Evidence for an exotic magnetic transition in the triangular spin system FeGa${}_2$S${}_4$

We report positive muon spin relaxation measurements on the triangular lattice magnetic system FeGa${}_2$S${}_4$. A magnetic transition that, to the best of our knowledge, has not been previously detected by specific-heat and magnetic susceptibility measurements is found in zero field at $T^{*}\simeq 30$ K. It is observed through the temperature dependencies of the signal amplitude and the spin-lattice relaxation rate. This transition is therefore not a conventional magnetic phase transition. Since persistent spin dynamics is observed down to 0.1 K, the ground state cannot be of the canonical spin-glass type, which could be suggested from hysteresis effects in the bulk susceptibility below $T_f\simeq 16$ K. These results are compared to those found for the isomorph NiGa${}_2$S${}_4$. It is argued that the fate of the transition, which has been interpreted in terms of the $Z_2$ topological transition in this latter system, is probably different in FeGa${}_2$S${}_4$.

Figure 1

Examples of zero- and longitudinal-field $\mu$SR spectra recorded on a powder sample of FeGa${}_2$S${}_4$. Solid curves: Results of fits as explained in the text. The reduced signal amplitude, i.e., the reduced effective initial asymmetry, for the $2.8$ K spectrum is clearly seen graphically. Note that these data result from the weighted subtraction of counts recorded in two opposite detectors, implying that their shape and amplitude are not dependent of any fit parameter.

Figure 2

The three upper panels show the temperature dependencies of the effective initial asymmetry $a_0\eta$, the spin-lattice relaxation rate ${\lambda }_Z$, and the stretched-exponential power $\beta$ measured in a powder sample of FeGa${}_2$S${}_4$ in zero or 10 mT longitudinal fields. The solid curves for ${\lambda }_Z\left(T\right)$ above $T^{*}$ result from a fit to a model explained in the main text. The adequacy of the model can be judged in the inset, which displays ${\lambda }_Z/T^3$ vs $1/T$. The temperature dependencies of the susceptibility ${\chi }_{\mathrm{dc}}$ given in cgs or Système International (SI) units and the ratio $C_M/T$ of the magnetic specific heat over the temperature (Ref. 18) are shown in the lower two panels. The susceptibility has been measured in an external field of 0.1 T after a ZFC or FC protocol. For reference dashed lines are plotted at 30 and 40 K.

Figure 3

Comparison of spectra measured at 0.15 K for FeGa${}_2$S${}_4$ after zero-field cooling. The first spectrum is measured in zero field and the second after the subsequent application of a 10 mT longitudinal field.