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As a candidate material for applications such as magnetic memory, polycrystalline antiferromagnets offer the same robustness to external magnetic fields, THz spin dynamics, and lack of stray field as their single crystalline counterparts, but without the limitation of epitaxial growth and lattice matched substrates. Here, we first report the detection of the average Neel vector orientiation in polycrystalline NiO via spin Hall magnetoresistance (SMR). Secondly, by applying strain through a piezo-electric substrate, we reduce the critical magnetic field required to reach a saturation of the SMR signal, indicating a change of the anisotropy. Our results are consistent with polycrystalline NiO exhibiting a positive sign of the in-plane magnetostriction. This method of anisotropy-tuning offers an energy efficient, on-chip alternative to manipulate a polycrystalline antiferromagnets magnetic state.

Soft x-ray Magnetic Circular Dichroism spectroscopy at the Dy and Tb M 4 , 5 and the Fe L 2 , 3 edges was performed on a sputter deposited polycrystalline Terfenol-D (Tb0.3Dy0.7Fe2) film on sapphire substrates at temperatures from 100 to 300 K to evaluate the elementwise contribution to the magnetocrystalline anisotropy and coercive field. The elemental spin and orbital magnetic moments were calculated using the x-ray Magnetic Circular Dichroism sum rules. As temperatures decreased, the Tb and Fe moments plateau at 200 K with values of 7.6 μB/atom and 1.8 μB/atom, respectively, while the Dy moment increases to 8.9 μB/atom at 100 K. Between 300 and 200 K, the change in magnetic anisotropy is dominated by thermally induced magnetoelastic effects while for temperatures below 200 K magnetocrystalline anisotropy (MCA) changes are dominant. The MCA changes below 200 K appear to be due to increases in the Dy orbital moment with decreasing temperature in this temperature regime.

Programming magnetic fields with microscale control can enable automation at the scale of single cells ≈10 µm. Most magnetic materials provide a consistent magnetic field over time but the direction or field strength at the microscale is not easily modulated. However, magnetostrictive materials, when coupled with ferroelectric material (i.e., strain-mediated multiferroics), can undergo magnetization reorientation due to voltage-induced strain, promising refined control of magnetization at the micrometer-scale. This work demonstrates the largest single-domain microstructures (20 µm) of Terfenol-D (Tb0.3 Dy0.7 Fe1.92 ), a material that has the highest magnetostrictive strain of any known soft magnetoelastic material. These Terfenol-D microstructures enable controlled localization of magnetic beads with sub-micrometer precision. Magnetically labeled cells are captured by the field gradients generated from the single-domain microstructures without an external magnetic field. The magnetic state on these microstructures is switched through voltage-induced strain, as a result of the strain-mediated converse magnetoelectric effect, to release individual cells using a multiferroic approach. These electronically addressable micromagnets pave the way for parallelized multiferroics-based single-cell sorting under digital control for biotechnology applications.

Sputter-deposited thin film nitinol constructs with various micropatterns were fabricated to evaluate their effect on the vessel wall in vivo when used as a covering for commercially available stents. Thin film nitinol constructs were used to cover stents and deployed in non-diseased swine arteries. Swine were sacrificed after approximately four weeks and the thin film nitinol-covered stents were removed for histopathologic evaluation. Histopathology revealed differences in neointimal thickness that correlated with the thin film nitinol micropattern. Devices covered with thin film nitinol with a lateral × vertical length = 20 × 40 µm diamond pattern had minimal neointimal growth with well-organized cell architecture and little evidence of ongoing inflammation. Devices covered with thin film nitinol with smaller fenestrations exhibited a relatively thick neointimal layer with inflammation and larger fenestrations showed migration of inflammatory and smooth muscle cells through the micro fenestrations. This “proof-of-concept” study suggests that there may be an ideal thin film nitinol porosity and pore geometry to encourage endothelialization and incorporation of the device into the vessel wall. Future work will be needed to determine the optimal pore size and geometry to minimize neointimal proliferation and in-stent stenosis.

