Improved magnetic properties of self-assembled epitaxial nickel nanocrystallites in thin-film ceramic matrix

Nanocrystalline nickel particles were embedded in amorphous alumina and crystalline TiN matrices using a pulsed laser deposition process to investigate the effect of texturing on magnetic properties of nickel nanocrystallites. The crystalline quality of both the matrix and magnetic particles was investigated by cross-sectional high-resolution transmission electron microscopy. The embedded Ni nanocrystals were found to be epitaxial in the case of the TiN matrix and polycrystalline in Al₂O₃ amorphous matrix. The Ni nanocrystals on TiN/Si grow epitaxially because the TiN acting as a template grows epitaxially on Si substrate via domain epitaxy. On the other hand, Ni nanocrystals in the Al₂O₃ matrix are polycrystalline because of the amorphous nature of the alumina matrix. Magnetization versus temperature measurements have shown that the blocking temperature, above which the samples lose magnetization-field (M-H) hysteretic behavior, of the Ni-TiN sample (approximately 190 K) is significantly higher than that of Ni-Al₂O₃ sample (approximately 30 K) with a similar size distribution of embedded magnetic particles. A comparison of the values of coercivity (H_c) of the two samples, measured from M-H data, indicates that epitaxial Ni nanocrystals also exhibit significantly higher coercivity than polycrystalline Ni particles in amorphous alumina matrix. The high values of T_B and H_c of Ni-TiN samples with respect to T_B of Ni-Al₂O₃ samples are believed to be associated with preferred alignment of nanocrystallites.