Journal of Magnetism and Magnetic Materials 242­245 (2002) 352­354 Study of in-plane magnetic anisotropy in Co-based thin-film media R. Murao*, C. Okuyama, K. Takahashi, A. Kikuchi, Y. Kitamoto, S. Ishida Yamagata Fujitsu Ltd., Ko 5400-2, Higashine Higashine-City, Yamagata 999-3701, Japan Abstract The relationship among macroscopic in-plane magnetic anisotropy, the crystal structure of both the Co-based magnetic layer and the Cr under-layer, and the surface morphology of the textured substrate was studied. In the highly oriented media, the preferred orientation of the c-axis of Co to the circumferential direction and the distortion of the Cr crystal lattice were observed. In-plane magnetic anisotropy is induced when the grain of the under-layer is smaller than the texture grooves. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetic recording media; Crystal structure; Anisotropy induced; X-ray diffraction 1. Introduction thickness was varied within the range of 20­200 nm; the thickness of the Co-based magnetic layer was 26 nm Generally, in Co-based thin-film media on textured (Ms: 0.48 T). NiP coated substrates, macroscopic in-plane magnetic The magnetic properties were measured both in the anisotropy is induced. In this case, HC is higher in the circumferential and in the radial directions with a circumferential direction than in the radial direction. vibrating sample magnetometer. The crystal structure Several models for the origin of this in-plane magnetic of the Co-alloy and the Cr layer was analyzed with an anisotropy had been reported, such as the effect of X-ray diffractometer (Cu-Ka line), using the y22y inverse magnetostriction induced in Co crystal grain due scanning method and the glazing angle incidence X-ray to anisotropic stress [1] and the crystal orientation of diffraction method (GIXD; glazing angle: 0.31). Using Co-alloy [2,3]. However, there is no common under- the GIXD method, we were able to analyze the standing of the mechanism that causes this phenomen- crystallographic data of planes perpendicular to the on. To clarify the origin of the magnetic anisotropy, we substrate surface. The GIXD measurements were done studied the relationship between the in-plane magnetic from the circumferential and the radial direction to anisotropy and the crystal structure of both the analyze the planes perpendicular to each direction. For magnetic layer and under-layer. the GIXD measurements of the Cr layer, samples without a magnetic layer were used. The microstructure of the film was observed from both the in-plane and 2. Experimental method cross-sectional direction witha transmission electron microscope (TEM). The surface morphology of the On mechanically textured NiP/Al substrates with Ra textured substrate was observed withan atomic force 1.7 nm, a Cr under-layer, a Co-based magnetic layer and microscope (AFM). protective layer were formed sequentially using the DC magnetron sputtering method. The substrate tempera- ture just before sputtering was 1801C. The under-layer 3. Results and discussion *Corresponding author. Tel.: +81-237-43-8250; fax: +81- 237-43-7149. Fig. 1 shows the dependence of the thickness of the Cr E-mail address: murao@yfl.fujitsu.com (R. Murao). layer on HC and the orientation ratio. In this study, the 0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 1 5 7 - X R. Murao et al. / Journal of Magnetism and Magnetic Materials 242­245 (2002) 352­354 353 orientation ratio refers to the ratio of HC in the 1.5 circumferential direction to HC in the radial direction (Hcir C =Hrad C ). HC in bothdirections increases as the Cr thickness increases, however, Hcir 1.4 C =Hrad C decreases. From the GIXD analysis of the Co-alloy layer of high Hcir ) C =Hrad C media, it was found that the diffraction rad 1.3 intensity of the Co(0 0 2) perpendicular to the circumfer- C ential direction (Icir /H Coð0 0 2Þ) is stronger than that to the cir C 1.2 radial direction (Irad Coð0 0 2Þ). This means the number of (H Co-alloy grains in which the c-axis is oriented to the circumferential direction is larger than that to the radial 1.1 O.R. direction. Fig. 2 shows the relationship between Hcir C =Hrad C and the Co(0 0 2) intensity ratio 1.0 (Icir Coð0 0 2Þ=I rad Coð0 0 2Þ). They show a good correlation, and Hcir C =Hrad C increases monotonously with Icir 0 Coð0 0 2Þ=I rad Coð0 0 2Þ: This result suggests that the preferred 0 2 3 4 orientation of the easy magnetization axis of the Co- alloy to the circumferential direction induces macro- IcirCo(002)/IradCo(002) scopic in-plane magnetic anisotropy. We have found Fig. 2. Co(0 0 2) intensity ratio (Icir Coð0 0 2Þ=I rad Coð0 0 2Þ) dependence that the Co(0 0 2) interplanar spacing is almost constant on HC orientation ratio (Hcir C =Hrad C ). as the Cr thickness changes. In contrast, for the Cr layer, the Cr(1 1 0) interplanar spacing in the circumferential direction (dcir Crð1 1 0Þ) becomes shorter than that in the 4.0 radial direction (drad Crð1 1 0Þ) as the Cr thickness decreases, and the Cr crystal lattice becomes slightly distorted from the BCC structure. Fig. 3 shows the dcir Crð1 1 0Þ=drad Crð1 1 0Þ dependence of 3.0 