S. Porthun

Magnetic force microscopy of thin film media for high density magnetic recording

This paper discusses various aspect of magnetic force microscopy (MFM) for use in the field of high density magnetic recording. After an introduction of the most important magnetic imaging techniques, an overview is given of the operation and theory of MFM. The developments in instrumentation, MFM tips, quantification of MFM data, high resolution imaging and application of external fields is discussed. Examples are given of the applications of MFM, such as the characterization of heads, magnetic structures (bits) in longitudinal and perpendicular recording media, bits in magneto-optic films and domain structures in soft magnetic materials.

QUANTITATIVE MAGNETIC FORCE MICROSCOPY ON PERPENDICULARLY MAGNETIZED SAMPLES

We present a transfer-function approach to calculate the force on a magnetic force microscope tip and the stray field due to a perpendicularly magnetized medium having an arbitrary magnetization pattern. Under certain conditions, it is possible to calculate the magnetization pattern from the measured force data. We apply this transfer function theory to quantitatively simulate magnetic force microscopy data acquired on a CoNi/Pt multilayer and on an epitaxially grown Cu/Ni/Cu/Si(001) magnetic thin film. The method described here serves as an excellent basis for (i) the definition of the condition for achieving maximum resolution in a specific experiment, (ii) the differences of force and force z-derivative imaging, (iii) the artificial distinction between domain and domain wall contrast, and finally (iv) the influence of various tip shapes on image content.

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.

Magnetic force microscopy using electron‐beam fabricated tips

We used a new concept of tip preparation for magnetic force microscopy (MFM) proposed recently based on coating electron beam deposited carbon needles with appropriate magnetic thin film materials. In combining the advantages of electron beam fabricated needles with those of already widely used thin film tips, high resolution MFM tips can be prepared routinely and reproducibly on all type of cantilevers. Due to the fabrication procedure, which is described in detail, the effective magnetic tip volume is formed by a homogeneous, magnetically isolated, high aspect ratio thin film element favoring a single domain tip behavior. To reinforce the inherent shape anisotropy an additional uniaxial anisotropy is induced along the tip axis by applying an external field during the deposition of the ferromagnetic alloy. Because of the parallel side walls and the rounded tip end, most of the stray field will emanate from the apex region, resulting in a high field density and only little influence on the magnetization within the sample of interest. By using a material with high saturation magnetization, the thickness of the magnetic layer can be drastically reduced, leading to an increased resolution. To demonstrate the potential of the new concept various MFM observations on different types of materials for magnetic storage technology have been performed without changing the presented basic preparation parameters.

Electron beam fabrication and characterization of high- resolution magnetic force microscopy tips

The stray field, magnetic microstructure, and switching behavior of high‐resolution electron beam fabricated thin film tips for magnetic force microscopy (MFM) are investigated with different imaging modes in a transmission electron microscope (TEM). As the tiny smooth carbon needles covered with a thermally evaporated magnetic thin film are transparent to the electron energies used in these TEMs it is possible to observe both the external stray field emanating from the tips as well as their internal domain structure. The experiments confirm the basic features of electron beam fabricated thin film tips concluded from various MFM observations using these tips. Only a weak but highly concentrated stray field is observed emanating from the immediate apex region of the tip, consistent with their capability for high resolution. It also supports the negligible perturbation of the magnetization sample due to the tip stray field observed in MFM experiments. Investigation of the magnetization distributions within the tips, as well as preliminary magnetizing experiments, confirm a preferred single domain state of the high aspect ratio tips. To exclude artefacts of the observation techniques both nonmagnetic tips and those supporting different magnetization states are used for comparison.

Calculation of playback signals from MFM images using transfer functions

Magnetic force microscopy has proven to be a suitable tool for analysis of high-density magnetic recording materials. Comparison of the MFM image of a written signal with the actual read-back signal of the recording system can give valuable insight in the recording properties of both heads and media. In a first order approach one can calculate a ‘signal’ by plotting the line integral over the track width along the track direction (Glijer et al., IEEE Trans. Magn. 32 (1996) 3557). The method however does not take into account the spatial frequency dependence of the transfer functions of both the MFM and the readback system. For instance the gap width of the head (limiting the high frequency signals) and the finite length of the MFM tip (limiting the sensitivity for low frequencies) are completely disregarded (Porthun et al., J. Magn. Magn. Mater. 182 (1998) 238). This type of problem involving spatial frequencies can be very elegantly solved in the Fourier space. The response of the MFM is described by the force transfer function (FTF) as introduced by (Porthun et al. (J. Magn. Magn. Mater. 182 (1998) 238) and Hug et al. (J. Appl. Phys. 83 (1998) 5609), which describes the relation between the MFM signal and the sample stray field at the height of the tip. From this stray field an ‘effective surface charge distribution’ can be calculated, by means of the field transfer function (HTF). The same function HTF can be used to calculate the stray field at the height of the head. From this stray field the playback voltage can be calculated, resulting in the playback transfer function (PTF). In order to do this the Karlquist model had to be extended to three dimensions.

Domain wall theory and exchange stiffness in Co/Pd multilayers

The stripe model of domain structure in multilayers is studied by micromagnetic simulation. The results indicate a strong reduction of the effective domain wall energy (by dipolar effects). Domain width measurements on sputtered Co/Pd multilayers are compared with the theory. The estimated exchange stiffness is comparable with that of bulk Co. The effects of interface roughness and of interlayer exchange are discussed.

Domain structures in Co/Pd multilayers

The observations of submicron domain structure of Co/Pd multilayers at various parts of the M-H loop and after different magnetization cycles designed to approach the global-equilibrium domain width are reported. The wall energy densities were estimated from comparison of the measured equilibrium domain width with the anhysteretic model predictions and also compared with the wall energy densities determined from the slope of major M-H loop. >

Structural and magnetic characterization of very thin Co-Cr films

In this paper we describe the nucleation and growth of thin Co-Cr layers on different substrates and seedlayers. Especially the presence and formation of the initial layer is considered, and corresponding growth mechanisms are proposed. The magnetic properties of these layers have been analyzed by in-plane VSM and by anomalous Hall effect (AHE) measurements. Even layers as thin as 5 nm are shown to exhibit a perpendicular magnetic anisotropy. For a high coercive Co-Cr film the formation of dot-like domains is shown by the Bitter-colloid/SEM technique.

Magnetic force microscopy on thin film magnetic recording media

In order to understand the magnetic behaviour of thin film magnetic recording media high resolution magnetic observation techniques are very essential to see magnetic details smaller than 100 ran size. Magnetic Force Microscopy (MFM) is used for analysing thin magnetic films and multilayers as well as for investigation of written bits in high density recording media. Continuous attention is given to increase the performance of the used magnetic tips.