Particle size dependence of atomic ordering and magnetic properties of L1₀-FePd nanoparticles

L1₀-type FePd nanoparticles with mean sizes between 4 and 15 nm in diameter have been fabricated by electron beam evaporation. Coercivity and remanence of FePd nanoparticles with sizes less than about 8 nm were quite low compared to the values for 10-nm-sized FePd nanoparticles. Both coercivity and remanence increased with decreasing the temperature, indicating the large suppression of the thermal agitation of the magnetic moment at lower temperatures. Superparamagnetic behavior appeared in zero-field-cooled (ZFC) and field-cooled (FC) temperature dependence of magnetization for about 6-nm-sized FePd nanoparticles. Disappearance of the hard magnetic properties in smaller sized FePd nanoparticles is attributed to the reduction of the long-range order (LRO) parameter of the L1₀-structure as well as the thermal agitation of magnetic moment.