Preparation of ferrimagnetic Mn4N compound films at low substrate temperature by reactive facing target sputtering (abstract)

Abstract

It has become attractive to prepare films of transition metal nitride for both the studies of ferrimagnetism and their practical applications. Mn4N is well known as one of the typical ferrimagnetic materials with an effective moment of Bohr magnetons of 1.0–1.2 μB per unit cell. However, high temperature processing is required to synthesis such materials. We have succeeded in depositing Mn4N films at a relatively low substrate temperature Ts of around 300°C by reactive facing target sputtering (r-FTS).1 In this study, Al and Cu underlayers were deposited to enhance the epitaxial effect between (111) preferred texture in such layers and the (111) texture of Mn4N crystallites in order to deposit Mn4N films with better crystallinity at lower Ts. Mn4N films were deposited on Si wafer substrates by sputtering a pair of sintered Mn disks in a mixture of Ar and N2 gases. Total gas pressure was set at 2 mTorr and the ratio in pressure of N2 gas to the RN2 mixture ranged from 0% to 40%. Ts was varied in the range of 40–400°C. Al and Cu layers with thicknesses of about 500 Å were used as underlayers. Mn–N films with (111) oriented Mn4N crystallites could be prepared at RN2 of 10% and Ts above 270°C. Those films revealed the saturation magnetization 4πMs of about k40 emu/cc. Excessive RN2 and Ts above 400°C caused the formation of another phase nitride, such as Mn3N2, which caused the degradation of 4πMs. Basically, Mn atoms in Mn4N crystallites form fcc lattices. Since Al and Cu atoms form fcc lattices and their lattice constant is almost the same as that of Mn4N, Al and Cu thin layers were used as underlayers. Substrate biasing of about −160 and −70 V during deposition of Al and Cu layers, respectively, was very effective in attaining (111) orientation in those films. Mn nitride films deposited at Ts as low as about 200°C on (111) oriented Al underlayers were composed of (111) oriented Mn4N crystallites which revealed 4πMs of about 24 emu/cc at room temperature. (111) oriented Cu layers were not as effective in depositing Mn4N films at low Ts. Consequently, Al underlayers were suitable to deposit films composed of (111) oriented Mn4N crystallites at low Ts of around 200°C.