Size effect on $L1$ ordering and magnetic properties of chemically synthesized FePt and FePtAu nanoparticles

There is growing evidence that FePt nanoparticles become increasingly difficult to chemically order as the size approaches a few nanometers. We have studied the chemical ordering of FePt and FePtAu nanoparticle arrays as a function of particle size. Monodisperse $Fe49Pt51$ and $Fe48Pt44Au8$ nanoparticles with a size about $6nm$ were synthesized by the simultaneous decomposition of iron pentacarbonyl and reduction of platinum acetylacetonate and gold (III) acetate in a mixture of phenyl ether and hexadecylamine (HDA), with 1-adamantanecarboxylic acid and HDA as stabilizers. The nanoparticles were dispersed in toluene, films of the particles were cast onto silicon wafers from the dispersion, and the films were annealed in a tube furnace with flowing $Ar+5%H2$⁠. The magnetic anisotropy and switching volumes were determined from time- and temperature-dependent coercivity measurements. By comparing with $3‐nm$ FePt and FePtAu nanoparticles of comparable composition, the phase transformation is easier for the larger particles. Under the same annealing conditions, the larger particles have higher anisotropy and order parameter. Additive Au is very effective in enhancing the chemical ordering in both small and large particles, with x-ray diffraction superlattice peaks appearing after annealing at $350°C$⁠. Dynamic remnant coercivity measurements and magnetic switching volumes suggest particle aggregation at the higher annealing temperatures in both small and large particles.