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Brangham, Jack T.

Publications and source records attributed to Brangham, Jack T..

8 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Long lifetime of thermally excited magnons in bulk yttrium iron garnet

Spin currents are generated within the bulk of magnetic materials due to heat flow, an effect called intrinsic spin Seebeck. This bulk bosonic spin current consists of a diffusing thermal magnon cloud, parametrized by the magnon chemical potential (μ m ), with a diffusion length of several microns in yttrium iron garnet (YIG). Transient optothermal measurements of the spin-Seebeck effect (SSE) as a function of temperature reveal the time evolution of μ m due to intrinsic SSE in YIG. The interface SSE develops at times <2 ns while the intrinsic SSE signal continues to evolve at times >500 μs, dominating the temperature dependence of SSE in bulk YIG. Time-dependent SSE data are fit to a multitemperature model of coupled spin/heat transport using the finite-element method (FEM), where the magnon spin lifetime (τ) and magnon-phonon thermalization time (τ mp ) are used as fit parameters. From 300 to 4 K, τ mp ) varies from 1 to 10 ns, whereas tau varies from 2 to 60 μ s with the spin lifetime peaking at 90 K. At low temperature, a reduction in τ is observed consistent with impurity relaxation reported in ferromagnetic resonance measurements. Finally, these results demonstrate that the thermal magnon cloud in YIG contains extremely low-frequency magnons (similar to 10 GHz), providing spectral insight to the microscopic scattering processes involved in magnon spin/heat diffusion.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Nonsinusoidal angular dependence of FMR-driven spin current across an antiferromagnet in Y 3 F e 5 O 12 / NiO / Pt trilayers

Out-of-plane angular (θH) dependence of inverse spin Hall effect (ISHE) voltage (V ISHE ) generated by spin pumping has been investigated in Y 3 Fe 5 O 12 (YIG)/NiO/Pt trilayers. A simple sinusoidal angular dependence of V ISHE has been viewed as a signature of spin pumping. Surprisingly, we observe an extensive plateau in the V ISHE vs. θH plots with a pronounced peak feature at θH ~ 45 to 60 degrees when the measurement temperature is close to the Néel temperature (T N ) of NiO. This phenomenon can be understood as arising from the competition between the exchange coupling at the YIG/NiO interface, the easy-plane and in-plane easy-axis anisotropies of NiO, and the effect of the applied magnetic field. We conclude, while insulating antiferromagnetic films can efficiently transmit spin currents and show promise for integration in spintronic devices, the underlying physics of spin ordering and dynamics is richer than currently understood.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Thickness and angular dependent ferromagnetic resonance of ultra-low damping Co 25 Fe 75 epitaxial films

Metallic ferromagnets with ultra-low damping are highly desirable for charge-based spintronic applications. In this work, we systematically investigate the magnetic dynamics of Co 25 Fe 75 epitaxial films with a Gilbert damping constant as low as 7.1×10 -4 . The in-plane angular dependence of ferromagnetic resonance (FMR) was measured on various thicknesses Co 25 Fe 75 films grown on MgO and MgAl 2 O 4 , from which the mechanisms for FMR linewidth broadening can be distinguished and quantified. The thickness dependencies of the magnetic anisotropy and inhomogeneous broadening of the linewidth are good indicators of crystal quality and magnet uniformity. Additionally, it is shown that anisotropic two-magnon scattering is induced by defects at the surfaces.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Metallic ferromagnetic films with magnetic damping under 1.4 × 10 -3

Low-damping magnetic materials have been widely used in microwave and spintronic applications because of their low energy loss and high sensitivity. While the Gilbert damping constant can reach 10 -4 to 10 -5 in some insulating ferromagnets, metallic ferromagnets generally have larger damping due to magnon scattering by conduction electrons. Meanwhile, low-damping metallic ferromagnets are desired for charge-based spintronic devices. In this article, we report the growth of Co 25 Fe 75 epitaxial films with excellent crystalline quality evident by the clear Laue oscillations and exceptionally narrow rocking curve in the X-ray diffraction scans as well as from scanning transmission electron microscopy. Remarkably, the Co 25 Fe 75 epitaxial films exhibit a damping constant <1.4 × 10 -3 , which is comparable to the values for some high-quality Y 3 Fe 5 O 12 films. This record low damping for metallic ferromagnets offers new opportunities for charge-based applications such as spin-transfer-torque-induced switching and magnetic oscillations.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2017

Strong Modulation of Spin Currents in Bilayer Graphene by Static and Fluctuating Proximity Exchange Fields

Two-dimensional materials provide a unique platform to explore the full potential of magnetic proximity-driven phenomena, which can be further used for applications in next-generation spintronic devices. Of particular interest is to understand and control spin currents in graphene by the magnetic exchange field of a nearby ferromagnetic material in graphene–ferromagnetic-insulator (FMI) heterostructures. Here, we present the experimental study showing the strong modulation of spin currents in graphene layers by controlling the direction of the exchange field due to FMI magnetization. Owing to clean interfaces, a strong magnetic exchange coupling leads to the experimental observation of complete spin modulation at low externally applied magnetic fields in short graphene channels. Additionally, we discover that the graphene spin current can be fully dephased by randomly fluctuating exchange fields. This is manifested as an unusually strong temperature dependence of the nonlocal spin signals in graphene, which is due to spin relaxation by thermally induced transverse fluctuations of the FMI magnetization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Exceptionally high magnetization of stoichiometric Y 3 Fe 5 O 12 epitaxial films grown on Gd 3 Ga 5 O 12

The saturation magnetization of Y 3 Fe 5 O 12 (YIG) epitaxial films 4 to 250 nm in thickness has been determined by complementary measurements including the angular and frequency dependencies of the ferromagnetic resonance fields as well as magnetometry measurements. The YIG films exhibit state-of-the-art crystalline quality, proper stoichiometry, and pure Fe 3+ valence state. The values of YIG magnetization obtained from all the techniques significantly exceed previously reported values for single crystal YIG and the theoretical maximum. This enhancement of magnetization, not attributable to off-stoichiometry or other defects in YIG, thus opens opportunities for tuning magnetic properties in epitaxial films of magnetic insulators.

36 MATERIALS SCIENCE↗