Single crystal martensitic phase of structural properties-related magnetic behaviour of FeCrNi thin films: in-plane magnetic anisotropy under different substrate rotation speeds

Ternary FeCrNi thin films were sputtered on polyimide substrates from the source (austenitic AISI 304 stainless steel) under the substrate rotation speeds of 0, 15, 30 and 45 rpm, respectively. The films with 50 nm thickness were sputtered at 0.9 nm/s. To the elemental measurements, while the Fe content slightly varied, Ni increased and Cr content decreased as the rotation speed increased. And, all films have a single crystal of (110) peak at the angle of 2θ ≈ 44.7° which is body-centred tetragonal (bct) martensitic α′-phase. Also, the peak intensity of (110) slightly varied as a result of changes of FeCrNi martensitic alloy contents in the films. Moreover, increasing substrate rotation speed resulted in a decrease in grain size. The morphologic analysis by a scanning electron microscope and an atomic force microscopy displayed a more homogenous structure and the decrease of film roughness parameters with increasing of rotation speed, respectively. For magnetic measurements, the saturation magnetisation, M_s increased from 995 to 1172 emu/cm³ and the coercivity, H_c decreased from 88 to 52 Oe with the increase of rotation speed. In the films, the increases of the martensitic structure formations and the decrease of the grain size were observed to have probably caused the increase of the M_S values and the decrease of the H_C values, respectively. As observed from the XRD results, ferromagnetic behaviour of the films may be due to the martensitic phase since austenitic phase has a weak paramagnetic behaviour as a source material at room temperature. From the parallel and perpendicular hysteresis loops, the films can also be obtained to have uniaxial in-plane anisotropy with increasing rotation speed. It is seen that the film properties can be easily varied by changing rotation speed for potentially new device applications such as spintronics, magnetic hetero-structures, magnetic separators, etc. on flexible substrates.