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: T_c≈265 K with increasing temperature, T_c≈245 K with decreasing temperature. The spin–lattice relaxation time of the unpaired electron spin in the Co²⁺ ion (T_1e) 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 T_1e as ε=180 and 330 cm^-1 above and below T_c, 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 T_1e was not seen at either temperature.

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