Structure and soft magnetic properties of Fe-N thin films RF-sputtered on heated substrate

Abstract

The film structures and soft magnetic properties have been investigated for Fe-N films prepared by RF reactive sputtering at a substrate temperature of 473 K. The substrate heating during film deposition is effective in improving the soft magnetic property of the Fe-N film. As the ratio of the N2 gas pressure to the (Ar+N2) gas pressure, R(=PN2/Ptotal) in the sputtering gas increases, the film structure changes from the α-Fe single phase to a mixture of α-Fe and γ′-Fe4N. The lowest coercive force (Hc=63.6 A/m) and the highest initial permeability (μ′ = 1500 at 1 MHz) are obtained for the film prepared under a sputtering condition of R=8%. This film maintains a high magnetic flux density (Bs=2.16 T) which is the same as that of bulk Fe. The excellent soft magnetic property is obtained when the film is a mixture of fine grains of α-Fe and γ′-Fe4N without any preferential orientation. At R=3-5%, the Bs value of the film exceeds that of pure Fe, and the maximum value is Bs=2.44 T. This is attributed to the nitrogen incorporation in the α-Fe matrix.