ESR measurement of triple chain magnet Cu3(OH)4SO4 in millimeter wave region

Cu₃(OH)₄SO₄, which was obtained by the hydorothermal synthesis, was studied by our high frequency electron spin resonance (ESR) measurements. It has an interesting magnetic structure which consists of triple chains of Cu²⁺ spins (s=1/2). It undergoes the 3D-transition below 5.3 K. In the ordered state, the magnetic moment of the central chain becomes zero. This interesting behavior is called as "idle spin". We have carried out the high frequency high field ESR measurements in the millimeter wave region to obtain the information about the 3D-ordering and the magnetic anisotropy of Cu₃(OH)₄SO₄. We have observed antiferromagnetic modes at 1.8 K, which suggests the existence of the 3D-transition. The energy gap, which has been analyzed by the conventional 2- sublattice AFMR theory, is estimated to be 2.6 K. We have also succeeded in observing the change of the AF modes in the high magnetic field region. The critical Held is about 5T.