SUBMILLIMETER EPR SPECTROSCOPY OF VAN-VLECK PARAMAGNETS HOLMIUM NICOTINATE DIHYDRATE AND KY3F10-HO

Results of EPR measurements of the Van Vleck paramagnets (monoclinic h olmium nicotinate dihydrate and activated cubic complex fluoride KY3F1 0:Ho3+) are presented. Spectra were taken with a wide-band spectromete r operating in the frequency range 65-535 GHz at liquid helium tempera ture for magnetic fields up to 1 T. Transformations of the hyperfine s tructure of the EPR spectra with the frequency of the radiation and th e directions of the constant and alternating magnetic fields have been studied and compared with results of calculations based on a diagonal ization of the complete electronic-nuclear Hamiltonian. Crystal field splittings between the ground and first excited singlets of 7.3 cm(-1) in holmium nicotinate and 5.8 cm(-1) in KY3F10:Ho have been estimated using extrapolations of the resonance frequency dependences on magnet ic field to zero field strength.