Effects of Ta addition in CoCrTa intermediate-layer on thin film magnetic recording media

Abstract

It is known that the magnetic performance can he significantly improved by inserting a thin intermediate layer of CnCrTa between a CoCrTaPt magnetic layer and a Cr alloy underlayer in magnetic thin tilm media [1-3]. A number of investigations have been done on the effects of Ta addition on COCffaPt and CoCtTa magnetic thin films 14.51. It is generally agreed that Ta enhances Cr segregation at column boundaries but not Ta segregation. In addition, CO grain isolation can also he achieved by preferential inter-grain diffusion of underlayer materials to the CO alloy grain boundaries [2,3]. In this paper, we are interested in studying effects of Ta content in the range of 0-1 1 at% in CoCrTa intermediate-layer on thin film magnetic recording media. In the present work, the thin film media deposited on glass substrate have the same structure and fixed thickness for NiAl seedlayer, CrV underlayer and CoCrTa intermediate layer: NiAI(5Onm)/CrV(30nm)/CoCfla(Znm)/CoCfl~. For each of composition of CnCrTa layer. the thickness of the CoCflaPt magnetic layer was varied at 10nm. 12nm. 14m and 18nm. All the thin film media were prepared by DC sputtering with the same sputtering process using an Innotec four-target sputter machine. ConaCr~ target was used for the intermediate layer deposition. Ta composition was adjusted by placing Ta chips on the target. Chemical compositioa of CoCrTa was analyzed by energy disperse x-ray spectroscope (EDX). Microstructure and morphology were investigated by transmission electron microscope (TEM). Magnetic properties were characterized by a vibrating sample magnetometer (VSM). Stnrcturd properties were studied by x-ray diffraction (XRD). When Ta content is in the range of 0-5 at% in the CoCrTa intermediate layer, and the thickness of magnetic layer is 12 tun, the thin film media can possess good magnetic performance such as H, up to around 4000 Oe, and M,t value below 0.4 memu/cm2. In plane TEM bright field image of the above mentioned sample is shown in Figure 1. The visually estimated average grain size of the magnetic layer is ahout 12 nm and very uniform. Figure 2 shows the magnetic layer thickness dependence of H, undcr different Ta content in CoCrTa intermediate layer. The H, increases as magnetic layer thickncss increases from 10 nm tu 12 nm, thereafter H, decreases. It is clearly seen in Fig. 2, that the higher Td content in CoCrTa intermediate layer, the greater decrease in H, when the thickness of the magnetic layer is higher than 12 nm. The result may he related to the change of interface due to Ta addition in intermediate layer. But Ta content in intermediate layer has no intluence on M,t value when the thickness of magnetic layer is fixed. The M,t value of the samples with the CoCrTas intermediate layer decreaqes from 0.56 to 0.32 memu/cm2 with decreasing magnetic layer thickness from 18 nm to IO nm. For all of the samples, the S and S* value is around 0.77 and 0.85, respectively. The change in H, as a function of Ta content in intermediate layer is shown in Figure 3. As Ta content is in the range of 0-5 at% in the intermediate layer, a high H, can he achieved for different thickness of the magnetic layer. H, decreases dramatically once Ta content is over 5 at% in the intermediate layer. This feature may be explained from the result of XRD as shown in Fig. 4. There is obvious change in CO alloy texture and peak intensity with changing Ta content in the CoCrTa intermediate layer. Both Co(100) and Co(l0l) peaks exist with no shift when Ta content is increaed from 0 to 5 at%, however, Co(l0l) texture is decreased significantly at 7 at% of Ta and CO alloy peaks disappeared at 9 at% and 11 at% of Td. The lattice constant a and c for hcp CO phase and a for bcc Cr phase were calculated. It is found that the misfit of Co(lOl)/Cr(llO) planes and Co(lOO)/Cr(211) planes are respectively ahont 3.5 5% and 5.7 %, when Ta content is in the range of 0-5 at%, but the misfits are correspondingly over 15 % and 21 % at 7 at% of Ta, because Ta addition enlarges c lattice parameter of CO phase. In summary, the Ta content in the CoCrTa intermediate layer has effectively influence on the magnetic performance of the thin film media. The effects can be attributed lo the change of interface and the lattice mismdtch between CO and Cr planes due to Ta addition in the intcrmediate layer.