J. Lohau

Quantitative determination of effective dipole and monopole moments of magnetic force microscopy tips

We present experimental results on the characterization of commercially available magnetic force microscopy (MFM) thin film tips as a function of an external magnetic field. Well defined magnetic stray fields are produced using current carrying rings with radii ranging between 603 and 2369 nm fabricated by electron-beam lithography directly imaged by MFM. Treating the MFM tip as a point probe, the analysis of the image contrast as a function of both the magnetic stray field and the lift height allows for a quantitative determination of effective magnetic dipole and monopole moments of the tip as well as their imaginary location within the real physical tip. Our systematic study gives a quantitative relationship on how absolute values of the magnetic dipole and monopole moments and their location within the tip depend on a characteristic decay length of the z component of the magnetic field being detected. From this we can estimate the effective tip volume of the real physical thin film tip relevant in MFM...

Magnetization reversal and coercivity of magnetic-force microscopy tips

An experimental technique is presented that allows determining the magnetization reversal and coercivity of magnetic-force microscopy (MFM) tips. An Ω-shaped current carrying gold ring with a radius of 2400 nm fabricated by electron-beam lithography and a lift-off technique is charged with a constant current in order to produce a magnetic stray field in the z direction that is detected by MFM. While an oscillating MFM tip is continuously raster scanned across the center of the current ring, an external magnetic field is applied in the z direction and increased in magnitude in order to reverse the tip magnetization during imaging. Thus, the corresponding changes in the measured image contrast exclusively describe the magnetization reversal and coercivity of the particular part of the tip that is used for imaging. We have investigated commercially available thin-film tips and we find that the hysteresis loops measured with MFM may be significantly different as compared to hysteresis loops measured by means ...

Quantitative determination of the magnetization and stray field of a single domain Co/Pt dot with magnetic force microscopy

The z-component of both the magnetization and the stray field of a nanometer sized single domain magnetic Co/Pt multilayer dot with perpendicular magnetic anisotropy is determined quantitatively within the point probe approximation by magnetic force microscopy (MFM). The MFM tip used is calibrated by probing omega-shaped nanosized current rings fabricated by electron-beam lithography. Since the stray field geometry of the dot and the current rings are similar, the calibrated tip can be used to determine quantitatively the magnetization and the stray field of the dot with perpendicular magnetic anisotropy.

Magnetization reversal and coercivity of a single-domain Co/Pt dot measured with a calibrated magnetic force microscope tip

The magnetization reversal and the coercivity of a nanofabricated single-domain 230 nm diam Co/Pt multilayer dot with perpendicular magnetic anisotropy are measured quantitatively with magnetic force microscopy (MFM). During MFM imaging, a variable external magnetic field ranging between −1 kOe<H<1 kOe is applied in the z direction in order to simultaneously reverse the magnetizations of both the single-domain Co/Pt dot and the MFM tip. The hysteresis loop of the Co/Pt dot can be extracted unambiguously from the resulting MFM image contrast, since we have used a calibrated MFM tip, for which the magnetization reversal has been determined independently also by MFM using a nanofabricated current carrying ring.

Electron-beam lithography with metal colloids: Direct writing of metallic nanostructures

We report on the fabrication of metallic nanostructures in the sub-100 nm region by means of electron-beam lithography with metal colloids. A thin organometallic film consisting of surfactant stabilized Pd-colloids (⩽3 nm) is directly patterned by electron-beam irradiation. Non-exposed colloids are easily removed by rinsing the sample with appropriate dissolvers. The metallic character of the nanostructures is checked by resistance measurements. We find the morphology and the resistance behavior of the present nanostructured Pd-colloids to be similar to those of granular thin Pd/C films. Accordingly, the metal content of the nanostructures fabricated with Pd-colloids can be estimated.

Magnetization reversal of nanostructured Co/Pt multilayer dots and films studied with magnetic force microscopy and MOKE

Large scale periodic arrays of magnetic nanostructures consisting of e.g. Co/Pt multilayer dots with perpendicular magnetic anisotropy are possible candidates for future high-density magnetic storage media. We have fabricated periodic dot-arrays by optical interference lithography with Ar/sup +/ ion lasers operating at wavelengths of 457 nm and 244 nm, respectively. Periodicities range from 125 nm to 1100 nm with dot diameters between 70 nm and 740 nm covering a total area of up to 20 cm/sup 2/ with a maximum dot density of about 4.1/spl times/10/sup 10/ dots/in/sup 2/. The global magnetic properties of these Co/Pt dot-arrays and of corresponding homogeneous films are investigated by the magneto-optical Kerr effect (MOKE) in polar geometry. Results of the magnetization reversal are compared to magnetic force microscopy (MFM) investigations in external magnetic fields of up to H=1 kOe perpendicular to the sample. A simple MFM-data analysis is described which allows to determine hysteresis loops from a series of MFM images obtained in various external magnetic fields. This then allows to measure the coercivity H/sub c/ of a given sample on small length scales. A comparison with MOKE hysteresis loops shows good agreement, although the unknown hysteretic behavior of the tip cannot be separated. The MFM-image analysis described therefore stands as a semi-quantitative method, which is however useful to measure relative changes of the coercivity on small length scales.

Electron-beam lithography using a scanning transmission electron microscope CM12 (Philips)

We report on the modification of a conventional scanning transmission electron microscope (CM12, Philips) for electron-beam lithography. The hardware as well as the entirely home designed lithography software are described in detail. Special attention is paid to the reduction of the proximity effect by means of our lithography software using a fairly simple exposure model. Various lithography results are shown including nonmagnetic as well as ferromagnetic nanostructures with linewidths as small as 20 nm. Problems occurring when fabricating magnetic nanostructures and their solutions are presented and discussed.