CRITICAL BEHAVIOUR OF THE FERROMAGNET (NH4)2 Cu (Cl1-xBrx)4 2H2O A. Paduan-Filho, Y. Ridente, S. Zacarelli and C. C. Becerra J. Phys. Colloques, 49 C8 (1988) C8-823-C8-824 DOI: 10.1051/jphyscol:19888370