Magnetic properties of multicore magnetite nanoparticles prepared by glass crystallisation

A glass with the composition 13K₂O*13Al₂O₃*16B₂O₃*43SiO₂*15Fe₂O_3−x was melted and rapidly quenched in water. This leads to the formation of phase-separated droplets with diameters from 100 to 150 nm. Magnetite crystals with a size of 10–20 nm precipitate within these droplets. The magnetite containing phase-separated regions can be separated from the glass by dissolving the SiO₂-rich amorphous glass matrix through boiling the pulverized glass in a concentrated aqueous sodium hydroxide solution. The residual, magnetite containing phase-separated droplets match multicore magnetite nanoparticles (McNP). The magnetite nanoparticles show superparamagnetic behaviour and as McNP, lead to a higher effective magnetic radius than single crystals. Magnetisation measurements of the McNP indicate that the particles show a narrow hysteresis, but the ratio of remanent to saturation magnetisation is not high enough for uniaxial anisotropy. The additionally performed temperature-dependent magnetorelaxometry (TMRX) measurements show peaks at 13 and 39 K in the distribution of the magnetic moment relaxation. The obtained inter-particle distance of the magnetite within the McNP is smaller than 5 d _C (core diameter), leading to strong magnetic interactions.