THE PERCOLATION LIMIT FOR THE DISAPPEARANCE OF FERROMAGNETISM IN MELT-SPUN CO-B-C AMORPHOUS-ALLOYS

Low-temperature magnetic properties of the Co56-xB32+xC12, X=0, 2, 4, and 4.5 ternary amorphous alloys are investigated by using low-field a c magnetic susceptibility measurement techniques and also a supercondu cting quantum interference device magnetometer. The ac susceptibility data show that these alloys undergo a paramagnetic to ferromagnetic st ate transition at cryogenic temperatures. The Curie temperature T(c) d ecreases linearly with decreasing Co content at a rate 31 K per Co at %, suggesting a percolation limit for the disappearence of ferromagnet ism at around 50 at % Co. A scaling analysis performed on magnetic iso therms obtained near T(c); even for the sample almost close to the per colation limit, yields critical exponent values consistent with the 3D -Heisenberg model for an ideal ferromagnet. Thus in these Co-based amo rphous alloys, long-range order persists to the concentrations almost at the percolation limit and the mean-field theory is found to describ e well the paramagnetic to the ferromagnetic state phase transitions.