Magnetic Microstructures of Perpendicular Magnetic-Recording Media
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MRS BULLETIN/OCTOBER 1995
instead of a domain-wall motion, chemical stability under various environmental conditions, and a small surface roughness. In order to achieve the desired magnetic anisotropy and coercivity, a columnar morphology (small diameter) with an hep [0001] texture and exchange-decoupled, columnar boundaries—to create a magnetic microstructure for single domain switching columns with high coercivity—should be obtained. An overview of the preparation, microstructure, and magnetic properties of Co-Cr thin films is given in Reference 8. Depending on the deposition parameters, a so-called initial layer (with an in-plane magnetization) can be present. To obtain high coercivity, the grains are magnetically isolated with a nonferromagnetic Co-Cr composition (at% Cr >26) by compositional separation. In addition to appearing in the columnar boundaries, compositional separation can also be found in the column itself,9 the so-called chrysanthemum-like pattern (CP). The main features of the CP structure are Co- and Cr-rich stripes, with spatial periodicity of 3-7 nm, perpendicular to the column boundaries. Results obtained by atom-probe field-ion microscopy (APFIM) have shown how the concentration fluctuations in the grains (columns) are distributed.10 Over a distance of 40 nm in the planar direction of a Co-Cr column, differences of 30 at% Cr and 7 at% Cr have been measured. The latter composition is ferro-
magnetic while the composition above 26 at% Cr is paramagnetic. The results obtained by compositional separation studies support the anomalous Hall effect measurements (see later) obtained on our samples. The magnetic structures (magnetic domains and written bits) in Co-Cr films are strongly related to the microstructure, morphology, and compositional separation. A simple schematic structure (see Figure 1) of sputtered Co-Cr films consisting of equal-size columns is used as a model to discuss the possible magnetization-reversal phenomena. Figure la shows the microstructure with equal columnar size and shape, and a columnar boundary. In this case, the interaction between the columns occurs by exchange and magnetostatic interactions. The boundary can only hinder the movement of the domain wall, which means, in principle, an increase in Hc. For higher Hc's, we need to break up the exchange forces between the columns. This can be realized by compositional separation or separation (by voids) of the columns, which has been also shown in the case of Co-Cr-X media for LMR. In the case of compositional separation, a nonferromagnetic Cr-rich composition is present between the columns (Figure lb). The more complicated CP structure of one column in combination with a Crenriched column boundary is shown in Figure lc. These complicated structures have many consequences for the switching behavior of the magnetization and, moreover, for the coercivity. The reversal of the magnetization depends on the exchange and magnetostatic interactions between the columns or cluster of columns. For a reasonable S/N, it is necessary
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