Defect induced ferromagnetism in MgO and its exceptional enhancement upon thermal annealing: A case of transformation of various defect states.
MgO particles in few micron size are synthesized through sol-gel method at different annealing temperatures such as 600 0C (MgO-600), 800 0C (MgO-800) and 1000 0C (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 TC (Curie temperature)around65 K (±5 K). Unexpectedly, the saturation magnetization (Ms) 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. Positron annihilation lifetime measurement study indicated presence of divacancy (2VMg + 2VO) cluster in case of the low temperature annealed compound, which underwent dissociations into isolated mono vacancies of Mg and O at higher annealing temperature. EPR study showed both singly charged Mg vacancies (VMg-1) and oxygen vacancies (VO+) are responsible for the ferromagnetic ordering. It also showed that at lower annealing temperature the contribution from VMg-1 was very low while at higher annealing temperature, it was increased significantly. PL study showed that most of the F+ centers were present in their dimer form i.e. F22+ centers. DFT calculation inferred that this dimer form has higher magnetic moment than the monomer. After a careful consideration of all these studies, for the first time of its kind of report, this thermally tunable unusual magnetism phenomenon was attributed to a transformation mechanism of one kind of cluster vacancy to another.