Recently it has been shown that the areal density of Co-Cr media for perpendicular recording using a single pole head can be in the order of 5-10 GbitJ inch 2 [1]. In this laboratory demonstration a track pitch of 0.5 Ilm (50 kTPI) and a bit length of 260 nm (lOO kFRPI) was achieved. Using the same recording setup , in this paper a MFM image is shown using 300 kFRPI linear bit density and a track width of 0.4 Ilm. With these densities, microstructural features like columnar size, compositional separation of Co and Cr, magnetic coupling and reversal behaviour become more and more important. This paper presents experimental results obtained from Co-Cr thin films by using MFM, study of artificially etched micro-strips and measuring anomalous Hall effect from submicron structures, as well as results obtained from magnetostatic and micromagnetic simulations.
The nature of the effective uniaxial magnetic anisotropy (Keg=K,-1/2 poMZ) is of interest in magnetic storage media, and can be determined from torque measurements. The first order fourier coefficient of the torque curves is field dependent and is given in a first order approximation by 111:where E is given by:This relation is often used to correct measured torque values for finite field error. We will show however torque measurements which are inconsistent with this relation. The inconsistency will be explained in terms of second order anisotropy effects and a dispersion in direction and magnitude of the uniaxial anisotropy.
ResultsIn figure 1 the field dependence of the first order Fourier coefficient is shown for different types of materials. Their slopes are listed in table 1 and compared to the slope as calculated by equation 1, using VSM data and the Keff extrapolated to infinite field. Only the permalloy sample exhibits an agreement. Note that the measured siope can even revcse iii sign.
Influence of K2The influence of %e second order anisotropy constant K2 is often neglected because it is only a few percent of K1. However for most perpendicular storage media Keff is around zero and therefore the influence of K2 can be considerable. In table 1 this is expressed in column 3 which shows that the agreement between theory and experiment for the alumite samples increased. For the Co-Cr samples the slope even reversed its sign. It can be concluded that with taking K2 into account the agreement increased for the sample with a negative K,w For positive Keff the agreement between experiment and theory decreases. (1) it is assumed that the film is homogeneous. In an inhomogeneous film K e~ vanes in direction and magnitude. When we neglect interaction it can be seen from relation (1) figure2 Field dependence of maximum torque.
Conclusion.The field dependence of torque curves is often not in agreement with theory. Correction of torque curves for the finite field error should preferably be performed by extrapolation of measurements at different fields values to infinite field. To explain the anomalous behaviour of field dependencies we have to consider the influence of second order anistropy, non-saturation and inhomogeneity in magnitude and distribution of anistropy. We will show simulations to illustrate the effect of inhomogeneity.
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