Soft magnetism of Fe–Co–N thin films with a Permalloy underlayer

A systematic investigation was carried out on the Fe–Co–N thin films with varying Permalloy underlayer thicknesses (in short Py–Co–N films), which show significantly improved magnetic softness when a very thin Permalloy underlayer is applied. The easy axis coercivities of the Py–Co–N films drop from 1.3 kA/m (16 Oe) to 640 A/m (8 Oe) with an appropriate Permalloy underlayer; and the corresponding hard axis coercivity drops from 320 A/m (4 Oe) to less than 80 A/m (1 Oe). The highest transverse permeability is observed in the Py–Co–N film with a Permalloy underlayer of 3.5 nm, which also shows the smallest dispersion angle α50 among the Py–Co–N films. The changes in the mean grain size, texture, and magnetostriction are determined to not be responsible for the observed magnetic softness in the Py–Co–N films. In contrast, a tantalum underlayer does not reduce the coercivities and dispersion angles in the Ta–Co–N films. Based on these results, an exchange-induced ripple reduction mechanism is proposed to explain the effects of the Permalloy underlayer on the Py–Co–N films.