Nanoscale Surface Patterning and Coercivity Reduction in NiFe 2O4 Thin Films

Biaxially textured nickel ferrite (NFO) thin films were grown by chemical solution deposition on c-plane sapphire substrates. Crystal structure and chemical composition was evaluated using X-ray Diffraction. Nanoimprint lithography (NIL) technique using a polydimethylsiloxane (PDMS) stamp was used imprint the films. A method for large scale precise patterning of was demonstrated. Quality of the transferred pattern was evaluated using atomic force and transmission electron microscopies. Magnetic measurements were performed using superconducting quantum interference device (SQUID) and showed large decrease of coercivity in patterned samples. Probable causes for coercivity reduction have been investigated and surface patterning has been shown to be the direct cause of the coercivity reduction phenomena. Coercivity reduction has been shown to translate to thicker films with layer-by-layer manufacturing method yielding better results. The effect of changing the surface pattern on the topography, crystallography and magnetic properties was investigated and different trends were observed for the measurements done with the magnetic field parallel and perpendicular to the film surface. Coercivity reduction was shown to increase with increasing feature size in the case of magnetic field oriented perpendicular to the film surface. For the field parallel to the film surface the opposite was observed. In all cases, the coercivity was reduced relative to the unpatterned film and relative to the base layer onto which the patterned film was deposited. All films showed a similar magnetic response as indicated by similarities in the curve shape. Crystallography measurements showed the imprint process did not affect the grain growth and orientation regardless of the surface feature size as indicated by all films having virtually identical diffraction patterns. The lower limit of surface patterning here was shown to be around 500 nm. Below 750 nm, the pattern quality degraded and the feature height reduced. The domain configurations of the plain and patterned films were investigated. Deviation from the expected domain configuration was found in the patterned films. The domain configuration of patterned films was similar regardless of the magnetic history of the sample. Micromagnetism and domain theory were used to investigate the energetics of different film configurations. Changing the domain sizes within values used in this study has been shown to have a very small effect on the energy of the system. Combining the theoretical prediction with the experimental measurements, the origin of the observed domain structure and coercivity reduction has been shown to be the surface topography induced change in the minimum energy configuration of the sample. This results in the minimization of the total sample magnetization through formation of stripe domains.