A. Carl

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...

FABRICATION OF LARGE SCALE PERIODIC MAGNETIC NANOSTRUCTURES

We report on the fabrication of large scale periodic magnetic nanostructures designed as possible candidates for a future application in magnetic data storage technology. The nanostructures are prepared on glass substrates by electron beam evaporation of Fe or Co onto photoresist masks, which are periodically patterned using optical interference lithography with an Ar+ laser (λ=457.8 nm) in combination with subsequent selective etching. We present our first results on the fabrication of periodic arrays of isolated magnetic dots with an average diameter of 600 nm and periodicity of 900 nm over areas of 5 cm2.

Magnetization reversal in nanostructured Co/Pt multilayer dots and films

We report on the magnetic properties of large scale periodic magnetic nanostructures designed as dot-arrays with periodicity ranging between 300 nm and 1100 nm, consisting of Co/Pt multilayer dots with respective diameters ranging between 230 nm and 740 nm. The nanostructures are fabricated by using optical interference lithography allowing for large area (cm/sup 2/) fabrication. The Co/Pt multilayer dots show a pronounced out-of-plane magnetic anisotropy. The magnetic properties are determined from magneto-optical Kerr effect measurements in polar geometry and results are compared to respective continuous Co/Pt multilayer thin films. The micromagnetic properties of both, films and nanostructures are investigated with magnetic force microscopy allowing for the application of an external perpendicular magnetic field of up to B=0.15 T.

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.

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 diffusion in percolating gold clusters

Static and dynamical properties of percolating gold films are investigated by means of computer simulations on digitized pictures taken from original TEM-micrographs. From the digitized pictures we calculate the percolation correlation length ξp and determine the rms distance r of a random walk as function of time. For r<ξp we find r(t∼t1dw with dw=2.62 close to dw = 2.87, the fractal dimension of the random walk in 2D percolation. Topological behaviour has been studied by means of determining the mass M on topological length scale l. For l<ξp we find M(l)∼ldl with dl=1.64 close to the theoretical value. We compare the results with the temperature dependence of the phase coherence length Lφ as determined from magnetoresistance (MR) measurements, where we find Lφ(T∼T−pdw with dw= 2.87±0.1 for Lφ<ξp. We conclude that the dynamical properties of our gold films are in agreement with percolation theory exhibiting anomalous diffusion for (Lφ, r)<ξp.

Magnetization measurements on the nanometer-scale

We calibrated the magnetic tip of a magnetic force microscope within the point-probe approximation, using lithographically prepared Ω-shaped current carrying gold rings of different radii R. Probing with this tip a nanosized single domain Co/Pt multilayer dot with perpendicular anisotropy allows quantitative determination of the z-component of the magnetization and the hysteresis loop of the dot.

Preparation and characterization of large scale periodic magnetic nanostructures (abstract)

We report on the preparation and characterization of large scale periodic magnetic nanostructures designed as possible candidates for a future application in magnetic data storage technology. The nanostructures are prepared on glass substrates by UHV electron beam evaporation of Fe, Co, or Co/Pt onto periodically patterned photoresist masks followed by a lift-off of the photoresist. The preceding patterning process of the photoresist is achieved by using an interference lithography process in combination with subsequent selective etching. For the exposure of the photoresist, we use two different experimental setups with Ar ion lasers operating at wavelengths of 457 and 244 nm, respectively. This allows us to control diameter as well as distances between magnetic dots ranging between 300 and 3000 nm. The structural characterization of magnetic nanostructures is performed by electron microscopy as well as atomic force microscopy. Magnetic force microscopy along with additional image calculations based on di...