Concentration Profile of Superparamagnetic Precipitates

Abstract

According to previous work the temperature variation of the initial susceptibility, χ0, of superparamagnetic Cu–Fe–Ni alloys deviates from Curie's law, Ms2/χ0 = CT (Ms = spontaneous magnetization). After tentatively correcting for magnetostatic interaction effects with the first order Debye term, the data could be analyzed quantitatively in terms of a temperature-dependent ferromagnetic particle volume which follows from a gradual concentration transition (interface) between precipitates and matrix. Based on this analysis the interface thickness, Δρ, was determined on 74Cu2Fe24Ni for the aging temperature Θ≡TA/TS≃0.95 (TS = critical temperature of the precipitation); Δρ≃12 Å. In this work the variation of Δρ with TA was studied. For 71Cu3Fe26Ni, Δρ decreases from 17A for Θ = 0.97 to 6.5 A for Θ = 0.915, following the theoretical relation Δρ ∝ (TS−TA)−1/2. For Θ = 0.85, Ms2/χ0 nearly follows Curie's law. This explains why no interface effect was detected on other alloys for Θ < 0.6. Furthermore, it is shown that the Debye interaction correction is applicable in all of the above mentioned work. Finally, it is indicated how the actual concentration profile of precipitates may be determined from experimental data of χ0(T).