Anomalously soft magnetic properties of boron-rich CoB thin films

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Abstract
Sequentially cosputtered, amorphous thin films with an average composition of 〈Co61B39〉 were found to have an anomalously high Curie temperature (>575 K) compared to those reported for their bulk counterparts. Melt-spun Co-B ribbons, for example, with only 30-at. % B(max) have a measured Tc ≊500 K. Tc of ≤300 K would be expected for ribbons with 39-at. % B. The sputtered films crystallized at ≊675 K, but became extremely soft when annealed in a rotating field at 500 K≤Tc. The coercivity of the as-deposited films was reduced from 1 to 2 Oe to approximately 0.01 Oe and the large uniaxial anisotropies of 5–10 Oe vanished; the annealed films were isotropic. Extremely large initial permeabilities of 8000–10 000 were measured to over 10 MHz. The observed anomalies indicate that the center regions of the columnar structure of the sputtered films were enriched by ≊10% Co compared to the boundaries.