Coercivity Reduction in Nickel Ferrite (NiFe₂O₄) Thin Films Through Surface Patterning

NiFe₂O₄ (NFO), a spinel ferrite with high electrical resistivity and favorable magnetic properties, is an interesting material for high-frequency signal and power electronic applications. Here, significant reduction in the coercivity of NFO films is obtained through surface patterning via nanoimprint lithography. Multilayered NFO films are grown on c-plane (0001) sapphire substrates using room temperature chemical solution deposition. Two film variants, layer-by-layer and bulk, are deposited. Prior to crystallization, films are patterned with a polydimethylsiloxane stamp. Good feature transfer to the thin-film surface is confirmed by atomic force microscopy and transmission electron microscopy. Θ-2Θ X-ray diffraction shows that both variants produce single-phase inverse spinel NFO, with better texture in the layer-by-layer samples. Magnetic measurements show substantial reduction in coercivity in the patterned samples due to the surface anisotropy-enhanced demagnetization field. The bulk patterned sample showed the lowest coercivity, ~18 Oe in-plane, albeit with reduced saturation magnetization, whereas the layer-by-layer patterned film maintained the same degree of texture and saturation as unpatterned films, with a ~80% reduction in coercivity. These results show that nanoimprint lithography of chemical solution deposition films is a cost-effective pathway to engineering the coercivity of NFO films while retaining desirable saturation magnetization and texture.