Biomedical subjects

D J Keavney

Publications and source records attributed to D J Keavney.

3 recordsLinked to original sources

PubMed2006

Magnetic vortex core dynamics in cylindrical ferromagnetic dots.

We report direct imaging by means of x-ray photoemission electron microscopy of the dynamics of magnetic vortices confined in micron-sized circular permalloy dots that are 30 nm thick. The vortex core positions oscillate on a 10 ns time scale in a self-induced magnetostatic potential well after the in-plane magnetic field is turned off. The measured oscillation frequencies as a function of the aspect ratio of the dots are in agreement with theoretical calculations presented for the same geometry.

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PubMed2005

Role of defect sites and Ga polarization in the magnetism of Mn-doped GaN.

We report a study of the Mn local structure, magnetism, and Ga moments in molecular beam epitaxy grown Mn-doped GaN films. Using x-ray absorption spectroscopy and magnetic circular dichroism, we find two distinct Mn sites and a Ga moment antiparallel to Mn. First-principles calculations reproduce this phenomenology and indicate that Mn preferentially populates Ga sites neighboring N split interstitial defects. These results show that defects may strongly affect the Mn ordering and magnetism, and that the GaN valence band is polarized, providing a long-range ferromagnetic ordering mechanism for Ga1-xMnxN.

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PubMed2003

Element resolved spin configuration in ferromagnetic manganese-doped gallium arsenide.

We report induced Ga and As moments in ferromagnetic Ga(1-x)MnxAs detected using x-ray magnetic circular dichroism at the Mn, Ga, and As L(3,2) edges. Across a broad composition range, we find As and Ga dichroism signals which indicate an As 4s moment coupled antiparallel to the Mn 3d moment, and a smaller parallel Ga 4s moment. The Ga moment follows that of Mn in both doping and temperature dependence. These results are consistent with recent predictions of induced GaAs host moments and support the model of carrier-mediated ferromagnetic ordering involving As-derived valence band states.

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