Phase Transitions of Fel2 in high magnetic field parallel to the spin direction,static field up to 150 kOe,pulsed field up to 250 kOe

Study of parallel and perpendicular susceptibilities shows that ferrous iodide presents at low temperature an antiferromagnetic order, with spins oriented along the anisotropy axis (c axis).Phase transitions of Fel₂ in a magnetic field parallel to c axis are studied by help of magnetization measurements. At low temperature (2.2 K) saturation is reached only for a magnetic field of 140 kOe. Results obtained in high static fields (Bitter and supraconductive coils allowing respectively 140 and 150 kOe) and in pulsed field are presented.At low temperature, two successive first order phase transitions are observed at 46 and 120 kOe. In the intermediate phase, the magnetization presents two minor discontinuities. An original phase diagram is given.The complexity of the Fel₂ behavior, in parallel magnetic field shows that the magnetic structure is not the same as the two sublattices one characteristic of FeCl₂ and FeBr₂. An estimate of the principal exchange coupling parameters and a study by neutron diffraction measurements (to be published) confirm an original magnetic structure.