Frequency conversion is an essential tool in modern communication devices. Traditionally, frequency conversion is achieved through parametric coupling via nonlinear inductors or capacitors whose reactance is modulated by a carrier wave. In this study, nonlinear acoustic Lamb wave devices are explored for simultaneous signal filtering and frequency conversion. Three sets of interdigitated transducers are fabricated on a piezoelectric aluminum nitride (AlN) thin film to provide a carrier wave, a low-power signal wave, and to receive a frequency converted mixed wave. Two devices are fabricated and tested to demonstrate frequency upconversion and downconversion by utilizing mechanical nonlinearity of AlN, and the results are compared to a nonlinear circuit model. The nonlinear circuit model is used to link experimentally observed phenomenon to the acoustic material's intrinsic nonlinearity. The nonlinearity of AlN reaches a maximum of 2.8% with a carrier wave power at 28 dBm. An analytical model is used to predict device performance along with physical dimensions. These analytical results show that nonlinear acoustic mixers and filters can approach sub-millimeter sizes, which is orders-of-magnitude smaller than conventional structures using nonlinear inductors and capacitors.

The soft magnetic, microstructural, and magnetostrictive properties of Fe81Ga19 (FeGa) film sputter deposited onto 2.5-nm Ta, Cu, and Ni80Fe20 (NiFe) underlayers were investigated. The films deposited with an underlayer showed increased in-plane uniaxial anisotropy and a decrease in in-plane coercivity. The smallest coercivity was observed in FeGa deposited with a NiFe underlayer at 15 Oe, compared to 84 Oe for films deposited directly on Si. In addition, an effective Gilbert damping coefficient (αeff) as low as 0.044 was achieved for a 100-nm FeGa film with a NiFe underlayer. The coercivity and αeff were shown to decrease further as a function of FeGa film thickness. The FeGa films were also able to retain or increase their saturation magnetostriction when deposited on an underlayer. This enhancement is attributable to the impact of the underlayer to promote an increased (110) film texture and smaller grain size, which is correlated to the lattice match of the underlayer of the sputtered FeGa film. Among the underlayers studied, NiFe promoted the best enhancement in the soft magnetic properties for FeGa thin films, making it an attractive material for both strain-mediated magnetoelectric and microwave device applications.

Nanomagnetic logic (NML) has attracted attention during the last two decades due to its promise of high energy efficiency combined with non-volatility. Data transmission in NML relies on Bennett clocking through dipole interaction between neighboring nanomagnetic bits. This paper uses a fully coupled finite element model to simulate Bennett clocking based on strain-mediated multiferroic system for Ni, CoFeB and Terfenol-D with perpendicular magnetic anisotropies. Simulation results demonstrate that Terfenol-D system has the highest energy efficiency, which is 2 orders of magnitude more efficient than Ni and CoFeB. However, the high efficiency is associated with switching incoherency due to its large magnetostriction coefficient. It is also suggested that the CoFeB clocking system has lower bit-density than in Ni or Terfenol-D systems due to its large dipole coupling. Moreover, we demonstrate that the precessional perpendicular switching and the Bennett clocking can be achieved using the same strain-mediated multiferroic architecture with different voltage pulsing. This study opens new possibilities to an all-spin in-memory computing system.

An ultra-thin (∼2.5 nm) non-magnetic Cu underlayer was found to have a significant effect on the microstructure, magnetic softness, and magnetostriction of sputter-deposited Fe81Ga19 (FeGa) thin films. Compared to the experimental control where FeGa was deposited directly on Si without an underlayer, the presence of Cu increased the in-plane uniaxial anisotropy of FeGa and reduced the in-plane coercivity by nearly a factor of five. The effective Gilbert damping coefficient was also significantly reduced by a factor of four, between FeGa on Si and FeGa on a Cu underlayer. The FeGa films on Cu also retained a high saturation magnetostriction comparable to those without an underlayer. The enhancement of the desirable magnetic properties for microwave applications is attributed to the Cu underlayer, promoting the (110) film texture and increasing the compressive film strain. The results demonstrated that the structural control is viable to simultaneously achieve the necessary magnetic softness and magnetostriction in FeGa for integration in strain-mediated magnetoelectric and microwave devices.

In this paper we explore parametric amplification on the acoustic wave platform for the purpose of developing nonlinear devices such as amplifiers and mixers. The advantage of acoustic wave platform over conventional electromagnetic platform is that the former obtains great leverage on short wavelength and low loss of acoustic wave at radio frequency to achieve devices with small footprints and high quality factor. To realize efficient parametric amplification a nonlinear surface acoustic wave (SAW) grating is employed to couple acoustic waves through different frequencies. Novel implementation of nonlinear SAW grating by coupling nonlinear permittivity of Barium Strontium Titanite (BST) to mechanical stiffness of Lithium Niobate has been proved by both theory and simulation. Parametric gain is realized with optimal grating length and tunable permittivity of commercial BST thin film.

