J.C. Lodder

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.

High resolution magnetic force microscopy using focused ion beam modified tips

Atomic force microscope tips coated by the thermal evaporation of a magnetic 30 nm thick Co film have been modified by focused ion beam milling with Ga+ ions to produce tips suitable for magnetic force microscopy. Such tips possess a planar magnetic element with high magnetic shape anisotropy, an extremely high aspect ratio of greater than 30:1, and an end radius of less than 25 nm. These tips have been used in a commercial atomic force microscope under ambient conditions to obtain 30 nm resolution magnetic images of an established CoNi/Pt multilayer reference sample.

Metal bonding during sputter film deposition

We studied the bonding between two flat Si substrates with thin metal films. The bonding was accomplished during thin film sputter deposition on contamination free surfaces of metal films. In this work we used Ti and Pt. Successful bonding of these metal films (each having a thickness of 10–20 nm) occurred at room temperature over the entire bonded area (12 mm × 12 mm). Self-diffusion, particularly at grain boundaries and film surface, was the mechanism for bonding. Suitable metal bonding only occurred if the film surface roughness is sufficiently smaller than the self-diffusion length of metals. Particularly in the bonding of Ti to Ti films, transmission electron microscope observation revealed that complete crystalline grains had been formed across the former interface between the single thin Ti films. The interfaceless bonding can be explained by recrystallization of the Ti lattice due to the high self-diffusion coefficient of Ti. This technique would be applied to bonding of wafers to fabricate thin film devices or microsystems. Moreover, this bonding technology can be used with many different thin film materials and various semiconductor substrates.

Reversal mechanism of submicron patterned CoNi/Pt multilayers

With laser interference lithography Co/sub 50/Ni/sub 50//Pt multilayers with perpendicular magnetic anisotropy have been patterned into regular matrices of submicron sized dots. Their magnetic properties have been studied with Vibrating Sample Magnetometry. Compared to continuous multilayers (H/sub c/=15 kA/m) the coercivity for the patterned multilayers is much larger (H/sub c/=115-270 kA/m). Though the hysteresis curves of 180 nm dots and 60 nm dots have identical shapes, virgin curves seem to indicate that the 180 nm dots are multidomain while the 60 nm dots are single domain. The latter has been confirmed with Magnetic Force Microscopy observations.

Correlation between anisotropy direction and pulse shape for metal evaporated tape

Measured replay pulses, recorded on metal evaporated tape, show a strong asymmetry and dependence on the direction of tape movement. It is found that pulse height and pulse width are unequal for both directions of tape movement. This is attributed to the nonlongitudinal easy-axis direction which causes differences in head-tape geometry for both directions, resulting in a different transition width. A simulation model is proposed. The differences in pulse asymmetry and pulse width between both directions of tape movement are very well reproduced by this mode. From X-ray diffraction measurements and the results of the simulations, it is concluded that not the columns but the crystallites act as magnetic entities for this kind of tape. >

Thermal fluctuation of magnetization in nanocrystalline FePt thin films with high coercivity

The effect of thermal fluctuations of magnetization on static and dynamic properties is discussed for nanocrystalline FePt films. The large magnetocrystalline anisotropy K/sub u/ of L1/sub 0/ type FePt results in large K/sub u/V/(kT) values of more than 70 even for very fine grain sizes of 7-8 nm, indicating the potential of this alloy film to resist thermal fluctuation of magnetization. It is successfully demonstrated that larger K/sub u/V/(kT) values of these films lead to lower magnetic viscosity. Annealing at higher temperature results in larger K/sub u/V/kT values and smaller V/sub act/. The remanent coercivity measured at high sweep rate by using pulsed magnetic fields indicates the high thermal stability of these alloy films at high frequencies. However, the results indicated that care should be taken to induce an adequate magnitude of K/sub u/ for the FePt alloy to be used as ultrahigh density recording media.

Barium ferrite films grown by laser ablation

Pulsed laser ablation (PLA) has been used to grow barium ferrite films on Al/sub 2/O/sub 3/ single crystal substrates. When deposition occurs in an oxidising atmosphere at high temperatures, the films are single BaFe/sub 12/O/sub 19/ phase, very well oriented with [001] texture, and exhibit a large perpendicular magnetic anisotropy. In this case, the microstructure and the anisotropy orientation are not influenced by variation of the oxygen pressure in the range 20-530 mtorr, but the coercivity and the remanence are affected. Films prepared without oxygen are isotropic. XRD measurements reveal the absence of the BaFe/sub 12/O/sub 19/ which becomes unstable. On the other hand, the presence of other phases such as Ba/sub 2/Fe/sub 6/O/sub 11/ and Fe/sub 2/O/sub 3/ is detected. The annealing study reveals the presence of a threshold temperature (629/spl deg/C) for the appearance of the magnetisation and the anisotropy orientation. This temperature also corresponds to the onset in growth of the crystalline structure of barium ferrite. In the latter case, the coercivity was found to be greatly reduced by the increase in grain size at higher annealing temperatures.

Metal evaporated tape: state of the art and prospects

Thin metallic films are the first choice for media in advanced rigid disk systems. For helical scan tape recorders thin metal films have become more important. The high signal-to-noise ratio per unit of track width allows very high densities. The preparation techniques and materials properties of the thin-film coating of metal evaporated (ME) are described. Important aspects are the size of the grains, the geometry of the columnar structure and the associated anisotropy. The consequences for the recording process are explained. Tribological and corrosion properties also help to determine the usefulness of a recording tape. Corrosion, wear, protection layers and lubricating organic films are briefly discussed.

On the determination of the internal magnetic structure by magnetic force microscope

In this contribution it is proven mathematically that it is in principle impossible to determine the magnetic charge distribution inside a magnetic material by a method which measures the stray field outside the sample, such as magnetic force microscopy (MFM). A general source of stray field, Eσ, is defined and it is shown that different solutions can be found for Eσ that result in the same stray field. It is also shown how both a perpendicular and a longitudinal medium can be described with the same Eσ. Using the equations for stray field, resulting from Eσ, it is also proven that performing the same MFM measurement at different scanning heights does not provide any new information on the stray field for sample; from a measurement at one (constant) height, the stray field at all other heights can be calculated. Moreover, the component of the field parallel to the same plane can be obtained from a measurement of the field component perpendicular to the sample plane.

Domains and magnetic reversals in CoCr

Magnetron sputtered CoCr layers with various thicknesses, coercivities and other magnetic properties have been studied by a digital enhanced magneto-optical Kerr microscope. The slope (T) at [dM/dH]M=φ is determined from the perpendicular hysteresis loop. Together with K 1 these values have been used for calculation of the characteristic stripe domain properties. The observed domain densities have been compared with the calculated densities based on a continuous or particular behaviour of CoCr. The relation between typical fields (like the nucleation field and surface coercivity), the observed domain configuration and the shoulder of the hysteresis loop are given. On the basis of the domain structure (from stripe to cluster-like) we conclude that the samples can be classified in low, medium and high coercive layers.