FeHfN and FeHfNO soft magnetic films for RF applications

In this work, recent improvements on high magnetization FeHfN films for RF-use and possibility of very high resistivity with introducing oxygen during deposition are investigated. The films are deposited at room temperature by reactive RF diode sputtering in a mixed Ar + N₂ atmosphere. N-incorporation leads to lattice expansion and grain size refinement. Up to N ∼ 10 at.%, the films consist of very fine nanostructure, encompassing excellent magnetic properties and retaining relatively high magnetization. The resistivity ρ increases almost linearly with N but remains at a moderate value for N ∼ 10 at.%. However, the soft magnetic properties rapidly degrade and the 4πMs strongly decreases while the film consists of amorphous-like microstructure with scattered large HfN precipitates. Having 4πMs = 19 kG, Hk = 20 Oe, and ρ = 130 μΩ-cm, a natural ferromagnetic resonance frequency occurs at 1.84 GHz with dc permeability μ´ close to 1000. The insertion of small O₂ flow rates is instrumental in increasing ρ above 1000 μΩ-cm. A few O₂ flow rate also allows a soft magnetic behaviour with Hk increasing with O₂. The N-incorporation in FeHfN results in promoting and increasing Hk, the O₂ addition seems to compete with the nitrogen effect.