Size effect of Fe nanoparticles on the high-frequency dynamics of highly dense self-organized assemblies

Molded Fe nanoparticle (NP) assemblies 4 mm × 8 mm × 0.3 mm ^t in size were fabricated by a uniaxial press from Fe NPs 3 nm to 22 nm in diameter, and their high-frequency dynamics and static properties were investigated. The freezing temperature of the magnetic moment of the assemblies increased from 25 K to over 400 K with an increasing Fe NP diameter, and for particles larger than 8.1 nm, the Fe NP assemblies showed ferromagnetic behavior even at room temperature. For particles smaller than 8.1 nm, on the contrary, the assemblies showed superparamagnetism at room temperature. From the complex magnetic susceptibility spectra of the assemblies normalized by the volume of the Fe NPs, X′ _Fe, and X″ _Fe, the minimum ferromagnetic resonance frequency, f _r (106 MHz), and maximum X′ _Fe at 1 MHz (123) were obtained at 8.1 nm. The f _r of the Fe NP assembly increased because of an enhanced magnetostatic interaction field between NPs larger than 8.1 nm. For NPs smaller than 8.1 nm, in the superparamagnetic phase, the thermal magnetic field increasingly dominated the effective magnetic field with a decreasing NP diameter. This indicates the existence of a critical diameter for the thermal fluctuation of the moment, which competes with the magnetostatic interactions between the Fe nanoparticles.