Microstructure and Magnetic Properties of CoCrTa Thin Films
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C.J. ROBINSON AND J.K. HOWARD IBM Magnetic Recording Institute, IBM Almaden Research Centre, 650 Harry Road, San Jose, Ca.95120-6099
ABSTRACT The addition of Ta into thin sputtered films of CoCr greatly affects the microstructure and magnetic properties. Grain size, crystallographic preferred orientation and hysteresis parameters have been studied using transmission electron microscopy, electron diffraction, X-ray diffraction, polar Kerr effect and vibrating sample magnetometry techniques. Crystallographic preferred orientation is enhanced and is accompanied by increased perpendicular magnetic anisotropy. An extremely fine-grained microstructure is produced giving rise to films which show no morphological growth features. Thus the observed magnetocrystalline anisotropy is not attributed to columnar growth. Orientation in these films can be further increased by using underlayers of non-ferromagnetic CoCrTa alloys. The mechanism by which this occurs is discussed.
INTRODUCTION Perpendicularly anisotropic thin films of CoCr alloys have received much interest for their application in vertical recording media [ 1,2] . In this work we present data on the microstructure and magnetic properties of the ternary system (Co85 Cr,5 ) 100_xTax where x=10-30. Previously CoCrTa media have been used and perpendicular recording results have been reported [3,41. We show here that the inclusion of Ta leads to a fine grain microstructure with no evidence of columnar growth morphology. The films show an initial transition region of growth similar to that reported in CoCr media [51 . The transition region itself has an easy axis of magnetization within the plane of the film with low coercivity Hc( , ). The overall perpendicular anisotropy and preferred crystallographic orientation are degraded by the presence of this region. It is shown that Ta-rich non-ferromagnetic underlayers of x=30 can be used to eliminate the transition region and enhance both the perpendicular anisotropy and Dreferred orientation in the films thereby resulting in a recording medium with improved magnetic properties. EXPERIMENTAL Thin films were prepared by cosputtering from two circular magnetron sputtering sources of target composition Co,5 Cr1 5 and Ta. The substrate material used was polyimide sheet of 75,u thickness mounted on a rotating planetary substrate holder. Samples of 1 cm 2 area were prepared for x-ray diffraction, vibrating sample magnetometry (VSM), and polar Kerr rotation analysis. Transmission electron microscopy (TEM), electron diffraction and Lorentz microscopy were performed on films deposited directly onto standard TEM grids coated with amorphous carbon support films. Sputtering was in Ar at a pressure of 2.7 x 10-' Pa. (2.0 x 10-t torr) with a residual gas pressure of < 6.6 x 10-5 Pa. (5.0 x 10-1 torr) and an Ar flow rate of :8.0 sccm. Substrate heating was achieved by radiation using quartz lamps situated facing the planetary assembly and substrate surface. Substrate temperature Ts during deposition was 200'C and the deposition rate was 0.2
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