The effect of substrate materials on the structuraland magnetic properties of Co-Cr recording media

Abstract

The investigations described in this thesis deal with the influence of substrates and seedlayers on the structural and magnetic properties of Co- Cr films. The increased importance of the substrates and seedlayers lies in the tendency that in Co-Cr thin film production for recording applications the medium thickness has decreased to about 0.1 ~m; therefore, in order to obtain Co-Cr films suitable for recording, the structural and magnetic properties should preferably be as homogeneous as possible. Every inhomogeneity at the substrate material / Co-Cr film interface should be avoided by tailoring the deposition conditions and by choosing the optimal substrate material. In chapter 2 a de scription is given of the methods that are used for characterization of the Co-Cr films and for electron microscopy sample preparation. The Bitter colloid technique that is used as a magnetic contrast observation technique is considered in detail and is compared to other domain observation techniques such as modified differential phase contrast microscopy (MDPC), magnetic force microscopy (MFM), scanning electron microscopy with polarization analysis (SEMPA) and Kerr microscopy. It is shown (chapter 3) that depending on the deposition conditions of the Co-Cr films two growth mechanisms can be discerned: from AES depth profiling measurements it was concluded that using germanium seedlayers an interdiffusive re action layer develops, containing Co, Ge and to a lesser degree Cr. The interdiffusive re action layer acts as a homogeneous nucleation layer, leading to hcp c-axis textured nucleation of Co-Cr crystallites from the first stage of growth and also as a relaxation layer in the case of a lattice misfit. Application of silicon nitride seedlayers results in a much more distinctive interface between the seedlayer and the Co-Cr film. No c-axis textured nucleation from the first stage of growth was found in this case, but with increasing thickness the Co-Cr film became increasingly textured in the (00.1] direction. The c-axis dispersion from the normal ofthe film at 0.1 ~m thickness was found to be determined by the dispersion of the first about 5 nm Co-Cr. The calculated strain in the fIlms was found to decrease an order of magnitude within the thickness range 5- 200 nm for Co-Cr films on a germanium seedlayer . Vibrating sample magnetometer and anomalous Hall effect measurements showed (chapter 4) that Co-Cr films of 5 nm thickness can exhibit a perpendicular anisotropy, depending on the choice of substrate materialor deposition conditions. By monitoring the in-plane remanence ratio (Mr!Ms)//, it was shown that well-textured Co-Cr films exhibit a perpendicular anisotropy at small (t 50 nm) thicknesses, but have a large in-plane remence ratio at intermediate thicknesses. This behaviour could be qualitatively explained by considering the presence / absence of the initial layer, the in-plane magnetization contributions in the domain walls and the developments of magneoelastic and magnetocrystalline anisotropy contributions with increasing thickness. Non-optimized deposition conditions or substrate materials result in Co-Cr fi1ms with inferior magnetic properties. In chapter 5 domain observations are given of well-textured low and high coercive Co-Cr fi1ms, obtained by Bitter colloid technique: bubbles and stripe domains were observed, in confirmation with earlier reports. MDPC observations show the existence of a small scale ( 1 ~m) domains. The ripple period seems to vary with its magnetic history .Written bits could reproduced using the Bitter colloid technique down to a Co-Cr fi1m thickness of about 25 nm, for fi1ms with a quality factor Q>l. Investigations in the depth direction of well-textured 0.1 ~m Co-Cr fi1ms by ion beam etching experiments revealed that the written magnetization penetrates the fi1m down to a fi1m thickness of at least 40 nm. The transition region between the written bits did not change in regularity on top and inside the fi1m. It was concluded (chapter 6) that Co-Cr f1lms of 0.1 ~m thickness with suitable properties for recording can be prepared with the right choice of deposition conditions and substrate materials; the preparation of the first 30 nm is of crucial importance in order to achieve this. Improved experimental magnetic contrast observation techniques wi1l lead to a better understanding of micromagnetic processes in Co-Cr fi1ms. Awaiting the developments in the fields of head design and tribology , Co-Cr fi1ms can establish themselves in high density magnetic storage applications.