Controlling annealing and magnetic treatment parameters to achieve high permeabilities in 55 Ni-Fe toroid cores

The 55 Ni-Fe alloy is a soft magnetic material capable of combining high permeability (e.g,. μ_0.4>100000 and μ _max>180000 in 0.08 mm strip measured at 60 Hz) and high saturation induction (1.5 T) compared to 78~80 Ni-Fe and 48~50 Ni-Fe alloys, after a proper thermal-magnetic treatment. The permeabilities of 55 Ni-Fe tape cores are very sensitive to annealing temperature and time, magnetic treatment temperature and time, as well as field direction and strength during the treatment. This paper summarizes these effects. The optimum treatments determined are: secondary recrystallization annealing at 1180°C for 6 hrs in dry H₂, followed by 24-hr uniaxial magnetic ordering treatment at 447°C in dry H₂ (or in dry N₂) with a circumferential DC field (>10-12 Oe) applied during treatment and cooling. The cooling rates after annealing or treatment are less critical and can be 50-650°C/h as long as the field during treatment is higher than 10-12 Oe