Defect induced ferromagnetism in MgO and its exceptional enhancement upon thermal annealing: a case of transformation of various defect states

MgO particles of few micron size are synthesized through a sol-gel method at different annealing temperatures such as 600 degrees C (MgO-600), 800 degrees C (MgO-800) and 1000 degrees 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-C (Curie temperature) around 65 K (+/- 5 K). Unexpectedly, the saturation magnetization (M-s) 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(Mg) + 2V(O)) 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 (V-Mg(1-)) and oxygen vacancies (V-O(+)) are responsible for ferromagnetic ordering. It also showed that at lower annealing temperatures the contribution from V-Mg(1-) 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 F-2(2+) 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.