Thermal Transport and Magnetotransport Properties of CuCr1-xMgxO2 with a Spin-3/2 Antiferromagnetic Triangular Lattice

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We have investigated the thermal conductivity (\kappa) 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_{\text{N}}) deduced from \kappa 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_{\text{N}}. \kappa below T_{\text{N}} is significantly suppressed by Mg substitution, suggesting the introduction of some disorder into the 120° Néel state. For the hole-doped CuCr0.97Mg0.03O2, a negative MR is observed above T_{\text{N}}, which is enhanced with a decrease in T toward T_{\text{N}}, while a component of positive MR appears below T_{\text{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_{\text{N}} and remains below T_{\text{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-by-disorder mechanism.
2013-01-15