S. Frank

Spin wave resonance in Ga1−xMnxAs

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.

Magnetoresistance in epitaxially grown degenerate ZnO thin films

The magnetoresistance of high-quality epitaxial doped ZnO(:Ga) thin films with various electron concentrations ranging from 3.2×1018 to 1.3×1020cm−3 has been measured. All samples investigated exhibit a negative magnetoresistance at low magnetic fields. Its magnitude systematically depends on carrier concentration and temperature. Low-doped samples switch the sign of the magnetoresistance and the conventional positive component dominates at high fields, whereas highly doped degenerate samples only show a negative component up to fields of 14.5 T. Therefore, the data are analyzed as the sum of a positive and negative contribution to the magnetoresistance applying a semiempirical expression to describe the observed behavior. The model takes into account third-order s–d exchange Hamiltonians describing the negative part and a two-band model for the positive contribution. Least-squares fits to the data are presented. Theory and experiment are in excellent agreement.

Curie temperature and carrier concentration gradients in epitaxy-grown Ga1−xMnxAs layers

We report on detailed investigations of the electronic and magnetic properties of ferromagnetic Ga1−xMnxAs layers, which have been fabricated by low-temperature molecular-beam epitaxy. Superconducting quantum interference device measurements reveal a decrease of the Curie temperature from the surface to the Ga1−xMnxAs/GaAs interface. While high-resolution x-ray diffraction clearly shows a homogeneous Mn distribution, a pronounced decrease of the carrier concentration from the surface towards the Ga1−xMnxAs/GaAs interface has been found by Raman spectroscopy as well as electrochemical capacitance–voltage profiling. The gradient in Curie temperature seems to be a general feature of Ga1−xMnxAs layers grown at low temperature. Possible explanations are discussed.

Effect of annealing on the depth profile of hole concentration in (Ga,Mn)As

The effect of annealing at

Ferromagnetic resonance in Ga1-xMnxAS

250\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}

Growth of GaMnAs under near-stoichiometric conditions

on the carrier depth profile, Mn distribution, electrical conductivity, and Curie temperature of (Ga,Mn)As layers with thicknesses

Enhancement of the Curie temperature in GaMnAs∕InGaMnAs superlattices

\ensuremath{\geqslant}200\phantom{\rule{0.3em}{0ex}}\mathrm{nm}

Epitaxy and characterization of GaMn(N)As for spin electronics

, grown by molecular-beam 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.

Electronic and magnetic properties of GaMnAs: annealing effects

We report on ferromagnetic resonance measurements of Ga1−xMnxAs thin films with Mn contents 0.022 ≤ x ≤ 0.051. For x ≥ 0.036 and the external magnetic field normal to the thin film, we observe several resonances, which we identify as spin wave resonances. The non-quadratic mode spacing can be consistently explained by a linear gradient in the magnetic properties of the films. From the measurements, the exchange constant A can be deduced for different Mn contents x.

DEPTH DEPENDENCE OF MAGNETIC AND ELECTRIC PROPERTIES IN GaMnAs LAYERS

We studied the effect of the V∕III flux ratio and substrate temperature on magnetotransport properties and lattice parameters of Ga0.96Mn0.04As grown by molecular-beam epitaxy. For all the substrate temperatures, the conductivities and Curie temperatures of the layers were found to increase as the V∕III flux ratio approaches 1. A Curie temperature as high as 95 K was achieved for the Ga0.96Mn0.04As samples grown at 240 °C and a V∕III ratio of about 1.5. The lattice parameter of Ga0.96Mn0.04As increased with decreasing V∕III ratio and/or increasing growth temperature. Possible reasons for the effect of the V∕III ratio on the magnetotransport properties and lattice parameter of GaMnAs are discussed.