Static and dynamic magnetic properties of spherical magnetite nanoparticles

TLDR: In this paper, a detailed study of static and dynamic magnetic behavior of Fe3O4 nanoparticles with average particle sizes ranging from 5 to 150 nm is presented, and the existence of surface spin disorder can be inferred from the decrease of saturation magnetization MS at low temperatures, as the average particle size is reduced.

Abstract: We present a detailed study of static and dynamic magnetic behavior of Fe3O4 nanoparticles with average particle sizes 〈d〉 ranging from 5 to 150 nm. Bulk-like properties such as saturation magnetization, hyperfine parameters, coercive field, and Verwey transition are observed in 150 nm particles. For decreasing particle size, the Verwey temperature, TV, shifts down to ∼20 K for 〈d〉=50 nm and is no longer observable for smaller particles. The smallest particles (〈d〉=5 nm) display superparamagnetic behavior at room temperature, with transition to a blocked state at TB∼45 K, which depends on the applied field. The existence of surface spin disorder can be inferred from the decrease of saturation magnetization MS at low temperatures, as the average particle size is reduced. This disordered surface did not show effects of exchange coupling to the particle core, as observed from hysteresis loops after field cooling in a 7 T magnetic field. For particles with 〈d〉=5 nm, dynamic ac susceptibility measurements show...