Non-Heisenberg two-dimensional ferromagnet in an external magnetic field Yu. A. Fridman, O. V. Kozhemyako and B. L. Eingorn Eur. Phys. J. B, 39 1 (2004) 49-54 Published online: 18 June 2004 DOI: 10.1140/epjb/e2004-00169-x