Anomalous magnetic behavior in FePt nanoparticles chemically synthesized with polymer protective agent

FePt nanoparticles protected by poly(N-vinyl-2-pyrrolidone) (PVP) as a protective agent are synthesized by a chemical route. Diameter dependence of coercivity at low temperatures below ∼20 K for the FePt nanoparticles synthesized at various reaction temperatures exhibits an anomalous magnetic behavior; the coercivity increases with decreasing particle-diameter, which is different from a particle-size dependence of coercivity explained by thermal fluctuation effect. We discuss the anomalous magnetic behaviors associating with a surface effect due to coordinate bonds between PVP and the surface of the FePt nanoparticles.