Sputtered thin films for high density tape recording

This thesis describes the investigation of sputtered thin film media for high density tape recording. As discussed in Chapter 1, to meet the tremendous demand of data storage, the density of recording tape has to be increased continuously. For further increasing the bit density the key factors are: increasing media coercivity and reducing the magnetic film thickness. To meet further requirements for recording, tape media with high magnetic anisotropy are desired. Since the shape anisotropy is limited, magnetocrystalline anisotropy is more suitable. Therefore, the motivation of this thesis is to produces a magnetic layer on a polymer substrate with high magnetic anisotropy and low film thickness. We applied the sputtering technology since this technique offers higher energy for the arriving atoms than vacuum evaporation. In addition, deposition of alloys with complex composition can also be made easily by sputtering. In Chapter 2, we describe the main experiment methods that we used in the thesis. The followed chapter presents some relevant aspects as well as results of oblique Co and Co alloy directly sputtered on polymer substrate. The first part of this chapter is dedicated to describe the mechanisms of oblique deposition such as shadowing and steering effect. In the second part of this chapter, experimental results of obliquely sputtered Co and Co-alloys on polymer substrate are presented. The pure Co films mainly consists of fcc structure and this gives a low anisotropy. In this case, the most important source of anisotropy is the shape anisotropy of the columns. At high angle of incidence (70), thick films exhibit well-separated columnar structure and due to the shape anisotropy this results in a moderate anisotropy which con- fine an in-plane easy axis in the longitudinal direction (Hc = 50 kA/m and Ha = 230 kA/m). In contradiction, at small thickness, the elongated nuclei caused by steering effect switches the in-plane easy axis to the transverse direction. At intermediate incident angle (50 degrees), the anisotropy is very small and directed in the longitudinal direction. As the consequence of the strong exchange coupling, stripe domains following the longitudinal direction are observed. We also investigated the effect of a commercial metal coating deposited on top of the polymer substrate on the properties of a sputtered Co layer. It has been found that such coating layer cannot alter the crystallographic structure of the Co. Thus also in this case the shape anisotropy is still the main source. Nevertheless, the coating layer has a certain effect in modifying the microstructure of the films, i.e the size of the nuclei. If we sputter a CoCrPt alloy directly on polymer, the film consists of mainly hcp phase, and consequently the magnetic anisotropy is much improved (Hc = 150 kA/m and Ha = 600 kA/m). In addition, the crystal size of the CoCrPt films is much smaller than that of Co films at the same thickness. This result indicates that composition modification is also an effective way to obtain films with desired properties. Chapter 4 deals with the properties of Co and Co alloys obliquely sputtered on different underlayers. In this chapter, we have shown that the introduction of Cr underlayer have a significant effect on the formation of hcp Co. When the thickness of Cr is high enough, the anisotropy and coercivity of the film is much higher than in the case of Co sputtered directly on polymer substrate. A good epitaxy relationship between Co and Cr is also found. The growth mode of Cr prefers (110) texture, which results in Co (10.1) texture with results in a c-axis orientation tilted out of the film plane. This texture is more pronounced in thick Co films. In case of thin films, the in-plane anisotropy is confined in the transverse direction. This effect is explained by the elongation of Cr grains in this direction. The epitaxial growth of Co on Cr then induced also an elongated structure of the Co grains. As a result, highly orientation of easy axis of Co in this direction is found. In the static configuration of sputtering, high coercivity (Hc of 200 kA/m) and high anisotropy constant (Ha of 4.6x105 J/m3) are obtained in the film sputtered at incident angle of 70with Co thickness of 20 nm and Cr thickness of 180 nm. Since the high anisotropy Co film can be obtained with low thickness it is interesting to realize this type of film as media for tape recording. We have attempted to produce experimental tapes with a high coercivity of 160 kA/m, low thickness of 20 nm and bi-directional recording behavior. The recording properties can also be further improved by the refinement of grains using NiAl seed layer. It is shown that the recording performances of our experimental tapes are closed with that of MICROMV tape, which is the most advanced ME tape commercially available. In the last part of this chapter, we also presented a study of CoCrPt/CoCrMn bilayer. This type of films exhibits excellent properties with a coercivity of 300 kA/m at a thickness of 30 nm, very small and well-defined crystal structure with crystal sizes of less than 10nm. This type of film can be a very promising candidate for high-density tape recording in the near future. Another somewhat different material, FePt, has been studied in Chapter 5. This material with a huge crystalline anisotropy has been chosen since it is also a potential candidate for ultra high-density hard disk recording. It shows by using proper sputtering conditions the hard mag- netic fct phase. Even at moderate annealing temperatures of 350 C - 400 C we proofed the presence of the fct phase. This hard magnetic phase acts at pinning sites preventing the domain wall movement of soft fcc phase. Therefore, high coercivities of 600 - 700 kA/m can be achieved. We also show that different factors such as Ar pressure and film thickness strongly influence the formation of fct phase. Increasing the Ar pressure promotes the formation of the fct phase. However, when the Ar pressure is too high the crystallinity of the films is degraded and the fct phase can not be formed. In our system, we found an optimum pressure of around 2x10-2 mbar. The thickness of the film also influences the ordering process: the transformation from fcc to fct structure takes place easier in thicker films. When the thickness of FePt is reduced to 10 nm, annealing temperature of 400 C is not sufficient to allow the formation of fct phase. In addition, the effect of Ag underlayer has been studied. Although Ag films sputtered at room temperature have (111) texture and consequently the (001) texture of FePt layer has not been promoted, it is obvious that the degree of ordering can be strongly enhanced by introduction of Ag underlayer. Ag underlayer can also provide pinning sites, which increase the coercivity and reduce the inter-grain exchange coupling. Although at this current state, our sputtered FePt films are not suitable for recording due to the existence of a strong exchange coupling, the obtained results would be benefit for further development of sputtered tape with very high density using novel FePt material. Finally, in Chapter 6 the main conclusions are given and the recommendations for future study are discussed. Last modified: Aug 27, 2004 by Hans.