XRD and VSM Analysis of Nanostructured Cu-Co Alloys

Results of an experimental study of Cu-Co alloys containing 0.3 to 5.0 wt% Co are presented. A series of solution annealed Cu-Co alloys were analyzed by x-ray diffraction (XRD) and vibrating sample magnetometry (VSM). Each Ka doublet was resolved by fitting a Pearson Type VII profile. Precise lattice parameter as a function of cobalt content deviated significantly from Vegard’s law but was in reasonable agreement with King’s analysis. VSM measurements showed that the alloys with cobalt I 2% were paramagnetic, and their initial magnetic susceptibility was a parabolic function of the cobalt content. Alloys containing cobalt in excess of 2% showed presence of a ferromagnetic phase. Solution annealed specimens containing 1.5% Co were aged at 700°C and 750°C for various times to precipitate sub-micron size Co-rich particles. In addition to XRD and VSM measurements, the precipitate particles were analyzed by magnetic force microscopy (MFM). The amount of magnetic precipitate determined from VSM measurements is in good agreement with Servi and Turnbull’s mathematical description of the solvus line. VSM and MFM results indicate that initially the precipitate particles are superparamagnetic that grow into single domain ferromagnetic particles. At the higher aging temperature, some of the single domain particles grow and become multi-domain. XRD measurements show an unexpected trend that has features coinciding with the magnetic transitions. Introduction Interesting magnetic properties are expected when geometric dimensions of magnetic particles dispersed in a nonmagnetic medium become comparable to the size of magnetic domains. The miniaturization of magnetic technologies has fueled a renewed interest in such sub-micron size magnetic particles. This paper presents an experimental study of Cu-Co alloys containing 0.3 to 5.0 wt% co. The Cu-Co phase diagram shows that the maximum solid solubility of Co in Cu is 7 wt% at 1112”C, and the solubility decreases sharply with decreasing temperatures becoming negligible below 500°C (ASM Handbook, 1992). Similarly, the solid solubility of Cu in Co becomes negligible below 500°C. The Cu-rich solid solution is face-centred cubic (FCC) and nonmagnetic while the Co-rich solid solution is magnetic and is either FCC or hexagonal close-packed (HCP) depending on its heat treatment history. When a Cu-Co alloy containing 2 wt% Co was solution treated in nitrogen at 1010°C for 10 minutes followed by quenching into an iced NaOH solution, magnetic measurements revealed that the cobalt was completely in solution (Becker, 1957). Upon subsequent aging at 650°C for aging times up to 130 minutes, magnetization curves Copyright (C) JCPDS-International Centre for Diffraction Data 1999