A. N. Farley

Magnetic properties of electrodeposited nanowires

Electrodeposited multilayered nanowires grown within a polycarbonate membrane constitute a new medium in which giant magnetoresistance (GMR) perpendicular to the plane of the multilayers can be measured. These structures can exhibit a perpendicular GMR of at least 22% at ambient temperature. We performed detailed studies both of reversible magnetization and of irreversible remanent magnetization curves for CoNiCu/Cu/CoNiCu multilayered and CoNiCu pulse-deposited nanowire systems with Co:Ni ratios of 6:4 and 7:3 respectively in the range 10 - 290 K, allowing the magnetic phases of these structures to be identified. Shape anisotropy in the pulse-deposited nanowire and inter-layer coupling in the multilayered nanowire are shown to make important contributions to the magnetic properties. Dipolar-like interactions are found to predominate in both nanowire systems. Magnetic force microscope (MFM) images of individual multilayered nanowires exhibit a contrast consistent with there being a soft magnetization parallel to the layers. Switching of the magnetic layers in the multilayered structure into the direction of the MFM tips stray field is observed.

Comparing the resolution of magnetic force microscopes using the CAMST reference samples

A set of reference samples for comparing the results obtained with different magnetic force microscopes (MFM) has been prepared. These samples consist of CoNi/Pt magneto-optic multilayers with di¤erent thicknesses. The magnetic properties of the multilayer are tailored in such a way that a very Þne stripe domain structure occurs in remanence. On top of this intrinsic domain structure, bits were written thermomagnetically using di¤erent laser powers. These samples have been imaged in six di¤erent laboratories employing both home-built and commercial magnetic force microscopes. The resolution obtained with these different microscopes, tips and measurement methods varies between 30 and 100 nm.

Resonant Torque Magnetometry A New In-situ Technique For Determining The Magnetic Properties Of Thin Film MFM Tips

A new high sensitivity Resonant Torque Magnetometry (RTM) technique has been developed enabling the determination of the magnetic properties of Magnetic Force Microscope (MFM) tips in-situ. A sensitivity of better than 10/sup -12/ emu (10/sup -15/ Am/sup 2/) has been achieved at ambient temperature and pressure. We have studied the micromagnetic properties of a variety of tip coatings including hard CoCr and CoPt films and soft permalloy. In-situ determination of the magnetic properties of MFM tips is vital if quantitative interpretation of MFM images is to be achieved especially when images are made in the presence of bias fields. The m(/spl mu//sub 0/H) loops obtained for CoPt tips have a shape similar to a standard magnetisation loop when the field is applied parallel to the tip axis. With the field applied perpendicular to the tip both CoPt and CoCr demonstrate behaviour consistent with partial demagnetisation of the axial moment, to which the RTM is sensitive.

Interactions between soft magnetic samples and MFM tips

This work considers the interactions between scanning probe microscope tips with thin magnetic coatings and soft thin films of permalloy. It is shown that the stray field from the tip can be large enough to cause perturbations of the magnetic structures in the sample being imaged. Results for various tip geometries are discussed.

Magnetic force microscopy of soft magnetic materials

The magnetic force microscopy of soft magnetic materials is a challenge due to the perturbing effect of the tip stray field on the samples magnetisation. Ferromagnetic thin film tips have been characterised by Lorentz electron microscopy and the tips employed in the study of a permalloy magneto-resistive sensor. Residual domains in the permalloy were detected and the location of domain walls related to the presence of imperfections in the edge of the sensor.

Imaging domains in Co/Pt multilayers by magnetic force microscopy

The magnetic microstructure in a Co/Pt multilayer has been imaged by magnetic force microscopy employing a novel tip structure. Magnetic tips have been fabricated by evaporating a thin film onto a single facet of the pyramidal tip of a silicon nitride cantilever. Nanolithographically defined elements have been used to simulate the micromagnetic structure of the tip and have been investigated by the Lorentz mode of electron microscopy. We demonstrate that stray fields from the Co/Pt multilayer can be sensed by the tip producing images of the magnetic microstructure with a fine structure on a scale of 30–40 nm.

Flux closure in magnetic force microscope tips

This work considers the effect of MFM tip magnetisation distribution on force gradient images of Co/Pt perpendicular anisotropy multilayer materials. In particular it is shown that a pyramidal tip with a Co coating which is thought to contain closure domain structures produces substantially different images from single domain CoPt coated tips. Modelling results are also presented which support the closure domain hypothesis. If is concluded that the small effective volume of the closure tip produces valuable results in terms of resolution and image clarity.

Potentiometric and magnetic force microscopy in multilayers

The simultaneous collection of magnetic, topographic and potentiometric data has been achieved using a modified magnetic force microscope (MFM). Use of a reference sample confirmed that there was no interference between magnetic and potentiometric channels. Images of domain walls and of the surface potential in a Co/Cu multilayer system have been obtained. Domain walls observed are consistent with those obtained using conventional MFM whilst potentiometry revealed a constant background with small areas of higher or lower potential.

Microscopy probes tiny magnets

Magnetic multilayers and films of granular materials have been the subject of intensive research since spin-dependent electronic effects, such as giant magnetoresistance, were discovered in the 1980s. Such materials consist of cobalt, nickel or iron combined with copper or gold. They could be used in non-volatile memories, which store information even when the power is switched off, and in magnetic recording heads.