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.

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The significant influence of magnetic X-type hexaferrite-C3N4 binary nanofillers on the shielding effectiveness of poly­(vinylidene fluoride) heterostructure composite materials has been displayed in the present report. The overall improvement in shielding effectiveness of these multiphase composite structures is immensely important for the prevention of electromagnetic pollution in the GHz frequency range. The uniqueness of this magnetic-semiconducting binary composite system helps improve the shielding effectiveness due to absorption (SEA) and reflection (SER) as well as total shielding effectiveness (SET) of the X-type hexaferrite-C3N4-PVDF heterostructure composite materials. The high magnetic loss in terms of domain wall resonance together with the eddy current loss produced by these X-type hexaferrite-C3N4 binary composite nanofillers inside the polar structure of β-phase reaches the PVDF matrix and the dielectric loss of the composite materials helps improve SEA to a maximum value of approximately −62 dB at 13.95 GHz as well as SET to a maximum value of approximately −73 dB in the X-band and approximately −88 dB in the Ku-band of the resultant composite structures. The high attenuation (>99.999999%) of these X-type hexaferrite-C3N4-PVDF heterostructure composite materials makes them the most useful for the applications in the microwave/GHz frequency region.

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