MgO particles of few micron size are synthesized through a sol-gel method at different annealing temperatures such as 600 °C (MgO-600), 800 °C (MgO-800) and 1000 °C (MgO-1000). EDX and ICP-AES studies confirmed a near total purity of the sample with respect to paramagnetic metal ion impurities. Magnetic measurements showed a low temperature weak ferromagnetic ordering with a T (Curie temperature) around 65 K (±5 K). Unexpectedly, the saturation magnetization (M) was found to be increased with increasing annealing temperature during synthesis. It was observed that with J = 1 or 3/2 or S = 1 or 3/2, the experimental points are fitted well with the Brillouin function of weak ferromagnetic ordering. A positron annihilation lifetime measurement study indicated the presence of a divacancy (2V + 2V) cluster in the case of the low temperature annealed compound, which underwent dissociations into isolated monovacancies of Mg and O at higher annealing temperatures. An EPR study showed that both singly charged Mg vacancies and oxygen vacancies are responsible for ferromagnetic ordering. It also showed that at lower annealing temperatures the contribution from was very low while at higher annealing temperatures, it increased significantly. A PL study showed that most of the F centers were present in their dimer form, i.e. as centers. DFT calculation implied that this dimer form has a higher magnetic moment than the monomer. After a careful consideration of all these observations, which have been reported for the first time, this thermally tunable unusual magnetism phenomenon was attributed to a transformation mechanism of one kind of cluster vacancy to another.

In the quest of materials with high temperature ferromagnetism and low temperature anti-ferromagnetism, we prepare Co3-xMnxTeO6; (0 < x ≤ 2) solid solutions. Room temperature structural investigations on these solid solutions as a function of Mn concentration using Synchrotron X-ray diffraction (SXRD) and X-ray absorption near edge structure measurements in corroboration with magnetism are presented. Phase diagram obtained from Rietveld Refinement on SXRD data as a function of Mn concentration indicates doping disproportionate mixing of both monoclinic (C2/c) and rhombohedral (R 3¯) structure for x < 0.5, while only R 3¯ structure for x ≥ 0.5. Further, it shows increase in both lattice parameters as well as average transition metal-oxygen (Co/Mn-O) bond lengths for x ≥ 0.5. Co and Mn K-edge XANES spectra reveal that both Co and Mn are in mixed oxidation state, Co2+/Mn2+ and Co3+/Mn3+. Relative ratios of Co3+/Co2+ and Mn3+/Mn2+ obtained using Linear combination fit decrease with increasing x (for x ≥ 0.5). These structural and spectroscopic evidences are used to provide possible interpretation of the observed paramagnetic to ferromagnetic transition at around 185 K followed by an enhanced antiferromagnetic transition ∼45 K for x = 0.5.

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The role of doping Fe on the structural, magnetic, and dielectric properties of frustrated antiferromagnet YMn1−xFexO3 (x ≤ 0.5) has been investigated. The neutron diffraction analysis shows that the structure of these polycrystalline samples changes from hexagonal phase (space group P63cm) to orthorhombic phase (space group Pnma) for x > 0.2. The frustration parameter decreases with Fe substitution. All the compounds are antiferromagnetic, and the magnetic structure is described as a mixture of Γ3 and Γ4 irreducible representation (IR) in the hexagonal phase, and the ratio of these two IRs is found to vary with Fe doping (x ≤ 0.2). A continuous spin reorientation as a function of temperature is observed in these samples. The magnetic ground state in the orthorhombic phase of the higher doped samples (x ≥ 0.3) is explained by taking Γ1 (GxCyAz) representation of Pnma setting. In YMnO3 suppression of dielectric constant ε′ is observed below TN indicative of magnetoelectric coupling. This anomalous behavior reduces in Fe doped samples. The dielectric constant is found to be correlated with the magnetic moment (M) obtained from neutron diffraction experiments and follows a M2 behavior close to TN in agreement with Landau theory.

A series of Co3−xMnxO4 (x=0.1–1.0) multiferroic cubic spinel ceramics were prepared to study the effect of Mn substitution at Co site on the crystal structures and dielectric properties. No significant change in the structural symmetry was observed with increasing x up to 1.0. A linear increase in lattice parameter with x is attributed to the substitution of Co3+ by Mn3+ (large ionic radii) at the octahedral sites. An antiferromagnetic-type ordering of Co3O4 changes to ferrimagnetic-type order after incorporation of Mn. The effect of Mn substitution on the dielectric constant and loss tangent was studied over a wide range of frequency (75 kHz–5 MHz) and temperature of 150–450 K. The measured value of room temperature ac conductivity at 1.0 MHz was found to increase from 2.0×10−6 to 4.4×10−4 Ω−1 cm−1 and follows power law (σac=Aωs) behavior. The dielectric constant ε′(ω) shows a weak frequency dispersion and small temperature dependence below 250 K for all ceramic samples. However, a strong temperature and frequency dependence on ε′(ω) was observed at higher temperature (>250 K). The temperature dependent ε′(ω) data show the existence of room temperature ferroelectricity in all prepared samples.