MICROSTRUCTURAL AND MAGNETIC CHARACTERIZATION OF RAPIDLY SOLIDIFIED AND ANNEALED PT-CO-B ALLOYS

Significant increases in intrinsic coercivity (H(ic)) of Pt-Co alloys have been obtained by the addition of boron and the application of rapid solidification processing. After rapid solidification by double anvil splat quenching with subsequent annealing at 650-degrees-C for 30 min, an alloy of Pt42Co45B13 (at. %) exhibits an H(ic) as high as 14 kOe. Transmission electron microscopy and Lorentz microscopy were used to interpret the magnetic data. Annealing of the Pt-Co-B influences the L1(0) superlattice structure and grain size of the matrix, the crystal structure and size of Co-boride precipitate, and the distribution of magnetic domain walls. A microstructural analysis shows that the maximum H(ic) occurs when Co borides, having the Co3B structure, are within the single magnetic domain size. The magnetic hardening mechanism in Pt-Co-B is believed to be a combination of inhibited magnetic domain nucleation and difficult reverse magnetic domain growth caused by the interaction of the magnetically anisotropic Co borides with the L1(0) Pt-Co matrix.