Low-temperature atomic ordering of oriented L1₀-FePtCu nanoparticles with high areal-density characterized by transmission electron microscopy and electron diffraction

Oriented and densely dispersed Ll₀-FePtCu nanoparticles have been directly synthesized by co-evaporation of Fe, Pt and Cu using rfmagnetron sputtering onto NaCl substrate kept at 563-613 K without any post-deposition annealing. Formation of the L1₀-type structure in the specimens fabricated as low a substrate temperature as 563 K (Fe₄₀Pt ₅₀Cu₁₀) was confirmed by electron microscopy and electron diffraction, while the coercivity measured at room temperature was very low and the intensity of the superlattice reflections was quite weak. The atomic ordering was promoted in the specimen fabricated at 613 K with a composition of Fe₃₇Pt₅₂Cu₁₁, resulted in a higher coercivity exceeding 1 kOe at room temperature as well as appearance of clear superlattice reflections. In addition to the evolution of atomic ordering, (100) oriented growth was enhanced as the substrate temperature increased. Particle size dependence of long-range order (LRO) is considered to be responsible for the decrease of coercivity with particle size reduction as well as thermal fluctuation of magnetization. High coercivity was obtained for specimens with Cu concentration near 10at% under the present sputtering condition. The LRO in the FePtCu ternary phase is considered to sensitively depend on me alloy concentration.