We show, by SQUID magnetometry, that in (Ga,Mn)As films the in-plane uniaxial
magnetic easy axis is consistently associated with particular crystallographic
directions and that it can be rotated from the [-110] direction to the [110]
direction by low temperature annealing. We show that this behavior is
hole-density-dependent and does not originate from surface anisotropy. The
presence of uniaxial anisotropy as well its dependence on the
hole-concentration and temperature can be explained in terms of the p-d Zener
model of the ferromagnetism assuming a small trigonal distortion.Comment: 4 pages, 6 Postscript figures, uses revtex
We report on the observation of the Ising quantum Hall ferromagnet with Curie temperature T(C) as high as 2 K in a modulation-doped (Cd,Mn)Te heterostructure. In this system field-induced crossing of Landau levels occurs due to the giant spin-splitting effect. Magnetoresistance data, collected over a wide range of temperatures, magnetic fields, tilt angles, and electron densities, are discussed taking into account both Coulomb electron-electron interactions and s-d coupling to Mn spin fluctuations. The critical behavior of the resistance "spikes" at T-->T(C) corroborates theoretical suggestions that the ferromagnet is destroyed by domain excitations.
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