A comparative study of the magnetic properties of bulk and nanocrystallineCo3O4

Abstract

A comparative study of the magnetic and electron paramagnetic resonance (EPR) parameters of bulkand Co3O4 nanoparticles (NP), synthesized by a sol–gel process, is presented. Both samplespossess the cubic phase with a slightly lower (by 0.34%) lattice parameter for theCo3O4 NP. The averagecrystallite size D = 17 nm determined by x-ray diffraction (XRD) for theCo3O4 NP is quite consistent with the electron microscopic observations. The bulkCo3O4 has particle sizein the 1–2 µm range. ANéel temperature of TN = 30 K (lower than the 40 K usually quoted in the literature) is determined fromthe analysis of the magnetic susceptibility versus temperature data for bulkCo3O4. ThisTN = 30 K is in excellentagreement with the TN = 29.92 K reported from specific heat measurements. TheCo3O4 NP powderexhibits a still lower TN = 26 K, possibly due to the associated finite size effects. The values of coercivity,Hc = 250 Oe, andexchange bias, He = −350 Oe, together with the training effect have been observed in theCo3O4 NP sample (cooledin 20 kOe). Both Hc and He approach zero as . For T>TN, the χ versus T data for both samples fit the modified Curie–Weiss law(χ = χ0+C/(T+θ)). Themagnitudes of C, θ and TN are used to determine the following: exchange constantsJ1ex = 11.7 K,J2ex = 2.3 K, and magneticmoment per Co2+ ion μ = 4.27 μB forbulk Co3O4;and J1ex = 11.5 K, J2ex = 2.3 Kand μ = 4.09 μB for Co3O4 NP. EPR yields a single Lorentzian line nearg = 2.18 in both samples butwith a linewidth ΔH that is larger for the Co3O4 NP. Details of the temperature dependence ofΔH, lineintensity I0, and disappearance of the EPR on approach toTN are different for the two samples. These effects are discussed in termsof spin–phonon interaction and additional surface anisotropy present inCo3O4 NP.