Thermal transport and magnetotransport properties of CuCr _1-xMg_xO₂ with a spin-3/2 antiferromagnetic triangular lattice

We have investigated the thermal conductivity (k) and magnetoresistance (MR) of non-doped and hole-doped delafossite CuCrO₂ with a spin-3/2 antiferromagnetic (AF) triangular sublattice. The phonon mean free path above the Néel temperature (T_N) deduced from k and lattice specific heat is almost identical to the magnetic correlation length, which indicates that, for both compounds, spin fluctuation enhanced in a geometrically frustrated lattice is strongly coupled with acoustic phonon above T_N. k below T_N is significantly suppressed by Mg substitution, suggesting the introduction of some disorder into the 120° Néel state. For the hole-doped CuCr_0.97Mg_0.03O₂, a negative MR is observed above T_N, which is enhanced with a decrease in T toward T_N, while a component of positive MR appears below T _N and the residual negative MR component is observed in a high magnetic field, indicating that spin fluctuation coupled with electrical conductivity is critically enhanced above T_N and remains below T _N. These results evidence that the 120° Néel state is partially disordered by a doped itinerant hole coupled with spin fluctuation, although AF transition is certainly promoted. The dynamic partial disorder may stabilize the Néel order through an order-bydisorder mechanism.