Control of the Neel vector in antiferromagnetic materials is one of the challenges preventing their use as active device components. Several methods have been investigated such as exchange bias, electric current, and spin injection, but little is known about strain-mediated anisotropy. This study of the antiferromagnetic L10-type MnX alloys MnIr, MnRh, MnNi, MnPd, and MnPt shows that a small amount of strain effectively rotates the direction of the Neel vector by 90 degrees for all of the materials. For MnIr, MnRh, MnNi, and MnPd, the Neel vector rotates within the basal plane. For MnPt, the Neel vector rotates from out-of-plane to in-plane under tensile strain. The effectiveness of strain control is quantified by a metric of efficiency and by direct calculation of the magnetostriction coefficients. The values of the magnetostriction coefficients are comparable with those from ferromagnetic materials. These results indicate that strain is a mechanism that can be exploited for control of the Neel vectors in this family of antiferromagnets., Comment: 10 pages including supplementary information. Currently in review at Applied Physics Letters

We report results of micro-Brillouin-Mandelstam light scattering spectroscopy of thermal magnons in the two-phase synthetic multiferroic structure consisting of a piezoelectric (PMN-PT) substrate and a Ni thin film with the thickness of 64 nm. The experimental data reveal the first two modes of the perpendicular standing spin waves (PSSW) spatially confined across the Ni thin film. A theoretical analysis of the frequency dependence of the PSSW peaks on the external magnetic field reveals the asymmetric boundary condition, i.e. pinning, for variable magnetization at different surfaces of the Ni thin film. The strain field induced by applying DC voltage to PMN-PT substrate leads to a down shift of PSSW mode frequency owing to the magneto-elastic effect in Ni, and corresponding changes in the spin wave resonance conditions. The observed non-monotonic dependence of the PSSW frequency on DC voltage is related to an abrupt change of the pinning parameter at certain values of the voltage. The obtained results are important for understanding the thermal magnon spectrum in ferromagnetic films and development of the low-power spin-wave devices., Comment: 15 pages, 6 figures

We used two different solid state synthesis routes in conjunction with Current Activated Pressure Assisted Densification (CAPAD) to produce lead-free 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (called BZT-BCT or BXT) ceramics. The materials are fully dense and have a wide range of grain sizes from nanocrystalline (90 nm) to more conventional micrometer size range (3 μm). We investigate the role of grain size and phase homogeneity on the dielectric and ferroelectric properties of BXT independent of porosity. One of the synthesis routes yields samples with comparably high switchable polarization, 2Pr at grain sizes ~7 times smaller (420 nm) than the former route.

Strain-coupled composite multiferroics, comprised of magnetostrictive and piezoelectric materials, promise tunable control of magnetism in applications such as rf resonators, sensors, and energy harvesters. Using polarized neutron reflectometry, the authors demonstrate that magnetostrictive and weakly magnetostrictive materials in a multilayer can be strain-coupled to an electric field over large length scales that enable designer functionality across a range of device applications.

The magnetoelastic behavior of multiferroic heterostructures-coupling of magnetic anisotropy or domain dynamics to structural deformations-has been intensively studied for developing materials for energy-efficient, spin-based applications. Here, we report on a large, interface-dominated magnetostriction in (Co/Ni)

Magnetic domain-wall motion driven by a voltage dissipates much less heat than by a current, but none of the existing reports have achieved speeds exceeding 100 m/s. Here phase-field and finite-element simulations were combined to study the dynamics of strain-mediated voltage-driven magnetic domain-wall motion in curved nanowires. Using a ring-shaped, rough-edged magnetic nanowire on top of a piezoelectric disk, we demonstrate a fast voltage-driven magnetic domain-wall motion with average velocity up to 550 m/s, which is comparable to current-driven wall velocity. An analytical theory is derived to describe the strain dependence of average magnetic domain-wall velocity. Moreover, one 180° domain-wall cycle around the ring dissipates an ultrasmall amount of heat, as small as 0.2 fJ, approximately 3 orders of magnitude smaller than those in current-driven cases. These findings suggest a new route toward developing high-speed, low-power-dissipation domain-wall spintronics.

