Magnetic and electronic properties ofCa1−xNaxCr2O4: Double-exchange interactions and ligand holes

Abstract

$\ensuremath{\beta}$-${\text{CaCr}}_{2}$${\text{O}}_{4}$ has a cycloidal magnetic structure and isostructural ${\text{NaCr}}_{2}$${\text{O}}_{4}$ shows unconventional colossal magnetoresistance below a canted antiferromagnetic (AFM) ordering temperature of ${T}_{\mathrm{N}}=125$ K. In this paper, magnetic and electronic properties of the solid-solution system ${\text{Ca}}_{1\ensuremath{-}x}$${\text{Na}}_{x}$${\text{Cr}}_{2}$${\text{O}}_{4}$, are investigated. The ${T}_{\mathrm{N}}$ value decreases with decreasing $x$ from $x=1$ to $\frac{2}{3}$. A spin-glass behavior appears for $0.1\ensuremath{\le}x\ensuremath{\le}0.6$. Interestingly, the spin-glass character is detected even for $\frac{2}{3}\ensuremath{\le}x\ensuremath{\le}0.95$ coexisting with the AFM ordering. Curie constant increases and Weiss temperature decreases with decreasing $x$. Curie constant is significantly suppressed from theoretical value, suggesting large quantum fluctuations and strong hybridization between Cr and O ions for higher-$x$ values. Substitution of Ca for Na introduces the ligand holes. The decrease in Weiss temperature points out that a single ligand hole mediates ferromagnetic interaction of approximately 400 K by the double-exchange mechanism. Saturation magnetization and magnetic susceptibility at 2 K show unusual $x$ dependencies, which suggests that not only the canting angle but also the periodicity of magnetic ordering depend on $x$. Electrical resistivity for $0.2\ensuremath{\le}x\ensuremath{\le}0.95$ follows the variable-range hopping mechanism, suggesting Anderson localization at the $x$ range.