Role of magnetic concentration in modulating the magnetic properties of ultra-small FePt nanoparticles

Abstract

Magnetic characterization of nanoscale materials is often hindered by the role that sample preparation techniques play in the determination of interparticle interaction strength. Well-dispersed d=2.6(4)nm FePt nanoparticles synthesized by a slight modification of a known polyol synthesis were employed to study this effect at the extreme lower limit of the nanoscale regime. Suspension in a diamagnetic matrix material at varying concentrations was used to characterize the relationships between average particle distance and representative magnetic properties (zero-field cooled/field-cooled (ZFC/FC) magnetization curves, hysteresis M(H) loops at 5K, and ac-susceptibility). By increasing the interparticle distance through diamagnetic dilution, the blocking temperature (TB), anisotropy energy barrier (Ueff), and coercive field (Hc) drop continuously until reaching a dilution ratio where the magnetic signals were limited by the competing diamagnetic contribution from the matrix material. Long-timescale blocking temperature and coercivity are relatively unaffected by particle dilution while the shorter timescale anisotropy energy barrier (Ueff) and attempt time (τ0) are strongly affected. These results demonstrate the need for well-defined sample preparation conditions when comparing materials properties, especially those with applications dependent on superparamagnetism.