No abstract

We have investigated the magnetization reorientation process of GaMnAs ferromagnetic films by changing external field direction in planar Hall effect (PHE) measurement. While the angular dependences of PHE data taken with clockwise and counterclockwise under strong magnetic field (i.e., above 400Oe) are completely overlapped without hysteresis, they are significantly different under small magnetic field (i.e., below 50Oe) by exhibiting nonabrupt hysteresis. We have analyzed such angular dependence of PHE using the magnetic free energy based on Stoner-Wohlfarth model. The behavior observed under the high field was well understood in terms of coherent rotation of magnetization in the form of single domain. However, the nonabrupt hysteric behavior observed with low field cannot be explained by a single domain picture and requires involvement of multidomain structures.

The distribution of magnetic domain pinning fields was determined in ferromagnetic GaMnAs films using the angular dependence of the planar Hall effect. A major difference is found between the pinning field distribution in as-grown and in annealed films: the former showing a strikingly narrower distribution than the latter. This effect, which we ascribe to differences in the degree of uniformity of magnetic anisotropy, provides a better understanding of magnetic domain landscape in GaMnAs, a subject of current intense interest.

We have fabricated micrometer-sized single-turn coils on top of charged CdSe/ZnSe quantum dot heterostructures by lithographical techniques. Current injection creates magnetic fields in the some 10 mT range, strong enough to modulate the hyperfine interaction. The very low coil inductance allows for generation of fast field transients. We demonstrate local control of the resident electron spin as well as read-out of the nuclear spin state on the 10 ns time scale by electrical current pulses.