MAGNETIC-PROPERTIES AND MAGNETIC HARDENING MECHANISM OF PT-CO-B ALLOYS

The intrinsic coercivity is found to be maximized in the Pt42Co45B13 t ernary alloy which is undercooled and rapidly solidified (quenched usi ng a 70 m/s wheel speed after undercooling), and then annealed (800-de grees-C for 2400 min). The same alloy, processed at slower cooling rat es and annealed in the same way, has a much larger scale microstructur e and a much lower resulting magnetic coercivity. The microstructure w hich would optimize the coercivity of this ternary alloy is a complete ly ordered L1(0) Pt-Co matrix with a submicron magnetic single-domain Co-boride precipitate. The L1(0) phase is highly anisotropic magnetica lly while the Co-boride precipitate is somewhat less so. Annealing tre atments designed to produce single-domain Co-boride precipitates enhan ce the coercivity. This suggests that the refined microstructure is re sponsible for the high coercivities found in the rapidly solidified an d annealed alloy. The magnetic domain wall thickness for a Co-boride p recipitate is determined from both experimental observation and theore tical calculation in order to evaluate its influence on the coercivity of the alloy. The effects of the pinning of domain walls and the barr ier to the nucleation of reverse domains on the coercivity are discuss ed. Both microstructural analysis and theoretical calculation indicate that the high coercivities in the Pt42Co45B13 alloy are due to the di fficult nucleation of reverse magnetic domains.