A study of electron spin dynamics and phase transition in [Co(H2O)6][SiF6] by single crystal 19F NMR spin–lattice relaxation time measurements

Abstract

The temperature dependence of the 19F NMR spin–lattice relaxation time, T1, was measured for a [Co(H2O)6][SiF6] single crystal in the temperature range 170–320 K. A large hysteresis for the transition was observed: Tc≈265 K with increasing temperature, Tc≈245 K with decreasing temperature. The spin–lattice relaxation time of the unpaired electron spin in the Co2+ ion (T1e) was estimated from 19F NMR T1, which is dominated by a dipole interaction between the 19F nuclei and the electron spins of the Co2+ ions. The energy differences between the electronic ground and excited states in the Co2+ ion involving the Orbach process were estimated from the temperature dependence of T1e as Iµ=180 and 330 cm-1 above and below Tc, respectively. The investigations of the angular variations of 19F NMR T1 were performed at 291 and 193 K. Although T1 for the rotation about the c axis was invariant, the variation of T1 was observed in a plane containing the c axis at both temperatures. An anisotropy of T1e was not seen at either temperature.