A comparative study of the magnetic properties of bulk and nanocrystalline Co3O4

A comparative study of the magnetic and electron paramagnetic resonance (EPR) parameters of bulk and Co3O4 nanoparticles (NP), synthesized by a sol-gel process, is presented. Both samples possess the cubic phase with a slightly lower (by 0.34%) lattice parameter for the Co3O4 NP. The average crystallite size D = 17 nm determined by x-ray diffraction (XRD) for the Co3O4 NP is quite consistent with the electron microscopic observations. The bulk Co3O4 has particle size in the 1-2 mu m range. A Neel temperature of T-N = 30 K (lower than the 40 K usually quoted in the literature) is determined from the analysis of the magnetic susceptibility versus temperature data for bulk Co3O4. This T-N = 30 K is in excellent agreement with the T-N = 29.92 K reported from specific heat measurements. The Co3O4 NP powder exhibits a still lower T-N = 26 K, possibly due to the associated finite size effects. The values of coercivity, H-c = 250 Oe, and exchange bias, He = -350 Oe, together with the training effect have been observed in the Co3O4 NP sample (cooled in 20 kOe). Both Hc and He approach zero as T --> T-N(-). For T > T-N, the. versus T data for both samples fit the modified Curie-Weiss law (chi =chi(0) + C/(T +theta)). The magnitudes of C, theta and T-N are used to determine the following: exchange constants J(1ex) = 11.7 K, J(2ex) = 2.3 K, and magnetic moment per Co2+ ion mu = 4.27 mu(B) for bulk Co3O4; and J(1ex) = 11.5 K, J(2ex) = 2.3 K and mu = 4.09 mu(B) for Co3O4 NP. EPR yields a single Lorentzian line near g = 2.18 in both samples but with a linewidth Delta H that is larger for the Co3O4 NP. Details of the temperature dependence of Delta H, line intensity I-0, and disappearance of the EPR on approach to TN are different for the two samples. These effects are discussed in terms of spin-phonon interaction and additional surface anisotropy present in Co3O4 NP.