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We investigated the static and dynamic aspects of the magnetic properties for single phase nanocrystalline BiFeO 3 with average crystallite size of 35 nm. The frequency dependence of the peak is observed in the real part of ac susceptibility χ′ ac vs T measurement and described well by the Vogel-Fulcher law as well as the power law. These analyses indicated the existence of cluster glass state with significant interaction among the spin clusters and results in cluster-glass like cooperative freezing at low temperature. The influence of temperature and magnetic field cooling on the exchange bias effect is investigated. A training effect is also observed. We have reported a significantly high ZFC & FC exchange bias of 200 Oe & 450 Oe at 300 K and 900 Oe & 2100 Oe at 5 K. The obtained results are interpreted in the framework of core-shell model, where the core of the BFO nanoparticles shows antiferromagnetic behavior and surrounded by CG-like ferromagnetic (FM) shell associated to uncompensated surface spins.

We report synthesis, characterization and magnetic properties of exchange-coupled L 1 0 -FePt/fcc-FePt nanomagnets. Structural and morphological characterization of exchange-coupled L 1 0 -FePt/fcc-FePt was carried out by powder X-ray diffraction, Mossbauer spectroscopy and transmission electron microscopy. Rietveld refinement of powder X-ray diffraction pattern has been used to quantify L 1 0 -FePt and fcc-FePt phases present in samples. Room temperature Mossbauer spectroscopy showed sextets of both L 1 0 -FePt and fcc-FePt phases with their respective hyperfine interaction parameters. Transmission electron microscopic (TEM and HRTEM) images confirmed nanocrystalline nature of exchange-coupled nanomagnets with particle size ranges from 15 nm to 50 nm after annealing for different time at 700 °C. Room temperature magnetic studies showed ferromagnetic nature of nanomagnets and maximum energy product (BH) max ~10.92 MGOe was obtained for sample containing 89.0% volume fraction of L 1 0 -FePt phase.

We report the room-temperature ferromagnetism in Cr-doped TiO 2 nanorods (NRs) synthesized via the hydrothermal method followed by annealing at 700 °C. The transmission electron microscopy showed that the diameter of the NRs is between 40 and 70 nm. X-ray diffraction, XPS and Raman technique are used to confirm the absence of magnetic contaminations of metallic Cr clusters or any other phases. To test the oxygen vacancy related ferromagnetism, annealing in oxygen, air and argon are performed. With the changing of annealing atmosphere from oxygen to argon, the saturation magnetization ( M s ) of NRs increases. These results strongly prove that the oxygen vacancies play an important role in inducing the ferromagnetism. The shifting and broadening of the most intense E g (1) mode at 144 cm −1 observed by Raman spectroscopy further confirms that the origin of the ferromagnetism in NRs is due to the oxygen vacancies.

In zero field cooled and field cooled measurements of nanosized MnFe2O4, two peaks were observed and the one at high temperature was attributed to the typical superparamagnetic blocking of moments of the particles. A weak maximum occurs at the temperature around 21 K, which is attributed to the freezing of moments at the surface of the particles. In the specific heat measurements, a sharp change was observed near the freezing temperature. The transition temperature observed in the specific heat was found to be field dependent.

Nanocrystalline γ′-Fe4−xNixN (0.2⩽x⩽0.8) compounds were synthesized by using a citrate precursor route. It was observed that lattice constants for γ′-Fe4−xNixN (0.2⩽x⩽0.8), decrease with increasing Ni atom concentration. The atmospheric oxidation of γ′-Fe4−xNixN (0.2⩽x⩽0.8) compounds result in the formation of Fe-oxide layer at the surface of the ultrafine particles in addition to pure nitride phase. The local magnetic structures of the iron atoms in the nitride materials are quite similar to those found in the case of dilute alloy systems. The role of Ni substitution in γ′-Fe4N as γ-Fe4−xNixN is investigated with regard to crystal structure and magnetic properties of the ultrafine nitride materials. The results suggest that the average magnetic moment per iron atom in ultrafine γ′-Fe4−xNixN compounds is affected by the Ni atom concentrations, superparamagnetic relaxation, presence of oxide layer and randomly canted spin structure at the particle surface. The observed larger values of coercivities in the ultrafine materials are on account of the reversal of magnetization by spin rotation mechanism.