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

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 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.

Investigating Cationic Distribution, Defect Structure and Phosphor Characteristics of Na2Sm2Ca6(PO4)6F2

“…Figure showed the room temperature recorded EPR spectrum of Na 2 Sm 2 Ca 6 (PO 4 ) 6 F 2 which showed a resonance signal for Lande g factor(g)= 2.046.Similar kind of resonance signal was also observed very recently in Apatite‐Type (Ba,Sr) 5 (PO4) 3 Br matrix and it was attributed to oxygen vacancy. We have also observed similar resonance signal for oxygen vacancy with g value close to the present samples . In a oxide based matrices the oxygen vacancies can exist in different charged states such as i) neutral i. e. V O 0 , which is formed when two electrons are trapped in doubly charged oxygen vacancy, ii) singly charged oxygen vacancy i. e. V O + .…”

Atomic-scale investigation of MgO growth on fused quartz using angle-dependent NEXAFS measurements

“…27 Thus, NEXAFS investigations were extended to angle-dependent measurements in the present study. [28][29][30] In recent years, MgO has attracted more interest due to its unusual magnetism [31][32][33][34] and optical behavior. [35][36][37] This material is also known for phenomena like spin-dependent electron reection, 38 multilevel switching characteristics, 39 and broadband laser emission.…”

Effect of synthesis conditions on room-temperature ferromagnetic properties of Mg-O nanoparticles

“…The authors have observed that although oxygen vacancies make MgO optically active, these vacancies have an opposite effect on the magnetism. In 2017 Pathak et al [27] performed some positron annihilation lifetime measurements which indicate the presence of divacancy (2V Mg + 2V O ) cluster along with Mg vacancies at the low annealing temperature. An EPR study showed that MgO compound possesses both singly negative charged Mg vacancies and singly positively charged oxygen vacancies and the spins of both of these vacancies are ferromagnetically coupled.…”