Self-assembled epitaxial and polycrystalline magnetic nickel nanocrystallites

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 were investigated by cross-sectional high-resolution transmission electron microscopy. The embedded Ni nanocrystals were found to be epitaxial in case of TiN matrix and polycrystalline in ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ 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 ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ 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 $\mathrm{(M–H)}$ hysteretic behavior, of Ni–TiN sample (∼190 K) is significantly higher than that of ${\mathrm{Ni–Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ sample (∼30 K) with a similar size distribution of embedded magnetic particles. A comparison of the values of coercivity ${\mathrm{(H}}_c)$ of the two samples, measured from $\mathrm{M–H}$ data, indicates that epitaxial Ni nanocrystals also exhibit significantly higher coercivity than polycrystalline Ni particles. The high values of $T_B$ and $H_c$ of Ni–TiN samples with respect to $T_B$ of ${\mathrm{Ni–Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ samples are believed to be associated with preferred alignment of nanocrystallites.