Detailed investigation of structural and magnetic properties of multiphase binary Pd-Co alloys prepared by modified polyol process

Abstract

Pd-Co based magnetic alloy nanoparticles were prepared by the modified polyol process and stabilized by polyvinylpyrrolidone (PVP) and 3-aminopropyl-trimethoxysilane (APES) capping agents and further reduction of metal salts with Sodium borohydride (NaBH4) at high temperature to form desired NPs. We have a detailed investigation of the effect of Pd concentration in the Pd-Co alloy NPs on structural and magnetic properties. XRD and Rietveld refinement analyses were confirmed that the multiphase structures of fcc-PdCo, fcc-Co, and hcp-Co phases coexist at low Pd loading samples. Over %50 of Pd loading resulted in a single fcc-PdCo phase with reduced lattice parameter to 4.0079 Å and d(111)-space to 2.31 Å. TEM and SEM images reveal well dispersed and uniformly distributed NPs with an average particle sizes of below 7 nm. The elemental compositions and the characteristic OH, CH, CO stretching peaks of capping agents were confirmed by EDS and FT-IR spectrums, respectively. M(T) and M(H) curves revealed that there are multi magnetic phase transitions in the Pd-Co structure as a function of Pd loading from superparamagnetic to ferromagnetic phase or back to superparamagnetic phase by reducing the temperature from 300 K to 5 K. We observed that the blocking temperature (TB) could not be detected due to 5% Pd loading below 300 K, while it reduced up to 55 K at high Pd loading of 62%. The coercive field (Hc) was increased to ~1900 Oe for Pd0.62Co0.38 sample due to the smallest particle size as 5.26 nm. The highest amount of Co resulted in maximum saturation magnetization (Ms) up to 65.5 emu/g for 5% Pd concentration. We measured the Mr/Ms ratios were less than 0.5, which is due to the internal stress that results in the uniaxial magnetic anisotropy in the structure. The maximum Keff and μf.u. values were found to be over 12.9 × 106 erg/cm3 and 0.72 μB at 5 K for 5% Pd concentration, respectively.