Field induced magnetic ordering in CsFeBr₃ studied by high field ESR

High field ESR technique was applied to the study of the singlet ground state system CsFeBr₃, to study the energy structure and the field induced magnetic ordering state, which occurs at low temperature and in a field with the configuration parallel to the c-axis. In low field paramagnetic regime, the complicated spectra were observed and the obtained frequency-field diagram at low temperatures can not be explained within the simple singlet-doublet energy scheme asserted from previous neutron experimental results, indicating that the exchange interaction, which brings a dispersive character on the excited doublet levels, may play an important role to understand the origin of observed ESR modes. One of the modes observed at 2.0 K altered its slope to almost twice one at the field of about 2.9 T, which corresponds to the level crossing field between singlet and the lower branch of doublet states. This behavior is reasonably explained as the direct transition between doublets due to the mixing around level crossing field and is consistent with recently developed theoretical prediction proposed by Lolezhuk and Mikeska. Further, in the high field regime, two broad absorption lines with each line width of about a few tesla were also observed. These two ESR modes depend remarkably on the frequency in such a way that both resonance fields coincide at about 160 GHz and separate linearly with large g-value of 4.8 from each other as the frequency is increased. We present these experimental results in detail and give some possible explanations.