The grain size of α-Fe2O3 decreases to ∼20 nm by 64 h mechanical milling of the bulk sample. X-ray diffraction pattern suggested identical crystal structure in bulk and mechanical milled samples. Magnetic study (at temperatures of 100–900 K and fields of 0–±15 kOe) showed many interesting features during the decrease in grain size in antiferromagnetic α-Fe2O3, e.g., suppression of Morin transition, enhancement in low temperature magnetization, magnetic blocking at high temperature, exchange bias effect, and unusual relaxation of magnetic spin moment. We understand the results in terms of core-shell spin structure of nanograins, where the core part essentially retained the magnetic structure of the bulk sample and the magnetic structure of the shell part is modified due to grain size reduction and surface modification during mechanical milling. Core-shell structure also plays an important role in exhibiting the increasing soft ferromagnetic character in the present hematite samples. The in field magnetic relaxation at room temperature revealed some interesting properties of the magnetic spin ordering in hematite system.

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We report magnetic behaviour of MnCr2O4, which belongs to a special class of spinel, known as chromite. Bulk MnCr2O4 shows a sequence of magnetic states, which follows paramagnetic (PM) to collinear ferrimagnetic (FM) state below TC ∼ 45 K and collinear FM state to non-collinear FM state below TS ∼ 18 K. The non-collinear spin structure has been modified on decreasing the particle size, and magnetic transition at TS decreases in nanoparticle samples. However, ferrimagnetic order is still dominating in nanoparticles, except the observation of superparamagnetic like blocking and decrease of spontaneous magnetization for nanoparticle. This may, according to the core-shell model of ferrimagnetic nanoparticle, be the surface disorder effect of nanoparticle. The system also show the increase of TC in nanoparticle samples, which is not consistent with the core-shell model. The analysis of the M(T) data, applying spin wave theory, has shown an unusual Bloch exponent value 3.35 for bulk MnCr2O4, which decreases and approaches to 1.5, a typical value for any standard ferromagnet, with decreasing the particle size. MnCr2O4 has shown a few more unusual behaviour. For example, lattice expansion in nanoparticle samples. The present work demonstrates the correlation between a systematic increase of lattice parameter and the gradual decrease of B site non-collinear spin structure in the light of magnetism of MnCr2O4 nanoparticles.

We report the ferromagnetism of La0.67Ca0.33MnO3 in bulk polycrystalline, nanocrystalline, and amorphous phases. The structural change from crystalline phase to amorphous phase exhibited a systematic decrease in TC (paramagnetic to ferromagnetic transition temperature) and spontaneous magnetization (MS). The experimental results suggested few more interesting features, e.g., appearance of large magnetic irreversibility in the temperature dependence of magnetization, lack of magnetic saturation at high magnetic field, blocking of magnetization below TB, and enhancement of coercivity. In addition, the magnetic phase transition near to TC changes from first order character in bulk sample to second order character in nanocrystalline and amorphous samples. We understand the observed magnetic features as the effects of decreasing particle size and increasing magnetic (spin-lattice) disorder. The magnetic dynamics of amorphous samples is distinctly different from the nanocrystalline samples and also found to be comparable with the properties of reported amorphous ferromagnetic nanoparticles.