We report ferromagnetic resonance experiments on Ga1−xMnxAs thin films. For the dc magnetic field perpendicular to the sample plane, we observe up to eight distinct resonances, which we attribute to spin wave modes. To account for the spacing of the resonances, we infer a linear gradient in the magnetic properties, which is ascribed to a linear variation of the uniaxial magnetic anisotropy with film thickness. Values of D=(1±0.4)×10−9 Oe cm2 for the spin stiffness and JMnMn≈1 meV for the exchange integral between Mn spins are obtained.
We show that upon exposure to a remote dc hydrogen plasma, the magnetic and electronic properties of the dilute magnetic semiconductor Ga1-xMnxAs change qualitatively. While the as-grown Ga1-xMnxAs thin films are ferromagnetic at temperatures T less, similar 70 K, the samples are found to be paramagnetic after the hydrogenation, with a Brillouin-type magnetization curve even at T=2 K. Comparing magnetization and electronic transport measurements, we conclude that the density of free holes p is significantly reduced by the plasma process, while the density of Mn magnetic moments does not change.
The vibrational and electronic properties of Ga 1Ϫx Mn x As layers with Mn fractions 0рxр2.8%, grown on GaAs͑001͒ substrates by low-temperature molecular-beam epitaxy, are investigated by micro-Raman spectroscopy and far-infrared ͑FIR͒ reflectance spectroscopy. The Raman and FIR spectra are strongly affected by the formation of a coupled mode of the longitudinal optical phonon and the hole plasmon. The spectral line shapes are modeled using a dielectric function where intraband and interband transitions of free holes are included. In addition to the coupled mode, the contributions of a surface depletion layer as well as a symmetry forbidden TO phonon have to be taken into account for the Raman spectra. Values for the hole densities are estimated from a full line-shape analysis of the measured spectra. Annealing at temperatures between 250 and 500°C results in a decrease of the hole density with increasing annealing temperature and total annealing time. Simultaneously, a reduction of the fraction of Mn atoms on Ga lattice sites is deduced from high-resolution x-ray diffraction.
The effects of hydrogen and deuterium on ferromagnetic GaAs doped with high concentrations of Mn (≈1021 cm−3) are studied. Secondary ion mass spectroscopy depth profiles show that D is incorporated in the same concentration as Mn. The epilayers change from metallic to semiconducting behavior upon hydrogenation. Fourier transform infrared absorption measurements show the As–H and As–D local vibrational modes characteristic for the complexes of hydrogen with group-II acceptors in GaAs.
The effect of annealing at 250 • C on the carrier depth profile, Mn distribution, electrical conductivity, and Curie temperature of (Ga,Mn)As layers with thicknesses ≥ 200 nm, grown by molecularbeam epitaxy at low temperatures, is studied by a variety of analytical methods. The vertical gradient in hole concentration, revealed by electrochemical capacitance-voltage profiling, is shown to play a key role in the understanding of conductivity and magnetization data. The gradient, basically already present in as-grown samples, is strongly influenced by post-growth annealing. From secondary ion mass spectroscopy it can be concluded that, at least in thick layers, the change in carrier depth profile and thus in conductivity is not primarily due to out-diffusion of Mn interstitials during annealing. Two alternative possible models are discussed.
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