Structural, static and dynamic magnetic properties of dextran coated γ-Fe₂O₃nanoparticles studied by⁵⁷Fe NMR, Mössbauer, TEM and magnetization measurements

Abstract

The structural and magnetic properties and spin dynamics of dextran coated and uncoated γ-Fe(2)O(3) (maghemite) nanoparticles have been investigated using high resolution transmission electron microscopy (HRTEM), (57)Fe nuclear magnetic resonance (NMR), Mössbauer spectroscopy and dc magnetization measurements. The HRTEM observations indicated a well-crystallized system of ellipsoid-shaped nanoparticles, with an average size of 10 nm. The combined Mössbauer and magnetic study suggested the existence of significant interparticle interactions not only in the uncoated but also in the dextran coated nanoparticle assemblies. The zero-field NMR spectra of the nanoparticles at low temperatures are very similar to those of the bulk material, indicating the same hyperfine field values at saturation in accord with the performed Mössbauer measurements. The T(2) NMR spin-spin relaxation time of the nanoparticles has also been measured as a function of temperature and found to be two orders of magnitude shorter than that of the bulk material. It is shown that the thermal fluctuations in the longitudinal magnetization of the nanoparticles in the low temperature limit may account for the shortening and the temperature dependence of the T(2) relaxation time. Thus, the low temperature NMR results are in accord with the mechanism of collective magnetic excitations, due to the precession of the magnetization around the easy direction of the magnetization at an energy minimum, a mechanism originally proposed to interpret Mössbauer experiments in magnetic nanoparticles. The effect of the surface spins on the NMR relaxation mechanisms is also discussed.

Magnetic studies of mesoporous nanostructured iron oxide materials synthesized by one-step soft-templating

“…This reduction of the saturation magnetization has been attributed a variety of mechanisms: 35 studied two samples of monodisperse iron oxide nanoparticles (7 nm and 9.6 nm) and suggest that 57 Fe is a useful tool for distinguishing between iron oxide phases. M. Fardis, et al 30 have studied the interparticle interactions for well-crystallized 10 nm γ- Probably the most significant result from the NMR measurements in this work is the identification of the crystallographically similar γ-Fe 2 O 3 and Fe 3 O 4 spinel phases in the mesoporous nanostructured materials. It is difficult to distinguish between these two phases with XRD, particularly in the case of nanoparticles where the peaks are broadened.…”

Synthesis and properties of hybrid hydroxyapatite–ferrite (Fe3O4) particles for hyperthermia applications

“…The maximum magnetization obtained at 5 K in the strongest external field of 50 kOe is equal to 17 emu/g. This value is approximately five times less than the saturation magnetization of the bulk maghemite sample (80 emu/g) [42]. This decrease in the saturation magnetization is explained by different factors, including the presence of a nonmagnetic surface layer, noncollinearity of the spin structure on the surface, or even by the spin glass state of the surface (see, for example, [42] and references therein).…”

“…This value is approximately five times less than the saturation magnetization of the bulk maghemite sample (80 emu/g) [42]. This decrease in the saturation magnetization is explained by different factors, including the presence of a nonmagnetic surface layer, noncollinearity of the spin structure on the surface, or even by the spin glass state of the surface (see, for example, [42] and references therein). It can be assumed that the small saturation magnetization in our case is most likely associated with significantly smaller sizes of Fe 3 O 4 nanoparticles, noncollinearity of magnetic moments in the surface layer, and the presence of surfactants on the surface.…”

Characterization techniques for nanoparticles: comparison and complementarity upon studying nanoparticle properties

“…10 nm nanosized samples at low T displayed similar NMR spectra, and thus similar hyperfine fields to the bulk material, implying that the samples had the same magnetic structure. 121 Gossuin et al characterized gadolinium hydroxide and dysprosium oxide NPs using XRD, magnetometry and NMR relaxometry. Nuclear magnetic relaxation dispersion profile represented the evolution of the longitudinal relaxation rate with respect to the magnetic field and provided interesting information about the longitudinal relaxation mechanism.…”