Characterisation of magnetisation ripple using Lorentz microscopy: Effect of ultra-thin Ni79Fe21 seed layers on magnetic properties of Ni45Fe55

Ni₄₅Fe₅₅ has long been a high moment alternative for Ni₈₀Fe₂₀, which is commonly used in hard disk drive (HDD) read-write head design. In this paper we investigate the effect of ultra thin Ni₇₉Fe₂₁ seed layers on controlling the physical and magnetic behaviour of Ni₄₅Fe₅₅. Transmission electron microscopy (TEM) was used to observe grain size variation. Introducing a seed layer of 0.5 nm thickness reduced the average grain size diameter by over 80% to 2 nm. Diffraction analysis shows texture exists along the [010] direction for the unseeded and 0.25 nm seed layer films. The addition of the Ni₇₉Fe₂₁ seed layer reduces film texture. Bulk magnetic measurements showed seed layers improved uniaxial anisotropy, with decreasing easy axis coercivity H_C, and an increase in the anisotropy field H_K. The Fresnel mode of Lorentz microscopy was used to image micromagnetic behaviour during in situ magnetising experiments. The nano-scale visualisation of the magnetic structure allows for not only the mapping of hysteresis behaviour, but quantitative characterisation of the material in the form of magnetisation ripple. This was characterised in two ways; the ripple dispersion and wavelength. The addition of Ni₇₉Fe₂₁ seed layers acted to significantly reduce ripple wavelength and dispersion angle. The dramatic effect of seed layer addition suggests that the ultra-thin film is a good candidate for controlling both the physical and magnetic properties of Ni₄₅Fe₅₅ films, whilst maintaining a high magnetic moment density.