Spin frustration and magnetic exchange in cobalt aluminum oxide spinels

We report on x-ray diffraction, magnetic susceptibility, electron-spin resonance, and heat-capacity studies of Co[Al_1-xCoₓ]₂O₄ for Co concentrations 0x1. In this spinel system only the A-site Co²⁺ cation is magnetic, while the nonmagnetic B-site Al³⁺ is substituted by the low-spin nonmagnetic Co³⁺, and it is possible to investigate the complete phase diagram from Co²⁺Al³⁺₂O₄ to Co²⁺Co³⁺₂O₄. All samples reveal large negative Curie-Weiss temperatures ΘCW of the order of -1100.3em0exK independent of concentration, which is attributed to a high multiplicity of the superexchange interactions between the A-site Co²⁺ cations. A pure antiferromagnetic state is found for $x=1.0$ and 0.9 with N\'eel temperatures TN=29.5 and 21.20.3em0exK, respectively, as evidenced by lambdalike anomalies in the specific heat. Compositions with 0.3x0.75 show smeared out strongly reduced magnetic ordering temperatures. At low temperatures, a T2.5 dependence of the specific heat is indicative of a spin-liquid state. For x0.2 a T² dependence of the specific heat and a spin-glass-like behavior of the susceptibility below Tf~4.70.3em0exK are observed. The high value of the frustration parameter f=ΘCW∕Tf>10 indicates the presence of strong spin frustration at least for x0.6. The frustration mechanism is attributed to competing nearest-neighbor and next-nearest-neighbor superexchange interactions between the A-site Co²⁺ ions.