| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Thin Films | Monday Sessions |
| Session TF-MoE |
| Session: | Nanostructured Surfaces & Thin Films I |
| Presenter: | Goran Rasic, North Carolina Central University, USA |
| Authors: | G. Rasic, North Carolina Central University, USA BV. Vlahovic, North Carolina Central University, USA J. Schwartz, North Carolina State University, USA |
| Correspondent: | Click to Email |
As technology advances the demand for smaller and faster devices with reduced power losses becomes increasingly important. For high-frequency devices, such as antenna and sensors, two loss mechanisms are relevant, electrical and magnetic. Electrical losses are reduced by forming laminate structures and/or choosing materials with high electrical resistivity. Magnetic losses, however, arise from the magnetic hysteresis of the material, and thus are intrinsic. At frequencies above 1 GHz, these losses become immense. To reduce magnetic losses, the hysteresis losses must be reduced without losing desirable magnetic properties; reduced coercivity is needed, while retaining high saturation magnetization.
Textured nickel ferrite (NiFe2O4) thin films were deposited onto a c-plane sapphire substrate using chemical solution deposition. Surface of the films was patterned with a polydimethylsiloxane (PDMS) stamp using a modified nanoimprint lithography technique. A series of pattern masters with periods ranging from 500 nm to 1500 nm was used for patterning. In addition, samples with different thicknesses were prepared. Atomic force microscopy showed that the pattern was faithfully copied from the pattern masters to the thin films. X-Ray diffraction revealed all samples to be textured single phase inverse spinel nickel ferrite with preferential orientation along the <111> easy axis direction. Magnetic measurements showed the magnetic field response in the patterned samples to be unchanged as compared to plain samples, indicating there was no loss in magnetic properties. Similarly, the patterned samples showed appreciable and consistent saturation magnetization values. The coercivity of all samples showed significant reduction as compared to the plain samples. In addition, two distinct regimes were identified. For the out-of-plane measurements, the coercivity reduction increases with increasing feature size, whereas for the in-plane measurements the opposite is observed. These polar trends for the in-plane and out-of-plane measurements allow us to further fine tune the material for specific applications by not only controlling the feature size but also the orientation of the film with respect to the field. Finally, the origin of the coercivity reduction was shown to be the surface morphology anisotropy by MFM measurements. Magnetic domains were shown to be strongly affected by the surface features and oppositely oriented, reducing magnetostatic energy and leading to coercivity reduction. These results, in combination with the theoretical investigation, confirmed the origin of the coercivity reduction to be a direct consequence of altered surface topography.