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Issue 19, 2017
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Defect induced ferromagnetism in MgO and its exceptional enhancement upon thermal annealing: a case of transformation of various defect states

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Abstract

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 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. A positron annihilation lifetime measurement study indicated the presence of a divacancy (2VMg + 2VO) 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 Image ID:c7cp01776f-t1.gif and oxygen vacancies Image ID:c7cp01776f-t2.gif are responsible for ferromagnetic ordering. It also showed that at lower annealing temperatures the contribution from Image ID:c7cp01776f-t3.gif 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 Image ID:c7cp01776f-t4.gif 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.

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

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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., 2017,19, 11975-11989
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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. S. Ghosh, B. Kanrar, P. K. Pujari and R. M. Kadam, Phys. Chem. Chem. Phys., 2017, 19, 11975
    DOI: 10.1039/C7CP01776F

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