$^{-3}$. Under an applied in-plane magnetic field, the G-type antiferromagnetic ground state undergoes a continuous spin-flop phase transition into in-plane, field-polarized ferromagnetism. The critical field for ferromagnetism slightly decreases with an increasing number of free carriers, yet the field evolution of the spin-flop transition is qualitatively similar across the doping range. Unexpectedly, we observe interfacial ferromagnetism with saturated Eu$^{2+}$ moments at the substrate interface at low fields preceding ferromagnetic saturation throughout the bulk of the degenerate semiconductor film. We discuss the implications of these findings for the unusual magnetotransport properties of this compound.

cond-mat.str-el↗

arXiv2020

Correlation-driven eightfold magnetic anisotropy in a two-dimensional oxide monolayer

Engineering magnetic anisotropy in two-dimensional systems has enormous scientific and technological implications. The uniaxial anisotropy universally exhibited by two-dimensional magnets has only two stable spin directions, demanding 180 degrees spin switching between states. We demonstrate a novel eightfold anisotropy in magnetic SrRuO3 monolayers by inducing a spin reorientation in (SrRuO3)1/(SrTiO3)N superlattices, in which the magnetic easy axis of Ru spins is transformed from uniaxial <001> direction (N = 1 and 2) to eightfold <111> directions (N = 3, 4 and 5). This eightfold anisotropy enables 71 and 109 degrees spin switching in SrRuO3 monolayers, analogous to 71 and 109 degrees polarization switching in ferroelectric BiFeO3. First-principle calculations reveal that increasing the SrTiO3 layer thickness induces an emergent correlation-driven orbital ordering, tuning spin-orbit interactions and reorienting the SrRuO3 monolayer easy axis. Our work demonstrates that correlation effects can be exploited to substantially change spin-orbit interactions, stabilizing unprecedented properties in two-dimensional magnets and opening rich opportunities for low-power, multi-state device applications.

cond-mat.mtrl-sci↗

arXiv2020

Rational Design Protocols for Size-Based Particle Sorting Microdevices Using Symmetry-Induced Cyclical Dynamics

In this paper, we describe the unification and extension of multiple kinematic theories on the advection of colloidal particles through periodic obstacle lattices of arbitrary geometry and infinitesimally small obstacle size. We focus specifically on the particle displacement lateral to the flow direction (termed "deterministic lateral displacement") and the particle-obstacle interaction frequency, and develop novel methods for describing these as a function of particle size and lattice parameters for arbitrary lattice geometries for the first time in the literature. We then demonstrate design algorithms for microfluidic devices consisting of chained obstacle lattices of this type that approximate any lateral displacement function of size to arbitrary accuracy with respect to multiple optimization metrics, prove their validity mathematically, and compare the generated results favorably to designs in the literature with respect to metrics such as accuracy, device size, and complexity.

physics.comp-ph↗

arXiv2019

Interfacial-Redox-Induced Tuning of Superconductivity in YBa$_{2}$Cu$_{3}$O$_{7-δ}$

Solid state ionic approaches for modifying ion distributions in getter/oxide heterostructures offer exciting potentials to control material properties. Here we report a simple, scalable approach allowing for total control of the superconducting transition in optimally doped YBa$_{2}$Cu$_{3}$O$_{7-δ}$ (YBCO) films via a chemically-driven ionic migration mechanism. Using a thin Gd capping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films, oxygen is found to leach from deep within the YBCO. Progressive reduction of the superconducting transition is observed, with complete suppression possible for a sufficiently thick Gd layer. These effects arise from the combined impact of redox-driven electron doping and modification of the YBCO microstructure due to oxygen migration and depletion. This work demonstrates an effective ionic control of superconductivity in oxides, an interface induced effect that goes well into the quasi-bulk regime, opening up possibilities for electric field manipulation.

cond-mat.mtrl-sci↗

arXiv2019

Using Acoustic Perturbations to Dynamically Tune Shear Thickening in Colloidal Suspensions

