High magnetization Co-GdOx superparamagnetic granular films as magnetic coating materials for high-sensitivity alternating magnetic force microscopy tip

The Coy(GdOx)1−y, CoyAg1−y, (Fe70Co30)y(AlOx)1−y superparamagnetic (SP) granular films were fabricated by magnetron co-sputtering technique and compared for their magnetic properties. The effect of the different matrix (GdOx, Ag, AlOx) is investigated for refining and separating magnetic particles in non-magnetic matrix by calculating the magnetic volume fraction (y), and the results indicate that the GdOx matrix can more effectively refine and separate magnetic particles. The Co0.44(GdOx)0.56 SP film with the maximum Co volume fraction (y = 0.44) approaches the highest magnetization of 608 emu/cm3 at 20 kOe, and initial susceptibility (1.74 × 10−5 H/m) as compared with Co-Ag, Fe70Co30-AlOx films and some systems of other researchers at room temperature (RT). The magnetic particle size and size distribution in these SP films is calculated by fitting with Langevin function and the results of calculation are consistent with the high-resolution transmission electron microscopy (HRTEM) results. To further confirm the SP behavior, the blocking temperature of the Co0.44(GdOx)0.56 film with the maximum Co volume fraction is measured as ∼145 K, which is below RT. Finally, the Co0.33(GdOx)0.67 thin film having a linear magnetization curve is used for the fabrication of the SP tip and further used for the A-MFM measurement on the bulk Sr-Ferrite (SrF) magnet. The fabricated Co0.33(GdOx)0.67 SP tip along with A-MFM shows an excellent result as compared to the state of art FePt hard magnetic tip using conventional MFM, and it can directly detect the intensity and identify the polarity of the stray field originating from a bulk Sr-Ferrite (SrF) permanent magnet without topography crosstalk in ambient atmosphere.