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We have characterized the structural and magnetic properties of low-temperature molecular-beam epitaxy grown Ge: Mn by means of high-resolution transmission electron microscopy ͑HR-TEM͒, energy dispersive x-ray spectroscopy, and superconducting quantum interference device ͑SQUID͒ magnetometry. We find a coherent incorporation of Mn 5 Ge 3 clusters in an epitaxially grown Ge: Mn matrix, which shows the characteristics of a diluted magnetic semiconductor phase of Mn-doped Ge. The clusters are preferentially oriented with the hexagonal ͓0001͔ direction parallel to the ͓001͔ growth direction of the Ge: Mn matrix, as determined from both HR-TEM and SQUID measurements.

General expressions for the longitudinal and transverse resistivities of single-crystalline cubic and tetragonal ferromagnets are derived from a series expansion of the resistivity tensor with respect to the magnetization orientation. They are applied to strained (Ga,Mn)As films, grown on (001)-and (113)A-oriented GaAs substrates, where the resistivities are theoretically and experimentally studied for magnetic fields rotated within various planes parallel and perpendicular to the sample surface. We are able to model the measured angular dependences of the resistivities within the framework of a single ferromagnetic domain, calculating the field-dependent orientation of the magnetization by numerically minimizing the free-enthalpy density. Angle-dependent magnetotransport measurements are shown to be a powerful tool for probing both anisotropic magnetoresistance and magnetic anisotropy. The anisotropy parameters of the (Ga,Mn)As films inferred from the magnetotransport measurements agree with those obtained by ferromagnetic resonance measurements within a factor of two.

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We present a systematic study on the influence of epitaxial strain and hole concentration on the magnetic anisotropy in ͑Ga,Mn͒As at 4.2 K. The strain was gradually varied over a wide range from tensile to compressive by growing a series of ͑Ga,Mn͒As layers with 5% Mn on relaxed graded ͑In,Ga͒As/GaAs templates with different In concentration. The hole density, the Curie temperature, and the relaxed lattice constant of the as-grown and annealed ͑Ga,Mn͒As layers turned out to be essentially unaffected by the strain. Angledependent magnetotransport measurements performed at different magnetic-field strengths were used to probe the magnetic anisotropy. The measurements reveal a pronounced linear dependence of the uniaxial out-of-plane anisotropy on both strain and hole density. Whereas the uniaxial and cubic in-plane anisotropies are nearly constant, the cubic out-of-plane anisotropy changes sign when the magnetic easy axis flips from in-plane to out-of-plane. The experimental results for the magnetic anisotropy are quantitatively compared with calculations of the free energy based on a mean-field Zener model. Almost perfect agreement between experiment and theory is found for the uniaxial out-of-plane and cubic in-plane anisotropy parameters of the as-grown samples. In addition, magnetostriction constants are derived from the anisotropy data.

We present a detailed study of the magnetic properties of low-temperature molecular beam epitaxy grown Ge:Mn dilute magnetic semiconductor films. We find strong indications for a frozen state of Ge 1−x Mn x , with freezing temperatures of T f = 12 K and T f = 15 K for samples with x = 0.04 and x = 0.2, respectively, determined from the difference between field cooled and zero-field cooled magnetization. For Ge 0.96 Mn 0.04 , AC susceptibility measurements show a peak around T f , with the peak position T ′ f shifting as a function of the driving frequency f by ∆T ′ f /[T ′ f · ∆logf ] ≈ 0.06, whereas for sample Ge 0.8 Mn 0.2 a more complicated behavior is observed. Furthermore, both samples exhibit relaxation effects of the magnetization after switching the magnitude of the external magnetic field below T f which are in qualitative agreement with the field and zero-field cooled magnetization measurements. These findings consistently show that Ge:Mn exhibits a frozen magnetic state at low temperatures, and that it is not a conventional ferromagnet.

We have investigated the magnetic properties of a piezoelectric actuator/ferromagnetic semiconductor hybrid structure. Using a GaMnAs epilayer as the ferromagnetic semiconductor and applying the piezo stress along its ͓110͔ direction, we quantify the magnetic anisotropy as a function of the voltage V p applied to the piezoelectric actuator using anisotropic magnetoresistance techniques. As the magnetic anisotropy in GaMnAs substantially changes as a function of temperature T, the ratio of the magnetoelastic and the magnetocrystalline anistropies can be tuned from approximately 1/4 to 4. Thus, GaMnAs/piezoelectric actuator hybrids are an ideal model system for the investigation of different piezoelastic magnetization control regimes. At T = 5 K the magnetoelastic term is a minor contribution to the magnetic anisotropy. Nevertheless, we show that the switching fields of ͑ 0 H͒ loops are shifted as a function of V p at this temperature. At 50 K-where the magnetoelastic term dominates the magnetic anisotropy-we are able to tune the magnetization orientation by about 70°solely by means of the electrical voltage V p applied. Furthermore, we derive the magnetostrictive constant 111 as a function of temperature and find values consistent with earlier results. We argue that the piezo voltage control of magnetization orientation is directly transferable to other ferromagnetic/piezoelectric hybrid structures, paving the way to innovative multifunctional device concepts. As an example, we demonstrate piezo voltageinduced irreversible magnetization switching at T = 40 K, which constitutes the basic principle of a nonvolatile memory element.

We investigate the dependence of the spin-wave resonance ͑SWR͒ spectra of Ga 0.95 Mn 0.05 As thin films on the sample treatment. We find that for the external magnetic field perpendicular to the film plane, the SWR spectrum of the as-grown thin films and the changes upon etching and short-term hydrogenation can be quantitatively explained via a linear gradient in the uniaxial magnetic anisotropy field in growth direction. The model also qualitatively explains the SWR spectra observed for the in-plane easy-axis orientation of the external magnetic field. Furthermore, we observe a change in the effective surface spin pinning of the partially hydrogenated sample, which results from the tail in the hydrogen-diffusion profile. The latter leads to a rapidly changing hole concentration/magnetic anisotropy profile acting as a barrier for the spin-wave excitations. Therefore, short-term hydrogenation constitutes a simple method to efficiently manipulate the surface spin pinning.