Kristina Zuzek Rozman

Abstract:

The task of probing magnetic structures on a local scale, as opposed to bulk measurements which average over larger volumes, is one that lends itself well to the use of Magnetic Force Microscopy (MFM). A member of the scanning probe microscopy family. In this technique, a sharp magnetically sensitive tip mounted on a compliant cantilever beam is brought into close proximity to a magnetic sample. Varying interactions between the magnetic field of the sample and the stray field of the tip lead to changes in the status of the cantilever. As such magnetic force microscope with the lateral resolution less than 100 nm, and the ability to resolve both magnetic and topographic features of the sample, in order to better elucidate the interplay between the two. For the in-field MFM measurements on electroplated Fe-Pd and Co-Pt NWs a home-made stage was used. The maximum external field applied to the NWs was applied via permanent magnet with the maximum applied etrean field (Hmax~240 kA/m) which’s direction was varied. Using electrodeposition into anodic aluminium oxide membranes fcc Fe48Pd52 and hcp Co65Pt35 NWs with diameters of ≈ 200 nm and lengths of ≈ 3.5 µm were synthesised. Magnetic force microscopy on a single Fe-Pd NW revealed single-domain behaviour with the easy axis of magnetization along the long axis of the NW. The magnetization-switching behaviour of a single Fe-Pd NW studied with MFM suggested a square-shaped magnetization curve (M/Ms=1) with HC ≈ 3.2 kA/m. By using in-field MFM technique, the effect of dipolar interactions in Fe-Pd array of NWs still embedded in the AAO was examined and found that the dipolar interactions greatly reduce the remanence and the switching-field distribution of the Fe-Pd NW array. In the case of the Co-Pt NWs the texturing of the direction [100] was observed that suggests the uniaxial anisotropy perpendicular to NW long axis. The subsequent magnetic results obtained via “bulk” methods VSM and first order reversal curves (FORC), allowed us to conclude that Co-Pt NWs with hcp crystal structure had a uniaxial anisotropy perpendicular to NW long axis. This was found to further to result in a unique periodic domain structure observed with MFM. By using an adopted equation to calculate the period of the stripes or the domain width for the nanowires we estimated a domain width of Wcal ≈250 nm to minimize the total energy, which is in excellent agreement with the present observations. Finite-element calculations have also shown that the transverse magnetization configuration in such a type of domain pattern is the state with the lowest energy that can be applicable in the newest version of race track memory devices with perpendicular anisotropy.