TEM observations of magnetic domains and grain boundaries on spin-sprayed ferrite films exhibiting high permeability usable for gigahertz noise suppressors

Lorentz TEM observations of magnetic domain wall motion, as well as TEM observations of grain boundaries, were performed on spin-sprayed ferrite films #1 (Ni_0.17Zn_0.22Fe_2.61O₄) and #2 (Ni_0.19Zn_0.20Co_0.03Fe_2.58O ₄), both 0.5 μm in thickness. They exhibit much higher natural resonance frequencies than the bulk ferrite and thus have been applied to gigahertz noise suppressors. Films #1 and #2 exhibit prominent and weak in-plane uniaxial magnetic anisotropy, respectively, which is induced along the liquid flow direction during spin-spraying. Both films have columnar crystallites with 100-200 nm widths aligned perpendicular to the film plane, and the boundaries of the crystallites have no pores or impurity phases. Therefore, the crystallites are magnetically exchange-coupled, which is responsible for the unusually high permeability and high natural resonance frequencies of the films. Under zero bias magnetic field, film #1 exhibits mosaic-shaped magnetic domains, whereas film #2 exhibits magnetic domains elongated along the easy magnetization axis, both several hundred nanometers in width. For both films the domain structure remains unchanged when an in-plane bias DC magnetic field, H_dc, of up to 10 Oe is applied along the hard axis. Under a stronger H_dc, the domain structure prominently changes, and the domain walls disappear when H _dc exceeds ∼100 Oe. This confirms our previous finding that the initial permeability is ascribed only to magnetization rotation, with no contribution from domain wall motion