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Multicore magnetite nanoparticles prepared by glass crystallisation and their magnetic properties

A potassium alumina borosilicate glass with the composition 13K2O*13Al2O3*16B2O3*43SiO2*15Fe2O3-x was melted using Fe2O3 as raw material. The melt was dumped from a Pt-crucible with a downpipe in water which resulted in the formation of phase separated droplets with a size of around 100 - 150 nm. In this droplets, magnetite crystals with a size of around 10-20 nm were observed. These magnetite nanoparticles with superparamagnetic behaviour are arranged to larger aggregates. This leads to a higher effective magnetic radius. According to magnetisation measurements the particles show hysteresis. The ratio of remanent vs. saturation magnetisation is not as high as it is necessary for uniaxial anisotropy. It is possible to elude the phase separations by cooking the pulverized glass in concentrated sodium hydroxide. Additional temperature dependent magnetorelaxometry (TMRX) measurements show in the distribution of the relaxation of magnetic moments over the course of temperature two peaks at 13 and 39 K. According to an interparticle distance smaller than 5 dC (the core diameter) could that be a result of strong magnetic interactions. Other magnetic relaxation processes also explain this measured effect.