MAGNETIC ENERGY IN A 2D-HEISENBERG MIXED CRYSTAL WITH COMPETING EXCHANGE INTERACTIONS : K2CuxMn1-xF4 J. P. Jamet, J. Ferré, I. Yamada et M. Itoh J. Phys. Colloques, 49 C8 (1988) C8-1501-C8-1502 DOI: 10.1051/jphyscol:19888691