Grain Boundary Diffusion in Co/Cu and Co/Cr Magnetic Thin Films
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GRAIN BOUNDARY DIFFUSION IN Co/Cu AND Co/Cr MAGNETIC THIN FILMS JOHN G. HOLL-PELLERIN, S.G.H. ANDERSON, P.S. HO*, K.R. COFFEY, J.K. HOWARD**, AND K. BARMAK*** * Materials Laboratory for Interconnect and Packaging, University of Texas at Austin, BRC/MER Mail Code 78650, Austin, TX 78712-1100 **IBM, ADSTAR Division, 5600 Cottle Rd, 808/282, San Jose, CA 95193 ***Department of Materials Science and Engineering, Lehigh University, Whitaker Laboratory, 5 East Packer Ave., Bethlehem, PA 18015-3195
ABSTRACT X-ray photoelectron spectroscopy (XPS) has been used to investigate grain boundary diffusion of Cu and Cr through 1000 A thick Co films in the temperature range of 325 0 C to 400'C. Grain boundary diffusivities were determined by modeling the accumulation of Cu or Cr on Co surfaces as a function of time at fixed annealing temperature. The grain boundary diffusivity of Cu through Co is characterized by a diffusion coefficient, Dogb, of 2 x 104 cm 2/sec and an activation energy, Ea,,b , of 2.4 eV. Similarly, Cr grain boundary diffusion through Co thin films occurs with a diffusion coefficient, Do,gb, of 6 x 10-2 cm2/sec and an activation energy, Ea,hb' of 1.8 eV. The Co filn microstructure has been investigated before and after annealing by x-ray diffraction and transmission electron microscopy. Extensive grain growth and texturing of the film occurred during annealing for Co deposited on a Cu underlayer. In contrast, the microstructure of Co deposited on a Cr underlayer remained relatively unchanged upon annealing. Magnetometer measurements have shown that increased in-plane coercivity He, reduced rernanence squareness S, and reduced coercive squareness S* result from grain boundary diffusion of Cu and Cr into the Co films.
INTRODUCTION The increasing demands on thin film magnetic media continue to be focused on improving the signal-to-media noise (S/N) ratio in order to achieve ever higher areal recording densities, with particular emphasis placed on reducing noise. I Recent studies have shown that the use of a Cr interlayer between magnetic layers can affect the magnetic properties by influencing the grain orientation and the crystallographic texture of CoCr-based thin films. 1 ,2. 3 For example, Cr templates 150 A and 1500 A thick caused the grain size of CoPtCr films to double and created voids between the grains. The films produced with the thicker interlayer exhibited improved noise properties, which were attributed to either a reduction in exchange coupling between the grains or by a loss in preferred c-axis in-plane orientation. 3 Significantly, the intergrain exchange coupling could also be reduced by compositional segregation of non-magnetic materials to the grain boundaries.1 Segregation from a Cr interlayer into the grains and grain boundaries of a CoNiCr magnetic thin film has been proposed to explain the observed increase of in-plane coercivity H, as a function of time at constant temperature. However, direct evidence for grain boundary diffusion was not presented.4 To investigate the nature of grain bou
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