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 Co[Al_1-xCo_x]_2O_4 for Co concentrations 0<x<1. In this spinel system only the A-site Co^2+ cation is magnetic, while the non-magnetic B-site Al^3+ is substituted by the low-spin non-magnetic Co^3+, and it is possible to investigate the complete phase diagram from Co^2+Al^3+_2O_4 to Co^2+Co^3+_2O_4. All samples reveal large negative Curie-Weiss temperatures Theta_CW of the order of -110 K independent of concentration, which is attributed to a high multiplicity of the superexchange interactions between the A-site Co^2+ cations. A pure antiferromagnetic state is found for x = 1.0 and 0.9 with Neel temperatures T_N = 29.5 K and 21.2 K, respectively, as evidenced by lambda-like anomalies in the specific heat. Compositions with 0.3<x<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 < 0.2 a T^2 dependence of the specific heat and a spin-glass like behavior of the susceptibility below T_f = 4.7 K are observed. The high value of the frustration parameter f = |Theta_CW|/T_f > 10 indicates the presence of strong spin frustration at least for x < 0.6. The frustration mechanism is attributed to competing nearest neighbor and next-nearest neighbor superexchange interactions between the A-site Co^2+ ions.

arXiv comment19 pages, 9 figures, 46 references

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• Experimental evidence of a collinear antiferromagnetic ordering in the frustrated CoAl2O4 spinel
• Spin liquid in a single crystal of the frustrated diamond lattice antiferromagnet CoAl2O4
• Tuning magnetic frustration on the diamond lattice of the A-site magnetic spinels CoAl${}_{2-x}$Ga${}_x$O${}_4$: Lattice expansion and site disorder
• Evolution of magnetic states in frustrated diamond lattice antiferromagnetic Co(Al1-xCox)2O4 spinels
• Realization of anisotropic compass model on the diamond lattice of Cu${}^{2+}$ in CuAl${}_2$O${}_4$
• Cluster glass transition and relaxation in random spinel CoGa2O4
• Nonconventional magnetic order in frustrated diamond lattice antiferromagnet CoAl2O4 studied by neutron diffraction and classical Monte-Carlo simulation
• Spin wave theory study of neutron intensity, magnetic field, and anisotropy of Type IIA FCC antiferromagnet
• On the complexity of spinels: Magnetic, electronic, and polar ground states