MAGNETIC SOFTENING AND NANOCRYSTALLIZATION IN AMORPHOUS CO-RICH ALLOYS

In this work we have analyzed the changes of the magnetic properties i n the Co-based Metglas 2714A (made by Allied Chem. Corp., USA) caused by isothermal heat treatments in the range of temperature from room te mperature to conventional crystallization temperature T-cr = 550 degre es C. The nominal composition of the amorphous alloy is Co66Fe4Ni1B14S i15. The analysis is made on toroidal samples prepared by winding leng ths of amorphous ribbon of about 20 cm. The magnetic properties underg o variations depending on the treatment temperature, except for the sa turation magnetization which remains unchanged. For heat treatments of about half an hour around 500 degrees C, superior soft magnetic prope rties are obtained. Particularly, the initial permeability reaches val ues up to ten times the value of permeability in the as-received sampl e. Analysis by the transmission electron microscopy of the sample anne aled around 500 degrees C reveals the formation of a nanocrystalline p hase, with average grain size of 2 nm, embedded in a residual amorphou s matrix. The occurrence of permeability increases in concomitance wit h the formation of the nanocrystalline phase is ascribed to a drastic reduction in the local magnetocrystalline anisotropy randomly averaged out by the exchange interactions, similar to the case of the annealed Fe-based alloys containing Cu.