High field ESR measurements of CsFeCl3

CsFeCl3 is known to have a single ground state system with strong ferromagnetic interaction with the crystallographic c-axis and weak antiferromagnetic interaction in the plane perpendicular to c-axis. Under a magnetic field B applied parallel to the crystal c-axis, one of the states of the doublet excited state comes down to cross the ground state at 7.5 T. The temperature dependences of the ESR were measured with the frequency 120 GHz and 301 GHz. At the frequency 120 GHz, three absorptions were observed at 4.2K. One of them was observed below the magnetic field 7.5T. The other two absorptions were observed above the magnetic field 7.5T. Above 5 K, however only one below the magnetic field 7.5 T at 4.2 K. Above 5K, no absorption was observed below the magnetic field 7.5 T. On the other hand, above 4.2K, one absorption was observed the magnetic field 7.5T