Papers by Author: Shinichi Kikkawa

Abstract: Nanocrystals are important to attain high performance in optical & magnetic materials such as phosphors, laser emitters and information recording media. They are also required in future devices that involve magnetoresistance, logic gates, magnetic resonance and metamaterials. Nanocrystals of oxides and nitrides (and oxynitrides) were studied as nanosized powders, nanowires and dispersed granular thin films. Recent advancements of such nanocrystals prepared at Hokkaido University are introduced in this paper. Nanocrystals were prepared in transparent conducting oxides, white LED phosphor oxides and oxynitrides and magnetic iron nitride. Nanowires were grown in semiconducting gallium oxynitride and magnetic nanogranular thin films were prepared both in oxide and nitride.

Abstract: Sputter deposited Fe0.7Co0.3 nitride thin film had zinc blende structure. It was thermally decomposed completely back to the ferromagnetic Fe0.7Co0.3 alloy above 400°C. As-deposited nitride thin films obtained in cosputtering of (Fe0.7Co0.3)1-xAlx composite target with nitrogen sputter gas were solid solutions with zinc blende (x≤0.44) and wurtzite (x>0.5) type structure, respectively. The largest magneto resistance ratio of 0.24% was observed on the Fe0.7Co0.3 alloy particles dispersed in AlN thin film obtained by thermal decomposition of the nitride solid solution with x=0.66 at 500°C.

Abstract: The oxynitrides with composition A1-xBx(OyNz) (A: Nb and Ta, B: Al) were studied in preparation through citrate route and NH3 nitridation. In the case of Nb, the product of x = 0.5 prepared at 1000 °C was a new compound of a = 0.435 nm with rock salt structure. In this structure, Al and Nb atoms were distributed randomly in the cation sites. Anion sites were also randomly occupied by oxide and nitride ions with some amount of vacancy. The chemical composition was represented as Nb0.56Al0.44O0.38N0.37□0.25 from Rietveld refinement and oxygen/ nitrogen measurement. In the case of Ta, monoclinic Ta3N5 crystallized with a small amount of Ta4N5 impurity at x = 0.5. Aluminum compound was co-present as amorphous impurity.

Abstract: Iron nitrides thermally decompose to α-Fe releasing their nitrogen above 300°C. MR effect was found out in the thin films obtained by post-annealing of the following two kinds of sputter deposited iron nitride related films. (1) α-Fe particles dispersed in AlN granular film was obtained by an annealing of Al0.31Fe0.69N sputter deposited film in hydrogen. The MR=0.82% was found out in this nitride system. (2) Fe3O4 thin films were prepared by thermal decomposition of sputter deposited iron nitride films in low oxygen partial pressure. The iron nitrides were defect rock salt type γ΄˝-FeNx (0.5≤x≤0.7) and zinc blende type γ˝-FeNy (0.8≤y≤0.9) at the sputter nitrogen gas pressure of 1Pa and 6Pa. MR ratios of the oxide films were about 2%.