This article reports on reversible manipulation of room temperature ferromagnetism (RTFM) in nondoped bulk CeO2. The magnetization measurements establish that paramagnetic CeO2 is driven to a ferromagnetic state, without change in structure, when vacuum annealed at 600 °C. The Ce ions transform from 4+ to 3+ state, accompanied by evolution of oxygen vacancies (VO) during the RTFM transition, as determined by x-ray photoemission. The F+ centers (the electrons in singly occupied oxygen vacancies) play key role in the exchange mechanism. The transition shows complete reversibility where the RTFM is removed by removing the vacancies through re-heating the vacuum-annealed CeO2 in air.

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In this work, we demonstrate that room temperature ferromagnetism can be induced in pristine anatase TiO2 paramagnetic bulk powder through extended hydrogenation. Defect complexes, Ti3+–VO (Ti3+ ions accompanied by oxygen vacancies) are clearly identified in hydrogenated TiO2 by combining x-ray diffraction and photoemission spectroscopy. The observed ferromagnetic ordering is reversible that can be switched between “on” and “off” by inducing or removing, respectively, these defect complex. We convincingly elucidate that the factors (i) Ti 3d–O 2p hybridization (iii) F+ centers (the electrons in singly occupied oxygen vacancies), and (iii) oxygen vacancy assisted fragmentation of grains, compositely contribute to the ferromagnetic ordering.

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The electronic structure and magnetic properties of cobalt‐doped (7.5%) and manganese co‐doped (2.5%) ZnO polycrystalline samples have been investigated to understand the mechanism of room temperature ferromagnetism (RTFM) in dilute magnetic semiconductors. The samples in powder form were annealed in argon and hydrogen atmospheres followed by their repressing into pellets and reheating in air. Rietveld analysis of X‐ray diffraction patterns confirmed the single‐phase nature of the samples in the wurtzite type hexagonal (P63mc) ZnO structure. The X‐ray photoelectron spectroscopy (XPS) results indicate that the Co and Mn atoms are in +2 oxidation states, which incorporate at the Zn2+ site, with no signature of metallic clusters. The Co‐doped sample prepared in air displays a paramagnetic state while the sample annealed in Ar atmosphere shows a weak ferromagnetic ordering at 300 K. The co‐doping of Mn further enhances the ferromagnetic ordering, indicating that Co and Mn ions play an additive role in inducing the ferromagnetic ordering in the ZnO matrix. Interestingly, the Co‐ and (Co + Mn)‐doped ZnO samples annealed in hydrogen atmosphere show a huge increment in the magnetic moment, however, the Mn ions seem to stay passive towards the hydrogen induced magnetization. Notably, the samples reheated in air show suppression of the induced ferromagnetism (FM). The resistance measurements suggest that the additional carriers induced upon hydrogenation also play some role in mediating the exchange coupling. The O 1s XPS and the XRD results show clear evidence of oxygen depletion in the samples upon hydrogenation, followed by a recovery upon their reheating in air. The observed FM is explained in terms of composite effect of the oxygen vacancies and the carrier density. Our results point out that the ferromagnetic ordering could be switched between ‘on’ and ‘off’ by introducing (through hydrogenation) and by removing (through reheating in air) the oxygen vacancies in Co‐doped ZnO.

We have investigated the temperature-dependent changes in the near-E F occupied and unoccupied states of Pr 0.67 Ca 0.33 MnO 3 which shows the presence of ferromagnetic and antiferromagnetic phases. The temperature-dependent changes in the charge and orbital degrees of freedom and associated changes in the Mn 3d -O 2p hybridization result in varied O 2p contributions to the valence band. A quantitative estimate of the charge transfer energy (E CT ) shows a larger value compared to the earlier reported estimates. The charge localization causing the large E CT is discussed in terms of different models including the electronic phase separation.

Electronic and magnetic properties of Co-doped TiO2 polycrystalline pellets (Ti0.95Co0.05O2) have been investigated using x-ray diffraction, x-ray photoemission, magnetization, and resistance measurements. The as-synthesized and hydrogenated specimens crystallize in the anatase type tetragonal structure containing very small (∼4.4%) rutile phase. The dopant ions of Co are found to be divalent and well incorporated into TiO2 lattice, substituting the Ti site within the anatase phase, with no evidence of metallic Co or any other oxides of Co. The Co doping induces a weak ferromagnetic ordering in the diamagnetic TiO2 host matrix. Interestingly, when the Co-doped TiO2 is annealed in hydrogen atmosphere, it shows a giant enhancement in magnetization. However, an extended reheating in air causes this H-induced magnetization to vanish and the sample regains the as-prepared status. Our findings indicate that ferromagnetism originates from the doped matrix rather than any magnetic clusters and strongly correlated with oxygen vacancies in the doped TiO2. The induced ferromagnetic ordering is found to be a reversible process with regard to oxygen vacancy defects that could be induced or removed, respectively, upon introduction or removal of these defects.