We report a strong correlation between the location of Mn sites in ferrromagnetic Ga 1-x Mn x As measured by channeling Rutherford backscattering and by particle induced x-ray emission experiments and its Curie temperature. The concentrations of free holes determined by electrochemical capacitance-voltage profiling and of uncompensated Mn ++ spins determined from SQUID magnetization measurements are found to depend on the concentration of unstable defects involving highly mobile Mn interstitials. This leads to large variations in T C of Ga 1-x Mn x As when it is annealed at different temperatures in a narrow temperature range. The fact that annealing under various conditions has failed to produce Curie temperatures above ~110K is attributed to the existence of an upper limit on the free hole concentration in low-temperature-grown Ga 1-x Mn x As.

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Large, well-defined magnetic domains, on the scale of hundreds of micrometers, are observed in Ga1-xMn(x)As epilayers using a high-resolution magneto-optical imaging technique. The orientations of the magnetic moments in the domains clearly show in-plane magnetic anisotropy, which changes through a second-order transition from a biaxial mode (easy axes nearly along [100] and [010]) at low temperatures to an unusual uniaxial mode (easy axis along [110]) as the temperature increases above about T(c)/2. This transition is a result of the interplay between the natural cubic anisotropy of the GaMnAs zinc-blende structure and a uniaxial anisotropy which attribute to the effects of surface reconstruction.

This paper describes a systematic study of ferromagnetic resonance ͑FMR͒ carried out on a series of specimens of the ferromagnetic semiconductor Ga 1Ϫx Mn x As in thin film form. The GaMnAs layers were grown by low-temperature molecular beam epitaxy either on GaAs or on GaInAs buffers, the two buffers being used to obtain different strain conditions within the ferromagnetic layer. Our aim has been to map out the dependence of the FMR position on temperature and on the angle between the applied magnetic field and crystallographic axes of the sample. The analysis of the FMR data allowed us to obtain the values of the cubic and the uniaxial magnetic anisotropy fields-i.e., those which are associated with the natural ͑undistorted͒ zinc blende structure and those arising from of strain.

We show that the magnetization of a thin ferromagnetic (Ga,Mn)As layer can be modulated by picosecond acoustic pulses. In this approach a picosecond strain pulse injected into the structure induces a tilt of the magnetization vector M, followed by the precession of M around its equilibrium orientation. This effect can be understood in terms of changes in magnetocrystalline anisotropy induced by the pulse. A model where only one anisotropy constant is affected by the strain pulse provides a good description of the observed time-dependent response.