The spin dynamics in a molecular spin system on a two-dimensional (2D) distorted triangular lattice, ${\beta }^{\prime }\text{−}{\left(\mathrm{C}{\mathrm{H}}_3\right)}_4\mathrm{P}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ ($\mathrm{dmit}=1,3$-dithiol-2-thione-4,5-dithiolate, ${\mathrm{C}}_3{\mathrm{S}}_5$), has been investigated by virtue of muon spin relaxation $\left(\mu \mathrm{SR}\right)$. The muon spin relaxation rate increased with decreasing temperature within a narrow temperature range $(<0.8\mathrm{K})$ near the antiferromagnetic (AFM) transition temperature, $T_{\mathrm{N}}=39.3\mathrm{K}$. Muon spin precession signals appeared abruptly at this temperature. The AFM contribution to the $\mu \mathrm{SR}$ signal continuously increased just below $T_{\mathrm{N}}$ as temperature decreased. These peculiar features are discussed in terms of the crossovers specific to this frustrated system.

• Received 2 July 2004

DOI:https://doi.org/10.1103/PhysRevB.70.220404