Muon spin relaxation and rotation study on the solid solution of the two spin-gap systems (CH3)2CHNH3-CuCl3 and (CH3)2CHNH3-CuBr3

Muon-spin-rotation and relaxation studies have been performed on (CH${}_3$)${}_2$CHNH${}_3$Cu(Cl${}_x$Br${}_{1-x}$)${}_3$ with $x$=0.85 and 0.95, which are solid solutions of the two isomorphic spin-gap systems (CH${}_3$)${}_2$CHNH${}_3$CuCl${}_3$ and (CH${}_3$)${}_2$CHNH${}_3$CuBr${}_3$ with different spin gaps. The sample with $x$=0.85 showed a clear muon spin rotation under zero-field below $T_{\mathrm{N}}$=11.65K, indicating the existence of a long-range antiferromagnetic order. A critical exponent of the hyperfine field was obtained to be $\beta$=0.33, which agrees with 3D-Ising model. In the other sample with $x$=0.95, an anomalous enhancement of the muon spin relaxation was observed at very low temperatures indicating a critical slowing down due to a magnetic instability of the ground state.