We provide the first experimental evidence of oxygen vacancy defect induced ferromagnetism in undoped multiferroic CaMn7O12 (CMO) manganite synthesized from the facile chemical combustion method. The obtained nanocrystalline is characterized by various techniques like TGA, FTIR, XRD, SEM-EDX, AFM, UV-visible, XPS, and SQUID, etc. to confirm the phase purity and crystallinity of CMO. Surface roughness increases with sintering time attributed to the increase of surface oxygen vacancy defects. X-ray photoelectron spectroscopy was carried out to confirm the oxidation state of constituent elements and also provides direct evidence of enhanced oxygen vacancies. UV–vis optical absorption used to infer band gap shift from 1.68 to 1.38 eV, respectively, is also attributed to increases in oxygen vacancy defects. Multiple magnetic phase transition temperatures of 90, 50, and 42 K, respectively, were obtained from the derivative of magnetization. A systematic decrease of full widths at half maxima (fwhm) of dM/dT vs T curves with sintering time indicates strengthening of ferromagnetism (FM). Transition temperature does not change significantly with sintering time, indicating the extrinsic origin of FM. The results of the UV–vis, XPS, and AFM and strengthening of ferromagnetism all are corroborated with each other’s results and also attribute to enhanced oxygen vacancy concentration with sintering time. The origin of FM in undoped CMO manganite with sintering time results from bound magnetic polarons (BMPs) of enhanced iterant and the localized electron of oxygen vacancies trapped center at the surface or interfaces. Our finding also opens a new perspective for exploiting oxygen vacancy defect engineering at surfaces or interfaces in the design of exotic magnetic- and spintronics-based devices.

In the present work, the nanocomposite 0.9CaCu3Ti4O12–0.1BaTiO3 (CC-BT) is synthesized by a solid-state reaction method by sintering at 950°C for 12 h. X-ray diffraction analysis confirms the presence of both BaTiO3 and CaCu3–Ti4O12 phases in the composite ceramic. Transmission electron microscopy analysis of the composite demonstrates the formation of nanoparticles with an average particle size of 40 ± 5 nm. The surface morphology of the composite sintered at 950°C for 12 h obtained by scanning electron microscopy analysis indicates the evolution of large and small grains with a bimodol distribution. The average and root mean square roughness were found to be 1.41 nm and 2.24 nm respectively by atomic force microscopy studies. The dielectric constant of CC-BT ceramic was determined to be 6231 at 100 Hz and 500 K. The presence of the semiconducting grains and the insulating grain boundaries in the composite supports the internal barrier layer capacitance mechanism operative in CC-BT nanocomposite.