The effect of the condensation temperature on the formation of metastable phases in vapour-deposited Fe-Ni-Cr films and their magnetic properties

Abstract

The effect of vapour quenching on the size of the α + γ two-phase region in the system Fe-Ni-Cr was studied. Fe-Ni-Cr films of thickness 500 nm were deposited on silicon substrates by co-evaporation from a chromium and an Fe-Ni source at 2 nm s^-1. The substrate temperature was varied between 77 and 673 K. The Fe-Ni binary system (X_Cr/X_Ni = 0) and two vertical sections through the ternary Fe-Ni-Cr system at Cr:Ni ratios X_Cr/X_Ni of 0.37 and 1.38 were investigated. The iron content was between 40 and 100 at.%.

X-ray analysis at room temperature revealed that at X_Cr/X_Ni = 1.38 the body-centred cubic α phase, the face-centred cubic γ phase and the tetragonal σ phase were present whereas at X_Cr/X_Ni = 0 and at X_Cr/X_Ni = 0.37 only α and γ were found. At X_Cr/X_Ni = 0 and at X_Cr/X_Ni = 0.37 the width of the α + γ two-phase region was found at all temperatures to be considerably less than that in the equilibrium situation. Moreover for X_Cr/X_Ni = 0.37 the α + γ two-phase region disappeared between 373 and 173 K. The shape and position of the α + γ region could be explained by the assumption that α→α + γ or γ→α + γ transformations occured at high temperatures by diffusion and that an incomplete diffusionless γ→α martensitic transformation occured at low temperatures during film growth and/or subsequent cooling.

The single-phase α films exhibited a maximum of the saturation magnetization of 227 emu g^-1 at 87.5 at.% Fe, 7.5 at.% Ni and 5 at.% Cr. The coercivity ranged between 0.8 and 59 Oe. The highest values of H_c were found for the multiphase α + γ + σ and α + γ films.