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Magnetization Enhancement in Magnetite Nanoparticles Capped with Alginic Acid

We report on the effect of organic acid capping on the behavior of magnetite nanoparticles. The nanoparticles of magnetite were obtained using microwave activated process, and the magnetic properties as well as the electron magnetic resonance behavior were studied for the Fe3O4 nanoparticles capped with alginic acid. The capped nanoparticles exhibit improved crystalline structure of the surface which leads to an enhanced magnetization. The saturation magnetization Ms increases to ~75% of the bulk magnetization. The improved structure also facilitates quantization of spin-wave spectrum in the finite size nanoparticles and this in turn is responsible for unconventional behavior at low temperatures. In magnetic resonance these anomalies are manifested as an unusual increase in the resonant field Hr(T) and also as a maximum of the spectroscopic splitting geff parameter at low temperatures. The unconventional behavior of the nanoparticles also leads to pronounced upturn of magnetization at low temperatures and a deviation from the Bloch law M(T) T^3/2.