Influence of stress and texture on soft magnetic properties of thin films

This paper presents the relationship among stress, crystallographic texture, and soft magnetic properties of thin films. The magnetic properties considered are those affected by the formation of stripe domains and by the formation of magnetization ripple. In practice, one of these two undesirable domain structures is almost always the impediment to improving the soft magnetic properties. The theoretical analysis accounts for the contribution from stress through magnetostrictive anisotropy, and calculates the resulting total, stress-dependent anisotropy of the film. This anisotropy is then analyzed to yield the effective perpendicular and local in-plane anisotropy constants. These constants allow the calculation of the stripe domain onset thickness through Murayama's stripe domain theory, and the ripple coercivity through Hoffmann's ripple theory. The influence of stress and texture on the stripe domain onset thickness and ripple coercivity is theoretically calculated and experimentally verified for the examples of sputtered CoFe and FeAlN films, two of the most promising and widely studied materials for high-density recording head poles. The results indicate that the interactions between stress and soft magnetic properties depend on the details of composition, growth texture, growth morphology (whether the film is grown in columnar fashion or as equiaxial crystallites), grain size, and thickness of the film under consideration. On the basis of these results, the paper offers a systematic approach to choosing the appropriate composition, texture, and thickness so that low stresses simultaneously promote magnetic states in which stripe domains are suppressed and coercivities are low. The calculations presented here can be easily extended to other aspects of soft magnetic properties addressed by the ripple theory, such as initial permeability.