Spin-Disorder and Non-Degenerate Energy States in Geometrically Frustrated Materials

摘要

We have studied chalcogenide Ni$_{1-{rm x}}$Fe$_{rm x}$Ga$_{2}$ S$_{4}$ , showing geometrical frustration effect. M-H curves at 4.2 K reveal that the disordered spins cannot be rotated completely along the direction of high external field of 5 T, since these spins are strongly constrained in the triangular lattice. $H_{rm C}$ increases with increasing Fe concentration, being consistent with the enhanced antiferromagnetic (AFM) spin-spin interactions and the suppressed spin-fluctuation due to the increase in freezing temperature $T_{rm f}$. The specific heat $(C_{rm p}/T)$ measurement do not show any phase transformation between 2 and 160 K and there is no clear indication of gap in the temperature dependent $C_{rm p}$ curve between 0 and 7 T, because the atomic short-range ordering of the strained spin in the geometrically frustrated triangular lattice exists at low temperature. Though typical geometrically frustrated magnet shows degeneracy, Mössbauer analysis at 4.2 K shows that 5D of 3d orbit in the samples studied here is splitted into ${}^{5}T_{2{rm g}}$ and ${}^{5}E_{2{rm g}}$, and ${}^{5}T_{2{rm g}}$ is further splitted into singlet and doublet by large electric quadrupole splitting. This suggests that Fe $^{2+}$ ionic state has no absolute degenerate energy states in all samples due to Jahn-Teller effect.