Hollow NiFe2O4 nanoparticles are synthesized by self-templating process utilizing coupled interfacial chemical reactions and Kirkendall effect between the core (Ni33Fe67) and the shell (NiFe2O4) of the core/shell structure. Reaction temperature and time dependent structural and morphogical transformations are presented in detail. The kinetics of the transformation from (Ni33Fe67)/(NiFe2O4) nanoparticles to single phased NiFe2O4 hollow nanoparticles was studied by differential scanning calorimetry. Hollow morphology of the particles induces surface effects in the magnetic properties due to the formation of additional inner surfaces. Field cooled hysteresis loop exhibits significantly large shift due to unidirectional anisotropy resulting from the additional inner spin disordered surface along with the existing outer spin disordered surface. The enhancement in the surface anisotropy is also noticeable which leads to an increase in the blocking temperature of the particles with hollow morphology.

ZnO nanoparticles doped with Li (Zn 1Ày Li y O, y 0.1) have been investigated with emphasis on the correlation between their magnetic, electronic, and structural properties. In particular, defects such as interstitial Li and Zn atoms, substitutional Li atoms, and oxygen vacancies have been identified by X-ray photoelectron spectroscopy (XPS) and their respective roles in stabilization of the magnetic moment are discussed. X-ray diffraction (XRD) and XPS give clear evidence of Li presence at both substitutional and interstitial sites. XPS studies further show that the amount of substitutional Li defects (Li zn) and interstitial Li defects (Li i) vary non-monotonically with the Li concentration, with the Li i defects being noticeably high for the y ¼ 0.02, 0.08, and 0.10 concentrations, in agreement with the XRD results. Magnetization studies show room temperature ferromagnetism in these nanoparticles with the moment being largest for the particles with high concentration of interstitial lithium and vice versa. Both interstitial Zn (Zn i) defects and Zn-O bonds were determined from the Zn LMM Auger peaks; however, the variation of these with Li concentrations was not large. Oxygen vacancies (V o) concentrations are estimated to be relatively constant over the entire Li concentration range. We relate the Li i and Zn i defects to the formation and stabilization of Zn vacancies and thus stabilizing the p-type ferromagnetism predicted for cation (zinc) vacancy in the ZnO type oxides. V

We report stabilization of magnetic glassy state in non-stoichiometric nickel ferrite thin films prepared by pulse laser deposition. Details of electronic structure of the films are presented and compared with stoichiometric bulk counterpart. Hard x-ray photoelectron spectroscopy shows significant amount of oxygen vacancies and enhanced cationic inversion for thin films. Films show spin glass (SG) features which is contrary to the usual ferrimagnetic response of the bulk nickel ferrite. Films exhibit spin freezing temperature which is above room temperature in low fields (0.1 T) and shifts to lower temperature (∼250 K) in the presence of a large applied field of 3 T. An exceptionally large exchange bias (EB) of 170 Oe at a significantly higher temperature (∼50 K) is measured in cooling field of 3 T. In comparison, bulk samples do not show exchange bias and magnetic irreversibility vanishes in significantly weaker fields (i.e., few kOe). Role of oxygen vacancies is to induce spin canting by destabilizing indirect super exchange interaction. Consequently, the spin-glass like behavior occurs that is coupled with huge suppression in saturation magnetization in the thin films. Observation of exchange bias is explained to be due to oxygen vacancies (hence non-stoichiometry) which generates random anisotropy in exchange coupled grains.

Nickel ferrite thin films were synthesized by pulsed laser deposition. It was determined that the monotonic increase in saturation magnetization and the non-monotonic increase in electrical conductivity depend on the oxygen partial pressure during the growth of the thin films. A substantial reduction in magnetization was found which ranged between 0.4% and 40% of the bulk value as the oxygen partial pressure increased from 0.2 × 10−6 Torr to 500 mTorr during the deposition of the films. There was a three orders of magnitude increase in conductivity for the sample prepared under the most oxygen deficient environment (partial pressure of oxygen 0.2 × 10−6 Torr). These variations in saturation magnetization and conductivity are described within the framework of cation/oxygen vacancies in an inverse spinel nickel ferrite structure. The changes in the electronic structure due to the presence of the vacancies were investigated using x-ray photoelectron spectroscopy, which confirmed the formation of lower valent Ni for the samples prepared in an oxygen deficient atmosphere.