F. D. Saccone

Extrinsic origin of ferromagnetism in single crystalline LaAlO3 substrates and oxide films

Commercial LaAlO3 substrates were thermally cycled simulating a procedure similar to those followed during TiO2 and SnO2 dilute magnetic semiconductors’ film pulsed laser deposition. Ferromagneticlike behavior was found in some substrates, in which metallic iron impurities were detected by x-ray photoelectron spectroscopy and total reflection x-ray fluorescence measurements. A thorough experimental investigation, using high resolution techniques, showed that these impurities were introduced by the procedure used to fix the substrates to the oven silicon holders. It is suggested that magnetism observed previously in nominally pure SnO2 films is of extrinsic origin.

Influence of Nb Doping on Magnetic Properties of Nanocrystalline Nd-Fe-B Alloys

The influence of Nb doping was studied on ribbons of nominal composition Nd_yFe_86-x-yB₁₄Nb_x (y = 7, 8; x = 0, 2, 4). The alloys were prepared by melt-spinning and then annealed at 973 K for 20 minutes. The magnetic characterization revealed that niobium addition improves substantially the magnetic properties of the studied alloys. The structural characterization showed that, in most alloys, the magnetically hard phase Nd₂Fe₁₄B precipitated as the main phase. This was not the case for the ternaries (x = 0) and for the Nd₇Fe₇₇B₁₄Nb₂ alloys. Furthermore, the addition of Nb inhibited the precipitation of α-Fe, independently of the Nd content. From the complete set of samples studied in this work, we found that the sample with nominal composition Nd₈Fe₇₄B₁₄Nb₄ exhibited the highest coercive field H_C = 575 kA/m while Nd₇Fe₇₅B₁₄Nb₄ showed the highest maximum energy product, (BH)_max = 73 kJ/m³.

Structural and Magnetic Properties of Multilayered TiO

Two types of multilayered thin films of structure TiO2/FM/TiO2/FM/CoFe2O4 with FM = Fe or Py were grown by means of the pulsed layer deposition technique with a total thickness of less than 350 nm. Their structural properties were analyzed by X-ray diffraction and the Fe K-edge absorption was measured by X-ray fluorescence in grazing incidence geometry. By means of near edge absorption X-ray spectroscopy (XANES), we retrieved information about the octahedral Fe+3 environment within the cobalt ferrite (CoFe2O4) films as well as the role of titanium oxide (TiO2) as a good protective layer for these multilayered systems. The bulk magnetic properties of both samples exhibit certain similarities: at 10 K, the coercive field of their hysteresis loops is enhanced to approximately 1 T, with an intermediate magnetization drop occurring at approximately 2000 Oe reverse field strength, indicating the separated magnetization reversal behavior of pinned and unpinned grains within the FM layers. In contrast to this low temperature field dependence, the collective magnetic behavior is gradual upon field inversion when hysteresis loops are measured at or near room temperature. By using the FORC diagram technique at room temperature, we investigated this gradual transition in more detail and identified clear differences in between the two FM layer materials.