Icir Coð0 0 2Þ=I rad Coð0 0 2Þ: In this study, the Cr(1 1 0) interplanar spacing ratio (dcir Crð1 1 0Þ=drad Crð1 1 0Þ) was used to express the Co(002) degree of lattice distortion. Icir rad Coð0 0 2Þ=I rad Coð0 0 2Þ increases /I monotonously as dcir Crð1 1 0Þ=drad Crð1 1 0Þ decreases, and the Cr 2.0 lattice distortion becomes larger. Considering the crystal Co(002) cir growth of the HCP Co-alloy with the Co(1 1 0) plane I grown on the Cr(2 0 0) plane [4], it is consistent that the 1.0 3.0 Circumferential 00 0.997 0.998 0.999 1.000 2.5 dcirCr(110)/dradCr(110) (kOe) Fig. 3. Cr(1 1 0) interplanar spacing ratio dependence on 2.0 Radial H C Icir Coð0 0 2Þ=I rad Coð0 0 2Þ: 1.5 Co(0 0 2) prefers to orient to the circumferential Cr(1 1 0) rad 1.5 of the distorted Cr crystal, because the Co(0 0 2) C/H interplanar spacing corresponds to the short side of cir the rectangular (1 1 0) plane of a Co unit cell. These 1.25 H C results suggest that the distortion of the Cr crystal lattice influences the preferred orientation of the c-axis of 0 the Co-alloy and the appearance of the macroscopic in- 0 50 100 150 200 250 plane magnetic anisotropy. Cr thickness (nm) The distortion of the Cr crystal lattice is considered to Fig. 1. Dependence of Cr layer thickness on HC and HC be caused by the stress of anisotropic compressive stress orientation (Hcir C =Hrad C ). from the substrate surface, which is primarily induced by 354 R. Murao et al. / Journal of Magnetism and Magnetic Materials 242­245 (2002) 352­354 the cooling of the substrate surface after layer forma- 1.000 2.0 tion. Therefore, we analyzed the relationship between the substrate surface morphology and the grain growth of the Cr layer. In cross-sectional TEM images from the 0.998 1.8 radial direction, fine grooves witha roughness smaller O.R. Ra: 1.7 nm than Ra value were observed on the surface of the (110) 1.6 d Cr 0.996 Groove half-width (L0): (H textured substrates. In the highly oriented media, the Cr ra 11.1nm /d C grains are small and grow along these fine grooves. As Cr thickness: cir /H for the low oriented media with 200-nm thick Cr, large 0.994 20 to 200 nm Cr(110) 1.4 C rad Cr grains grow over several grooves. These results cir d ) suggest that the compressive stress that the Cr grains 0.992 1.2 receive from the substrate surface is anisotropic between the circumferential direction and the radial direction, if L0 the Cr grains are smaller than the fine grooves. The 0 0 relation between the distortion of Cr crystal and the 0 0.5 1 1.5 2 2.5 Cr grain diameter/L anisotropic surface morphology was supposed to play 0 an important role in the preferred orientation of c-axis. Fig. 4. Cr(1 1 0) interplanar spacing ratio and Hcir C =Hrad C as a A detailed analysis of the relationship between grain size function of the ratio of grain diameter and groove half-width and groove widthwas necessary to better discuss the L0: origin of the in-plane anisotropy. The morphology of the fine grooves was analyzed by the use of an AFM image of the substrate surface. In the AFM image of the substrate surface withRa 1.7 nm, fine grooves observed 4. Summary in the TEM images are formed on the deep scratches. To analyze the morphology of these fine grooves, compo- The origin of the macroscopic in-plane magnetic nents of the wave with a wavelength of over 30 nm were anisotropy that is induced on a mechanically textured removed from the image thorough a high-pass filter. The substrate is the preferred orientation of the Co c-axis to half-width of the fine groove (L0) was taken from the the circumferential direction. This preferred orientation filtered image; its measured L0 value was 11.1 nm. is strongly related to the distortion of the Cr crystal Fig. 4 shows the relationship between the ratio of the lattice. The lattice distortion is caused by the stress of grain diameter to the half-width of the fine groove anisotropic compression from the substrate surface, (GD=L0), dcir Crð1 1 0Þ=drad Crð1 1 0Þ and Hcir C =Hrad C : Both when the grain diameter is smaller than the half-width of dcir Crð1 1 0Þ=drad Crð1 1 0Þ and Hcir C =Hrad C exhibited drastic the fine grooves on the substrate surface. changes at GD=L0 ¼ 1: When GD=L0 o1, a large degree of Cr lattice distortion and a high Hcir C =Hrad C were observed. These results demonstrate that the anisotropic References stress from the substrate surface induces the distortion of the Cr crystal lattice and the anisotropic crystal- [1] A. Kawamoto, F. Hikami, J. Appl. Phys. 69 (1991) 5151. lographic orientation of the Co-alloy when the Cr [2] K.E. Johnson, M. Mirzamaani, M.F. Doerner, IEEE Trans. crystal grain is smaller than the half-width of the texture Mgn. 31 (1995) 2721. grooves. In contrast, when the Cr crystal grain is larger [3] C.A. Ross, M.E. Schabes, R. Ranjan, G. Bertero, T. Chen, than the half-width of the grooves, the morphology of J. Appl. Phys. 79 (1996) 5342. the grooves does not influence the Cr crystal lattice and [4] D.N. Lambeth, W. Yang, H. Gong, D.E. Laughlin, B. Lu, L.-L. Lee, J. Zou, P.S. Harllee, Mat. Res. Soc. Symp. Proc. the crystallographic orientation of the Co-alloy layer. 517 (1998) 181.