Piezoelectric resonators have been generally modeled with the electrostatic approximation, which does not predict any electromagnetic (EM) wave behavior. In this research, we develop and validate a fully coupled finite-element (FE) model to study the interaction between elastodynamics and electrodynamics. We simulate the EM radiation from a vibrating piezoelectric resonator excited by harmonic shears via d 15 coupling. Results show that the resonator resembles an infinitesimal dipole antenna in terms of the induced EM fields. Parametric studies reveal that radiated power declines with increasing permittivity that can be explained with a modified KLM circuit. Moreover, there exists a theoretical optimum of radiated power at a fixed electromechanical coupling strength, which dictates both the permittivity and piezoelectric coefficients. The radiation efficiency of the piezoelectric resonator is found to be superior to a current-driven electric dipole of the same size without matching network and ohmic loss taken into account, suggesting potential benefit of manipulating electric polarizations to generate EM radiation at microscale.

Time-varying magnetic flux can be induced from the dynamic mechanical strain of acoustic waves in multiferroic devices that are comprised of piezoelectric and magnetostrictive material. Such devices can be used to create electromagnetic radiation and to alleviate the platform effect associated with low-profile conformal antennas. In this paper, a bulk acoustic wave (BAW)-mediated multiferroic antenna structure is proposed. Its potential for efficient radiation of electromagnetic waves is evaluated by analytically deriving the lower bound of its radiation quality factor ( $Q$ factor). A one-dimensional (1-D) multiscale finite-difference time-domain (FDTD) technique is developed to predict the bilateral, dynamic coupling between the acoustic waves and electromagnetic waves. The simulation shows a decaying stress profile in the BAW resonator structure, which implies that the radiation of the electromagnetic waves acts as a damping load to the acoustic resonance. The simulated radiation $Q$ factor matches well with the analytical derivations and the agreement validates both the operating principle of the proposed antenna and the FDTD algorithm developed. The study concludes that efficient antennas may be realized at GHz frequencies with thin film multiferroic material that has thicknesses of the order of $10^{-5}$ wavelength.

In this work, we experimentally demonstrate deterministic electrically driven, strain-mediated domain wall (DW) rotation in ferromagnetic Ni rings fabricated on piezoelectric [Pb(Mg1/3Nb2/3)O3]0.66-[PbTiO3]0.34 (PMN-PT) substrates. While simultaneously imaging the Ni rings with X-ray magnetic circular dichroism photoemission electron microscopy, an electric field is applied across the PMN-PT substrate that induces strain in the ring structures, driving DW rotation around the ring toward the dominant PMN-PT strain axis by the inverse magnetostriction effect. The DW rotation we observe is analytically predicted using a fully coupled micromagnetic/elastodynamic multiphysics simulation, which verifies that the experimental behavior is caused by the electrically generated strain in this multiferroic system. Finally, this DW rotation is used to capture and manipulate micrometer-scale magnetic beads in a fluidic environment to demonstrate a proof-of-concept energy-efficient pathway for multiferroic-based lab-on-a-chip applications.

Strain-coupled multiferroic heterostructures provide a path to energy-efficient, voltage-controlled magnetic nanoscale devices, a region where current-based methods of magnetic control suffer from Ohmic dissipation. Growing interest in highly magnetoelastic materials, such as Terfenol-D, prompts a more accurate understanding of their magnetization behavior. To address this need, we simulate the strain-induced magnetization change with two modeling methods: the commonly used unidirectional model and the recently developed bidirectional model. Unidirectional models account for magnetoelastic effects only, while bidirectional models account for both magnetoelastic and magnetostrictive effects. We found unidirectional models are on par with bidirectional models when describing the magnetic behavior in weakly magnetoelastic materials (e.g., Nickel), but the two models deviate when highly magnetoelastic materials (e.g., Terfenol-D) are introduced. These results suggest that magnetostrictive feedback is critical for modeling highly magnetoelastic materials, as opposed to weaker magnetoelastic materials, where we observe only minor differences between the two methods’ outputs. To our best knowledge, this work represents the first comparison of unidirectional and bidirectional modeling in composite multiferroic systems, demonstrating that back-coupling of magnetization to strain can inhibit formation and rotation of magnetic states, highlighting the need to revisit the assumption that unidirectional modeling always captures the necessary physics in strain-mediated multiferroics.