arXiv subjects

Daniel B. Gopman

Publications and source records attributed to Daniel B. Gopman.

9 recordsLinked to original sources

arXiv2026

Interfacial exchange and magnetostatic coupling in a CoFeB/Thulium Iron Garnet heterostructure

We investigate the exchange coupling between a ferrimagnetic insulator (FI) thulium iron garnet (TmIG) deposited on a gadolinium gallium garnet (GGG) substrate, which shows perpendicular magnetic anisotropy and a ferromagnetic metal (FMM) stack with oxide capping that consists of CoFeB(x)/W(0.4 nm)/CoFeB(0.8 nm)/MgO(1 nm)/W(5 nm). Vibrating sample magnetometry, magneto-optical Kerr microscopy and first-order reversal curve studies coupled with micromagnetic simulations are used to analyze the coupling between these layers. Strong interlayer exchange coupling and magnetostatic coupling are observed in the samples where the relative strength between these interactions can be controlled by varying the thickness of the CoFeB layer. Films with CoFeB thickness x <=1 nm are strongly exchange coupled, whereas the magnetostatic coupling dominates when the thickness is increased to 3 nm or above. These findings have important implications towards realizing fast and energy efficient spintronic devices using a FI, as its coupling to the FMM layer can be used for effective electrical read out of the magnetic state of the FI.

cond-mat.mtrl-sci↗

arXiv2025

Strain Mediated Voltage Control of Magnetic Anisotropy and Magnetization Reversal in Bismuth Substituted Yttrium Iron Garnet Films and Meso-structures

We report on magnetic anisotropy modulation in Bismuth substituted Yttrium Iron Garnet (Bi-YIG) thin films and mesoscale patterned structures deposited on a PMN-PT substrate with the application of voltage-induced strain. The Bi content is selected for low coercivity and higher magnetostriction than that of YIG, yielding significant changes in the hysteresis loops through the magnetoelastic effect. The piezoelectric substrate is poled along its thickness, which is the [011] direction, by applying a voltage across the PMN-PT/SiO2/Bi-YIG/Pt heterostructure. In-situ magneto-optical Kerr effect microscopy (MOKE) shows the modulation of magnetic anisotropy with voltage-induced strain. Furthermore, voltage control of the magnetic domain state of the Bi-YIG film at a fixed magnetic field produces a 90° switching of the magnetization easy axis above a threshold voltage. The magnetoelectric coefficient of the heterostructure is 1.05x10^(-7)s/m which is competitive with that of other ferromagnetic oxide films on ferroelectric substrates such as La0.67Sr0.33MnO3/PMNPT and YIG/PMN-PZT. Voltage-control of magnetization reversal fields in 5-30 microns wide dots and racetracks of Bi-YIG show potential for energy efficient non-volatile memory and neuromorphic computing devices.

cond-mat.mtrl-sci↗

arXiv2022

Perpendicular magnetic anisotropy, tunneling magnetoresistance and spin-transfer torque effect in magnetic tunnel junctions with Nb layers

Nb and its compounds are widely used in quantum computing due to their high superconducting transition temperatures and high critical fields. Devices that combine superconducting performance and spintronic non-volatility could deliver unique functionality. Here we report the study of magnetic tunnel junctions with Nb as the heavy metal layers. An interfacial perpendicular magnetic anisotropy energy density of 1.85 mJ/m2 was obtained in Nb/CoFeB/MgO heterostructures. The tunneling magnetoresistance was evaluated in junctions with different thickness combinations and different annealing conditions. An optimized magnetoresistance of 120% was obtained at room temperature, with a damping parameter of 0.011 determined by ferromagnetic resonance. In addition, spin-transfer torque switching has also been successfully observed in these junctions with a quasistatic switching current density of 7.3*10^5 A/cm2.

physics.app-ph↗

arXiv2018

Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal

When a polarized light beam is incident upon the surface of a magnetic material, the reflected light undergoes a polarization rotation. This magneto-optical Kerr effect (MOKE) has been intensively studied in a variety of ferro- and ferrimagnetic materials because it provides a powerful probe for electronic and magnetic properties as well as for various applications including magneto-optical recording. Recently, there has been a surge of interest in antiferromagnets (AFMs) as prospective spintronic materials for high-density and ultrafast memory devices, owing to their vanishingly small stray field and orders of magnitude faster spin dynamics compared to their ferromagnetic counterparts. In fact, the MOKE has proven useful for the study and application of the antiferromagnetic (AF) state. Although limited to insulators, certain types of AFMs are known to exhibit a large MOKE, as they are weak ferromagnets due to canting of the otherwise collinear spin structure. Here we report the first observation of a large MOKE signal in an AF metal at room temperature. In particular, we find that despite a vanishingly small magnetization of $M \sim$0.002 $μ_{\rm B}$/Mn, the non-collinear AF metal Mn$_3$Sn exhibits a large zero-field MOKE with a polar Kerr rotation angle of 20 milli-degrees, comparable to ferromagnetic metals. Our first-principles calculations have clarified that ferroic ordering of magnetic octupoles in the non-collinear Neel state may cause a large MOKE even in its fully compensated AF state without spin magnetization. This large MOKE further allows imaging of the magnetic octupole domains and their reversal induced by magnetic field. The observation of a large MOKE in an AF metal should open new avenues for the study of domain dynamics as well as spintronics using AFMs.

