Magnetic Anisotropy in Quantum Hall Ferromagnets
  We show that the sign of magnetic anisotropy energy in quantum Hall ferromagnets is determined by a competition between electrostatic and exchange energies. Easy-axis ferromagnets tend to occur when Landau levels whose states have similar spatial profiles cross. We report measurements of integer QHE evolution with magnetic-field tilt. Reentrant behavior observed for the $\nu = 4$ QHE at high tilt angles is attributed to easy-axis anisotropy. This interpretation is supported by a detailed calculation of the magnetic anisotropy energy. 
T. Jungwirth, S.P. Shukla, L. Smrcka, M. Shayegan, and A.H. MacDonald