A comparative study of the magnetic and electron paramagnetic resonance (EPR) parameters of bulk and Co 3 O 4 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 Co 3 O 4 NP. The average crystallite size D = 17 nm determined by x-ray diffraction (XRD) for the Co 3 O 4 NP is quite consistent with the electron microscopic observations. The bulk Co 3 O 4 has particle size in the 1-2 μm range. A Néel 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 Co 3 O 4. This T N = 30 K is in excellent agreement with the T N = 29.92 K reported from specific heat measurements. The Co 3 O 4 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, H e = −350 Oe, together with the training effect have been observed in the Co 3 O 4 NP sample (cooled in 20 kOe). Both H c and H e approach zero as T → T − N. For T > T N , the χ versus T data for both samples fit the modified Curie-Weiss law (χ = χ 0 + C/(T + θ)). The magnitudes of C, θ 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 Co 2+ ion μ = 4.27 μ B for bulk Co 3 O 4 ; and J 1ex = 11.5 K, J 2ex = 2.3 K and μ = 4.09 μ B for Co 3 O 4 NP. EPR yields a single Lorentzian line near g = 2.18 in both samples but with a linewidth H that is larger for the Co 3 O 4 NP. Details of the temperature dependence of H , line intensity I 0 , and disappearance of the EPR on approach to T N are different for the two samples. These effects are discussed in terms of spin-phonon interaction and additional surface anisotropy present in Co 3 O 4 NP.

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The complex nature of magnetic ordering in the spinel Co 2 TiO 4 is investigated by analyzing the temperature and magnetic field dependence of its magnetization (M), specific heat (C p ), and ac magnetic susceptibilities χ and χ . X-ray diffraction of the sample synthesized by the solid-state reaction route confirmed the spinel structure whereas x-ray photoelectron spectroscopy shows its electronic structure to be

Transition metal containing ZnO powders (Zn 1−x M x O, 0 x 0.30; M = Ni, Mn, Co) have been synthesized by a sol-gel process using zinc acetate dihydrate, respective acetate and oxalic acid as precursors with ethanol as a solvent. The process essentially involves gel formation, drying at 80 • C for 24 h to provide the oxalate, and calcination at 500 • C for 2 h to undergo an exothermic reaction and yield Zn 1−x M x O powder. Their XRD patterns correspond to a wurtzite hcp structure similar to that of pure ZnO, but with the lattice parameters varying slightly with type and extent of doping. It is shown that the dissolution of nickel and cobalt in ZnO is less than 10 at.%, whereas that of manganese lies between 10 and 15 at.%. Other phases that emerge include NiO (hexagonal, a = 2.954 Å, c = 7.236 Å), ZnCo 2 O 4 (cubic, a = 8.094 Å) and ZnMnO 3 (cubic, a = 8.35 Å) in the Ni, Co and Mn containing ZnO systems, respectively. Observations of hysteresis loops both at 10 and 320 K and the nature of ESR spectra provide evidence for the ferromagnetic state in nickel containing ZnO powder. Besides, the deviation occurs in the magnetization versus temperature curves in zero field cooled (ZFC) and field cooled (FC) conditions (blocking temperature T B being 32 K for 5 at.% Ni). The magnetic behaviour of manganese and cobalt doped zinc oxide is, however, different, namely, (i) no hysteresis loops, (ii) decrease in magnetization with increase of Mn or Co content, and (iii) identical M-T curves under ZFC and FC conditions. The inverse susceptibility versus temperature curves of Zn 1−x Mn x O compounds reveal ferrimagnetism with Néel temperature T N of 4 K for x = 0.02, but antiferromagnetism for x = 0.15 and 0.25 with Curie-Weiss temperature of −43 and −30 K, respectively.