NUCLEAR MAGNETIC RESONANCE STUDY OF THE HEISENBERG FERROMAGNET Cu(NH4)2 Br4, 2 H2O J.-P. RENARD et E. VELU J. Phys. Colloques, 32 C1 (1971) C1-1154-C1-1156 DOI: 10.1051/jphyscol:19711413