Dependence of magnetic properties on atomic order in 77% Ni-14% Fe-5% Cu-4wt% Mo alloys

Abstract

We have studied the effect of atomic order on the lattice parameter,a, Curie temperature,Tθ, and the initial permeability,μi, of a series of molybdenum permalloys with varied Ni-Fe ratios and molybdenum concentrations. The ordering temperature range was between 380 and 600° C. The results of the effect of short-range order and long-range order on the lattice parameter after annealing for about 5 h between 420 and 470° C indicated a decrease of between 0.15 and 0.22 pm (0.04 to 0.06%) in the lattice parameter. No superlattice lines were detected. This may probably be due to the similarity in the atomic scattering factor of nickel and iron. It was also noted that both long-range and short-range ordering increased the Curie temperature of the ordered materials by about 1.4 to 6.6% due to the production of stronger and shorter Ni-Fe bonds. The number of Ni-Fe bonds, which controls the exchange force, was found to depend on the amount of order and molybdenum content in each material. The Curie temperature, which is a measure of the exchange force, is also an indirect means of measuring the degree of lattice ordering because the exchange integral is affected by metallurgical variables such as atomic ordering, composition, etc. Results of the isothermal annealing time on the initial permeability in the temperature range (380 to 600° C) indicated that between 380 and 460°C, maximum permeability was obtained at a critical degree of short-range ordering which is thought to correspond to a state when both the magnetostriction and anisotropy constants are close to zero. The maximum permeability was independent of ordering temperature in this range, although the time to reach this maximum decreases with increasing temperature. On annealing in the temperature range 460 to 500° C, the permeability reaches a maximum, the maximum permeability in this temperature range decreases with increasing temperature. As in the lower temperature range, the time to reach this maximum decreases with increasing temperature. Annealing between 500 and 600° C produced no maximum permeability. The permeability levelled off after an initial gradual increase. The activation energies for short-range order formation were found to be smaller (between 135 and 142 kJ mol-1) in alloys with molybdenum concentrations above 4 wt% Mo (>2.5 at.% Mo) and higher activation energies (between 196 and 210 kJ mol−1) in alloys with molybdenum concentrations below 4 wt% Mo (<2.5 at.% Mo) which suggests the formation of different ordered structures and mechanisms below and above 4 wt% Mo.