Soft magnetic properties of Co-based amorphous alloys with wide supercooled liquid region

New Co-based amorphous alloys with a wide supercooled liquid region above 40 K before crystallization and good soft magnetic properties were synthesized in the Co72-xFexZr8B20 and Co63Fe7Zr10-xMxB20 (M=Nb, Ta or W) systems. The supercooled liquid region defined by the difference between crystallization temperature (T-x) and glass transition temperature (T-g), Delta T-x(= T-x - T-g), is 39 K for Co56Fe16Zr8B20, 45 K for Co63Fe7Zr6Nb4B20, 40 K for Co63Fe7Zr4Ta6B20 and 44 K for Co63Fe7Zr6W4B20. The glass transition is also observed for the Co63F7M10B20 (M = Ta or W) alloys. The Co-based amorphous alloys with Delta T-x above 40 K exhibit low coercivity of 2.5 to 5.6 A/m, low magnetostriction of 1.7 x 10(-6) to 3.0 x 10(-6) and high permeability (mu(e)) of 10000 to 23000 at 1 kHz. The good mu(e) characteristics are maintained in a high frequency range and the mu(e) at 1 MHz is 5700 for Co56Fe16Zr8B20 and 6800 for Co63F7Zr6Ta4B20. The high frequency permeability is superior to those for conventional Co- and Fe-based amorphous alloys. The electrical resistivity (rho(n)) is as high as 1.7 to 2.0 mu Ohm m and hence the reduction of eddy current loss caused by the high rho(n) is presumed to be the origin of the better high-frequency permeability. The simultaneous achievements of good soft magnetic properties and high rho(n) are presumably due to the formation of a unique amorphous structure in which two kinds of atomic pairs of metal-metal (Co-Fe-Zr-M) and metal-metalloid (Co-Fe-Zr-M-B) types are homogeneously mixed on a subnanoscale, because the former type pair has good soft magnetic properties and the latter type pair has high rho(n). The good combination of high stability of the supercooled liquid and good soft magnetic properties is expected to induce future development as a new type of soft magnetic bulk amorphous alloy.