Low temperature FMR investigations on double surfactant water based ferrofluid

Magnetite nanoparticles were synthesized by coprecipitation of reverse micelles in an aqueous phase. XRD, TEM and VSM results confirm the average particle size 9-10 nm. The FMR measurements were performed in zero-field-cooled (ZFC) and field-cooled (FC) protocol. Raikher and Morais models were used for interpreting the resonance field and linewidth results. A value of 2.3×10^-2erg cm^-2for intrinsic surface anisotropy constant is observed as per the Raikher model. The higher melting point of water leaves the magnetic particles with a more disordered distribution of anisotropy axes of particles even in FC measurements. The angular variation of resonance field differentiates the magnetic behavior of system in 4-40 K (region I), 70-200 K (region II) and 200-260 K (region III). The value of effective magnetic anisotropy constant varied from 4.7×10⁴, 2.1×10⁴to 0 erg cm^-3through regions I, II to III. Linewidth analysis reveals that system undergoes spin-glass transition ~46 K. The fitting of linewidth data for region I and II indicate the presence of frozen and unfrozen surface spin states. Moreover, the role of applied magnetic field i.e. 1 T in field-cooled FMR spectra is reflected in interparticle distance parameter and magnitude of energy barriers related to the relaxation mechanisms. At 260 K fluid melts resulting in minimization of angular dependent anisotropy in resonance lines.