Colloidal suspensions in industrial processes often exhibit shear thickening that is difficult to control actively. Here, we use piezoelectric transducers to apply acoustic perturbations to dynamically tune the suspension viscosity in the shear-thickening regime. We attribute the mechanism of dethickening to the disruption of shear-induced force chains via perturbations that are large relative to the particle roughness scale. The ease with which this technique can be adapted to various flow geometries makes it a powerful tool for actively controlling suspension flow properties and investigating system dynamics.

cond-mat.soft↗

arXiv2019

Three-dimensional numerical modeling of surface acoustic wave devices: Acoustophoresis of micro- and nanoparticles including streaming

Surface acoustic wave (SAW) devices form an important class of acoustofluidic devices, in which the acoustic waves are generated and propagate along the surface of a piezoelectric substrate. Despite their wide-spread use, only a few fully three-dimensional (3D) numerical simulations have been presented in the literature. In this paper, we present a 3D numerical simulation taking into account the electromechanical fields of the piezoelectric SAW device, the acoustic displacement field in the attached elastic material, in which the liquid-filled microchannel is embedded, the acoustic fields inside the microchannel, as well as the resulting acoustic radiation force and streaming-induced drag force acting on micro- and nanoparticles suspended in the microchannel. A specific device design is presented, for which the numerical predictions of the acoustic resonances and the acoustophoretic repsonse of suspended microparticles in 3D are successfully compared with experimental observations. The simulation provides a physical explanation of the the observed qualitative difference between devices with an acoustically soft and hard lid in terms of traveling and standing waves, respectively. The simulations also correctly predict the existence and position of the observed in-plane streaming flow rolls. The presented simulation model may be useful in the development of SAW devices optimized for various acoustofluidic tasks.

physics.app-ph↗

arXiv2019

Spin Seebeck imaging of spin-torque switching in antiferromagnetic Pt/NiO heterostructures

As electrical control of Néel order opens the door to reliable antiferromagnetic spintronic devices, understanding the microscopic mechanisms of antiferromagnetic switching is crucial. Spatially-resolved studies are necessary to distinguish multiple nonuniform switching mechanisms; however, progress has been hindered by the lack of tabletop techniques to image the Néel order. We demonstrate spin Seebeck microscopy as a sensitive, table-top method for imaging antiferromagnetic order in thin films, and apply this technique to study spin-torque switching in NiO/Pt and Pt/NiO/Pt heterostructures. We establish the interfacial antiferromagnetic spin Seebeck effect in NiO as a probe of surface Néel order, resolving antiferromagnetic spin domains within crystalline twin domains. By imaging before and after applying current-induced spin torque, we resolve spin domain rotation and domain wall motion, acting simultaneously. We correlate the changes in spin Seebeck images with electrical measurements of the average Néel orientation through the spin Hall magnetoresistance, confirming that we image antiferromagnetic order.

cond-mat.mtrl-sci↗

arXiv2019

Exploring interfacial exchange coupling and sublattice effect in heavy metal/ferrimagnetic insulator heterostructures using Hall measurements, x-ray magnetic circular dichroism, and neutron reflectometry

We use temperature-dependent Hall measurements to identify contributions of spin Hall, magnetic proximity, and sublattice effects to the anomalous Hall signal in heavy metal/ferrimagnetic insulator heterostructures with perpendicular magnetic anisotropy. This approach enables detection of both the magnetic proximity effect onset temperature and the magnetization compensation temperature and provides essential information regarding the interfacial exchange coupling. Onset of a magnetic proximity effect yields a local extremum in the temperature-dependent anomalous Hall signal, which occurs at higher temperature as magnetic insulator thickness increases. This magnetic proximity effect onset occurs at much higher temperature in Pt than W. The magnetization compensation point is identified by a sharp anomalous Hall sign change and divergent coercive field. We directly probe the magnetic proximity effect using x-ray magnetic circular dichroism and polarized neutron reflectometry, which reveal an antiferromagnetic coupling between W and the magnetic insulator. Finally, we summarize the exchange-coupling configurations and the anomalous Hall-effect sign of the magnetized heavy metal in various heavy metal/magnetic insulator heterostructures.

