Zero-field NMR study on a spin-glass: Iron-doped2H—niobium diselenide

Abstract

We describe a zero-field NMR study attempting to resolve the controversy between phase-transition theories and nonequilibrium glasslike theories of the spin-glass transition. The spin dynamics of Fe spins in $2H-\mathrm{Nb}{\mathrm{Se}}_2{\mathrm{Fe}}_x$ is probed through its influence on the nuclear quadrupole resonance (NQR) of ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ nuclei. The spin-echo method is used. We measure intensity times temperature, $T_{1P}$ (spin-lattice relaxation parameter), and $T_2$ (spin-spin relaxation time) as a function of temperature. Our data reveal dramatic differences between non-spin-glass samples ($x=0, 0.25, 1, \mathrm{and} 5$ at.%) and spin-glass samples ($x=8, 10, \mathrm{and} 12$ at.%). Defining the spin-glass transition temperature $T_g$ by the cusp of the susceptibility-versus-temperature curve, we find well-defined minima at $T=T_g$ in intensity times temperature, $T_{1P}$ and $T_2$ as a function of temperature. Deduction of a correlation time of Fe spins from $T_{1P}$ and $T_2$ is complicated by the intensity changes which imply that one observes different groups of nuclei at different temperatures. We propose a two-correlation-time model, which utilizes the anisotropy of the exchange interaction, to overcome this difficulty. All of our NQR results and the model calculation of the correlation times of Fe spins are best described by the phase-transition picture of spin-glasses.