Particle-diameter dependence of the coercive field in FePt nanoparticles with a face-centered tetragonal structure

It has recently been reported that, in FePt nanoparticles (NPs) protected by poly(N-vinyl-2-pyrrolidone) (PVP), which possess a particle diameter (D) near the critical size (2–3 nm) for the emergence of the face-centered tetragonal (fct) structure, the coercive field (Hc) at room temperature increases with increasing D, whereas the Hc value below 10 K increases with decreasing D. We investigated the fct-ordering parameter (S) for PVP-coated FePt NPs with three mean D values (=2.0, 2.6, and 3.4 nm) in order to gain a better insight into the unique behavior of Hc. Furthermore, the temperature dependencies of Hc, saturation magnetization (Ms), and their product (HcMs) were also investigated. The behavior of Hc at high temperature is attributed to the decrease in the magnetic switching volume (V), while the enhancement of Hc at low temperatures with decreasing D is thought to originate from the decrease in the saturation magnetization, Ms, with decreasing D. The decrease in Ms with decreasing D is related to ...