cond-mat.mtrl-sci↗

arXiv2019

Nanoscale ferroelastic twins formed in strained LaCoO3 films

The coexistence and coupling of ferroelasticity and magnetic ordering in a single material offers a great opportunity to realize novel devices with multiple tuning knobs. Complex oxides are a particularly promising class of materials to find multiferroic interactions as they often possess rich phase diagrams and the interactions are very sensitive to external perturbations. Still, there are very few examples of these systems. Here we report the observation of twinning domains in ferroelastic LaCoO3 epitaxial thin films and their geometric control of structural symmetry that are intimately linked to the material electronic and magnetic states. A unidirectional structural modulation is achieved by selective choice of substrates possessing two-fold rotational symmetry. This modulation perturbs the crystal field splitting energy, leading to unexpected in plane anisotropy of orbital configuration and magnetization. These findings demonstrate the utilization of structural modulation to control multiferroic interactions and may enable a great potential for stimulation of exotic phenomena through artificial domain engineering.

cond-mat.mtrl-sci↗

arXiv2018

Ionic Tuning of Cobaltites at the Nanoscale

Control of materials through custom design of ionic distributions represents a powerful new approach to develop future technologies ranging from spintronic logic and memory devices to energy storage. Perovskites have shown particular promise for ionic devices due to their high ion mobility and sensitivity to chemical stoichiometry. In this work, we demonstrate a solid-state approach to control of ionic distributions in (La,Sr)CoO$_{3}$ thin films. Depositing a Gd capping layer on the perovskite film, oxygen is controllably extracted from the structure, up-to 0.5 O/u.c. throughout the entire 36 nm thickness. Commensurate with the oxygen extraction, the Co valence state and saturation magnetization show a smooth continuous variation. In contrast, magnetoresistance measurements show no-change in the magnetic anisotropy and a rapid increase in the resistivity over the same range of oxygen stoichiometry. These results suggest significant phase separation, with metallic ferromagnetic regions and oxygen-deficient, insulating, non-ferromagnetic regions, forming percolated networks. Indeed, X-ray diffraction identifies oxygen-vacancy ordering, including transformation to a brownmillerite crystal structure. The unexpected transformation to the brownmillerite phase at ambient temperature is further confirmed by high-resolution scanning transmission electron microscopy which shows significant structural - and correspondingly chemical - phase separation. This work demonstrates room-temperature ionic control of magnetism, electrical resistivity, and crystalline structure in a 36 nm thick film, presenting new opportunities for ionic devices that leverage multiple material functionalities.

cond-mat.mtrl-sci↗

arXiv2016

Topological transitions induced by antiferromagnetism in a thin-film topological insulator

Ferromagnetism in topological insulators (TIs) opens a topologically non-trivial exchange band gap, providing an exciting platform to manipulate the topological order through an external magnetic field. Here, we experimentally show that the surface of an antiferromagnetic thin film can independently control the topological order of the top and the bottom surface states of a TI thin film through proximity couplings. During the magnetization reversal in a field scan, two intermediate spin configurations stem from unsynchronized magnetic switchings of the top and the bottom AFM/TI interfaces. These magnetic configurations are shown to result in new topological phases with non-zero Chern numbers for each surface, introducing two counter-propagating chiral edge modes inside the exchange gap. This change in the number of transport channels, as the result of the topological transitions, induces antisymmetric magneto-resistance spikes during the magnetization reversal. With the high Neel ordering temperature provided by the antiferromagnetic layers, the signature of the induced topological transition persists in transport measurements up to a temperature of around 90 K, a factor of three over the Curie temperature in a typical magnetically doped TI thin film.

cond-mat.mes-hall↗