Spin frustration and magnetic exchange in cobalt aluminum oxide spinels

Abstract

We report on x-ray diffraction, magnetic susceptibility, electron-spin resonance, and heat-capacity studies of $\mathrm{Co}{[{\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}]}_{2}{\mathrm{O}}_{4}$ for Co concentrations $0\ensuremath{\leqslant}x\ensuremath{\leqslant}1$. In this spinel system only the $A$-site ${\mathrm{Co}}^{2+}$ cation is magnetic, while the nonmagnetic $B$-site ${\mathrm{Al}}^{3+}$ is substituted by the low-spin nonmagnetic ${\mathrm{Co}}^{3+}$, and it is possible to investigate the complete phase diagram from ${\mathrm{Co}}^{2+}\mathrm{Al}^{3+}{}_{2}{\mathrm{O}}_{4}$ to ${\mathrm{Co}}^{2+}\mathrm{Co}^{3+}{}_{2}{\mathrm{O}}_{4}$. All samples reveal large negative Curie-Weiss temperatures ${\ensuremath{\Theta}}_{\mathrm{CW}}$ of the order of $\ensuremath{-}110\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ independent of concentration, which is attributed to a high multiplicity of the superexchange interactions between the $A$-site ${\mathrm{Co}}^{2+}$ cations. A pure antiferromagnetic state is found for $x=1.0$ and 0.9 with N\'eel temperatures ${T}_{N}=29.5$ and $21.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively, as evidenced by lambdalike anomalies in the specific heat. Compositions with $0.3\ensuremath{\leqslant}x\ensuremath{\leqslant}0.75$ show smeared out strongly reduced magnetic ordering temperatures. At low temperatures, a ${T}^{2.5}$ dependence of the specific heat is indicative of a spin-liquid state. For $x\ensuremath{\leqslant}0.2$ a ${T}^{2}$ dependence of the specific heat and a spin-glass-like behavior of the susceptibility below ${T}_{f}\ensuremath{\sim}4.7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are observed. The high value of the frustration parameter $f=\ensuremath{\mid}{\ensuremath{\Theta}}_{\mathrm{CW}}\ensuremath{\mid}∕{T}_{f}>10$ indicates the presence of strong spin frustration at least for $x\ensuremath{\leqslant}0.6$. The frustration mechanism is attributed to competing nearest-neighbor and next-nearest-neighbor superexchange interactions between the $A$-site ${\mathrm{Co}}^{2+}$ ions.