Characterisation of amorphous MFM tip coatings

Magnetic Force Microscopy has emerged as one of the principal imaging techniques for viewing the micromagnetic structure of materials. Image contrast depends on a number of factors with the magnetic coating of the tip being one of the most important. Different coatings are used depending on, for example, the component of sample stray field which is to be observed or the magnetic hardness of the material under investigation. In this paper we present results from tip coatings which are extremely soft in character and whose imaging properties depend on the magnetic history of the tip. The stray fields from the tips were characterised by Lorenmicroscopy and the t i p themselves were used to image an MFh4 reference sample obtained from NIST. The tips studied were standard single crystal silicon nanoprobe tips sputter coated with a layer of amorphous ferromagnetic material of thickness 30 or 50nm. The coating composition is close to FeslSi,SB13~C~ (METGLAS"2605SC). The coercivity of comparable films grown on a Si coupon is -120Alm. Imaging of the stray fields from the tips was carried out using the differential phase contrast technique [l] which provides information on the magnetic induction distribution integrated along the electron path in the STEM. In the case of the MFM tips, linescans were taken at a distance of -50nm from the end of the tip. Imaging the tip in different orientations provides data for a tomographic reconstruction algorithm which extracts the three components of field in a plane equivalent to the specimen surface in an MFM [2]. The tips were studied in the as-deposited state and after being magnetised along the tip axis. Examples of the axial reconstructed field component for these two states are given in Figs. ](a) and 2(a). It is evident that the tip with the coating in the as-deposited state possesses a more localised field than the tip which has been magnetised. Quantitative measurements of the field can be made using this method and linetraces of the fields, as shown in Figs. I(b) and 2(b), confirm the much narrower field distribution in the case of the as-deposited film. In both cases the field appears consistent with a tip possessing axial magnetisation. This arises from the tip geometry which imposes strong shape anisotropy for the film on the tip, and the absence of any megnetocrystalline anisotropy in the coating. The tips have also been used to obtain images from a reference sample [3]. Images from the asdeposited and axially magnetised tips are given in Fig. 3. Magnetic contrast from the asdeposited tip appears to arise from anractive forces only. suggesting that the tip is switched hy a P