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Hoffmann, Axel

Publications and source records attributed to Hoffmann, Axel.

116 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Nanometer-Thick Yttrium Iron Garnet Films With Extremely Low Damping

Yttrium iron garnet (YIG) films that are in the nanometer thickness range and show extremely low damping are reported. The films were deposited via sputtering at room temperature and were then annealed in O 2 at high temperature. A 22-nm-thick YIG film showed a Gilbert damping constant α = (8.58 ± 0.21) × 10 -5 , which represents the lowest damping ever reported for nanometer-thick magnetic films. The film had a gyromagnetic ratio of |γ| = 2.83 MHz/Oe and a saturation induction of 4πM s = 1766 G, which are both very close to those of single-crystal YIG bulk materials. The film had a very smooth surface, with an rms surface roughness of about 0.13 nm.

current topics↗
DOE OSTI2013

Determination of the Pt spin diffusion length by spin-pumping and spin Hall effect

The spin diffusion length of Pt at room temperature and at 8 K is experimentally determined via spin pumping and spin Hall effect in permalloy/Pt bilayers. Voltages generated during excitation of ferromagnetic resonance from the inverse spin Hall effect and anisotropic magnetoresistance effect were investigated with a broadband approach. Varying the Pt layer thickness gives rise to an evolution of the voltage line shape due to the superposition of the above two effects. By studying the ratio of the two voltage components with the Pt layer thickness, the spin diffusion length of Pt can be directly extracted. We obtain a spin diffusion length of ∼1.2 nm at room temperature and ∼1.6 nm at 8 K.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Ge doped GaN with controllable high carrier concentration for plasmonic applications

Controllable Ge doping in GaN is demonstrated for carrier concentrations of up to 2.4 × 10{sup 20} cm{sup −3}. Low temperature luminescence spectra from the highly doped samples reveal band gap renormalization and band filling (Burstein-Moss shift) in addition to a sharp transition. Infrared ellipsometry spectra demonstrate the existence of electron plasma with an energy around 3500 cm{sup −1} and a surface plasma with an energy around 2000 cm{sup −1}. These findings open possibilities for the application of highly doped GaN for plasmonic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Magnetization Dynamics Through Magnetoimpedance Effect in Isotropic Co 2 FeAl/Au/Co 2 FeAl Full-Heusler Alloy Trilayer Films

We investigate the magnetization dynamics in low damping parameter α systems by measuring the magnetoimpedance effect over a wide range of frequencies, from 0.1 to 3.0 GHz, in Co2FeAl/Au/Co2FeAl full-Heusler alloy trilayer films grown by magnetron sputtering on glass and MgO substrates. We show that the film produced on the glass substrate presents high magnetoimpedance performance, while that grown on the MgO substrate has low magnetoimpedance performance. Since both films are polycrystalline and have isotropic in-plane magnetic properties, we interpret the magnetoimpedance results in terms of the low damping parameter α and strain effects in the films. Thus, we verified that our films present good magnetoimpedance performance and showed that high performance can be achieved even in films with isotropic in-plane magnetic properties, since they present low damping parameter α.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Influence of the Thickness of the Ferro- and Antiferromagnetic Phases on Magnetic Properties in Epitaxial Heterostructures Based on Exchange Biased La-Ca-Mn-O System

Metal oxides show fascinating physical properties such as ferromagnetism, antiferromagnetism, high temperature superconductivity, ferroelectricity, or even multiferroicity. For many possible electronic applications as well as fundamental studies, it is essential to fabricate epitaxial layered films and multilayers of these materials having complex lattice structures with sharp interfaces, preserving epitaxiallity through the whole structure. We have grown these kind of oxide heterostructures on single crystal (001) oriented SrTiO3 substrates by using an in-situ DC sputtering technique at high oxygen pressures. Specifically, we report the study of magnetic and transport properties in ferromagnetic/antiferromagnetic, F/AF, heterostructures based on the Ca-doped lanthanum manganite system. We artificially grew La2/3Ca1/3MnO3/La1/3Ca2/3MnO3 heterostructures, maintaining constant the total thickness of the sample and systematically varying the thicknesses of the antiferromagnetic layer (tAF) and ferromagnetic layer (tF). Magnetization measurements indicate a dependence of Curie temperature, exchange bias field, and magnetoresistance behavior with the tAF/tF ratio.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Observation of microwave-assisted magnetization reversal in perpendicular recording media

This letter reports microwave-assisted magnetization reversal (MAMR) in a 700-Gbit/in2 perpendicular media sample. The microwave fields were applied by placing a coplanar waveguide on the media sample and feeding it with narrow microwave pulses. The switching states of the media grains were measured by magnetic force microscopy. For microwaves with a frequency close to the ferromagnetic resonance (FMR) frequency of the media, MAMR was observed for microwave power higher than a certain threshold. For microwaves with certain high power, MAMR was observed for a broad microwave frequency range which covers the FMR frequency and is centered below the FMR frequency.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Anti-phase domains in cubic GaN

The existence of anti-phase domains in cubic GaN grown on 3C-SiC/Si (001) substrates by plasma-assisted molecular beam epitaxy is reported. The influence of the 3C-SiC/Si (001) substrate morphology is studied with emphasis on the anti-phase domains (APDs). The GaN nucleation is governed by the APDs of the substrate, resulting in equal plane orientation and the same anti-phase boundaries. The presence of the APDs is independent of the GaN layer thickness. Atomic force microscopy surface analysis indicates lateral growth anisotropy of GaN facets in dependence of the APD orientation. This anisotropy can be linked to Ga and N face types of the {l_brace}111{r_brace} planes, similar to observations of anisotropic growth in 3C-SiC. In contrast to 3C-SiC, however, a difference in GaN phase composition for the two types of APDs can be measured by electron backscatter diffraction, {mu}-Raman and cathodoluminescence spectroscopy.

36 MATERIALS SCIENCE↗