cond-mat.mtrl-sci↗

arXiv2016

Strain-assisted magnetization reversal in Co/Ni multilayers with perpendicular magnetic anisotropy

Multifunctional materials composed of ultrathin magnetic films with perpendicular magnetic anisotropy combined with ferroelectric substrates represent a new approach toward low power, fast, high density spintronics. Here we demonstrate Co/Ni multilayered films with tunable saturation magnetization and perpendicular anisotropy grown directly on ferroelectric PZT [Pb(Zr_xTi_1-x)O_3] substrate plates. Electric fields up to +/- 2 MV/m expand the PZT by 0.1% and generate at least 0.02% in-plane compression in the Co/Ni multilayered film. Modifying the strain with a voltage can reduce the coercive field by over 30%. We also demonstrate that alternating in-plane tensile and compressive strains (less than 0.01%) can be used to propagate magnetic domain walls. This ability to manipulate high anisotropy magnetic thin films could prove useful for lowering the switching energy for magnetic elements in future voltage-controlled spintronic devices.

cond-mat.mes-hall↗

arXiv2013

Bimodal switching field distributions in all-perpendicular spin-valve nanopillars

Switching field measurements of the free layer element of 75 nm diameter spin-valve nanopillars reveal a bimodal distribution of switching fields at low temperatures (below 100 K). This result is inconsistent with a model of thermal activation over a single perpendicular anisotropy barrier. The correlation between antiparallel to parallel and parallel to antiparallel switching fields increases to nearly 50% at low temperatures. This reflects random fluctuation of the shift of the free layer hysteresis loop between two different magnitudes, which may originate from changes in the dipole field from the polarizing layer. The magnitude of the loop shift changes by 25% and is correlated to transitions of the spin-valve into an antiparallel configuration.

cond-mat.mes-hall↗

arXiv2012

FLUXCAP: A flux-coupled ac/dc magnetizing device

We report on an instrument for applying ac and dc magnetic fields by capturing the flux from a rotating permanent magnet and projecting it between two adjustable pole pieces. This can be an alternative to standard electromagnets for experiments with small samples or in probe stations in which an applied magnetic field is needed locally, with advantages that include a compact form-factor, very low power requirements and dissipation as well as fast field sweep rates. This flux capture instrument (FLUXCAP) can produce fields from -400 to +400 mT, with field resolution less than 1 mT. It generates static magnetic fields as well as ramped fields, with ramping rates as high as 10 T/s. We demonstrate the use of this apparatus for studying the magnetotransport properties of spin-valve nanopillars, a nanoscale device that exhibits giant magnetoresistance.

physics.ins-det↗

arXiv2012

Audio Cards for High-Resolution and Economical Electronic Transport Studies

We report on a technique for determining electronic transport properties using commercially available audio cards. Using a typical 24-bit audio card simultaneously as a sine wave generator and a narrow bandwidth ac voltmeter, we show the spectral purity of the analog-to-digital and digital-to-analog conversion stages, including an effective number of bits greater than 16 and dynamic range better than 110 dB. We present two circuits for transport studies using audio cards: a basic circuit using the analog input to sense the voltage generated across a device due to the signal generated simultaneously by the analog output; and a digitally-compensated bridge to compensate for nonlinear behavior of low impedance devices. The basic circuit also functions as a high performance digital lock-in amplifier. We demonstrate the application of an audio card for studying the transport properties of spin-valve nanopillars, a two-terminal device that exhibits Giant Magnetoresistance (GMR) and whose nominal impedance can be switched between two levels by applied magnetic fields and by currents applied by the audio card.

physics.ins-det↗

arXiv2011

Asymmetric switching behavior in perpendicularly magnetized spin-valve nanopillars due to the polarizer dipole field

We report the free layer switching field distributions of spin-valve nanopillars with perpendicular magnetization. While the distributions are consistent with a thermal activation model, they show a strong asymmetry between the parallel to antiparallel and the reverse transition, with energy barriers more than 50% higher for the parallel to antiparallel transitions. The inhomogeneous dipolar field from the polarizer is demonstrated to be at the origin of this symmetry breaking. Interestingly, the symmetry is restored for devices with a lithographically defined notch pair removed from the midpoint of the pillar cross-section along the ellipse long axis. These results have important implications for the thermal stability of perpendicular magnetized MRAM bit cells.

cond-mat.mes-hall↗