Extensive structural and magnetic analyses of Hf 1−x Co x O 2 thin films grown by molecular beam epitaxy are reported. Nearly cobalt cluster-free film with x = 0.04-0.1 was obtained via 100°C growth, and Co ions are inferred to be located at interstitial site. Ferromagnetic behavior was observed up to 300 K in both magnetization curves and temperature dependence of the moment. Via post-oxygen-annealing studies, a qualitative correlation between saturation magnetization and oxygen vacancy concentration is established, consistent with the impurity-band exchange model, and that the occurrence of ferromagnetic insulator behavior in the Co doped HfO 2 is more probable than Co doped ZnO, TiO 2 , and SnO 2 systems for doping concentrations under cation percolation threshold.
Pure phase of [Co3(pyz)(HPO4)2F2] crystal powder was successfully synthesized. The structure consisted of neutral sheets of [Co3(HPO4)2F2], which were pillared through pyrazine ligand to form three-dimensional frameworks. Below 20 K, the magnetic hysteresis loops showed a ferromagnetic part superimposed with an antiferromagnetic (AF) part. With increasing field magnitude, the AF part showed sharp steps at 5 kOe. Micromagnetic simulation, taking into account the Co ions as AF coupled sublattices and as suggested by the negative Weiss temperature extrapolated by the inverse susceptibility versus temperature behavior, explained the hysteresis loops the temperature dependence qualitatively.
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