High field ESR in CsFeCl3

The spin state of Fe2+ in CsFeCl3 is usually treated as a fictitious spin S=1 with the singlet ground state due to the crystallographic single ion anisotropy. An anomalous magnetization has been observed under magnetic fields exceeding 33 T and applied parallel to the c-axis of the crystal. Such phenomenon not be explained within the framework of the fictitious spin S=1. Un order to study the relation between the anomalous magnetization and the excited state S=2, an ESR experiment has been performed using a pulse magnet with a maximum field intensity up to 40. The submillimeter waves with frequencies around 400 GHz have been delivered by the Gyrotron FU IV and Gyrotron FU IV A. In this paper, the frequency-field diagram corresponding to the anomalous magnetization is discussed