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

Abstract

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) around 65 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.

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Publication details

The article was received on 20 Mar 2017, accepted on 12 Apr 2017 and first published on 12 Apr 2017


Article type: Paper
DOI: 10.1039/C7CP01776F
Citation: Phys. Chem. Chem. Phys., 2017, Accepted Manuscript
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    Defect induced ferromagnetism in MgO and its exceptional enhancement upon thermal annealing: A case of transformation of various defect states.

    N. Pathak, S. K. Gupta, C. L. Prajapat, S. K. sharma, P. Ghosh, B. Kanrar, P. K. Pujari and R. M. Kadam, Phys. Chem. Chem. Phys., 2017, Accepted Manuscript , DOI: 10.1039/C7CP01776F

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