General overview »
Magnetic Nanoparticles »
Standardization »
Characterization and
analysis methods »
DC magnetization and AC
susceptometer analysis »
Medium and high frequency
AC susceptometry »
Mössbauer spectroscopy »
Electron microscopy »
XRD and SAXS »
SANS »
Electron microscopy »
Ferromagnetic resonance »
Dynamic light scattering and
electrophoretic light scattering »
Field-flow fractionation »
Magnetic modelling »
Magnetorelaxometry »
Magnetic particle spectroscopy »
Magnetic particle rotation »
Magnetic separation »
NMR R1 and R2 relaxivities »
Magnetic nanoparticle bio-detection »
Magnetic hyperthermia measurements »
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Mössbauer spectroscopy
Mössbauer spectroscopy relies on the resonant absorption of g-radiation by 57Fe nuclei in the sample and provides a fingerprint of the electrostatic and magnetic environment of the nuclei. The method can be used to assess the oxidation state, the symmetry and spin state and the magnetic ordering of the Fe atoms in a nanoparticle sample and hence it can be used to identify the magnetic phases in a sample. Moreover, for magnetically ordered materials, Mössbauer spectra recorded vs. temperature can be used to estimate the magnetic anisotropy energy and to quantify the thermal unblocking (superparamagnetism).
The splitting of the Mössbauer spectra in a 6-line spectrum is proportional to the average magnetic field acting on the nucleus over a time on the order of a nanosecond. At very low temperature, the magnetic moments of nanoparticles are thermally blocked and fixed along so-called easy directions defined by the magnetic anisotropy and the particle volume. However, upon increasing temperature the moments fluctuate near the easy directions with a magnitude that is inversely proportional to the product of the anisotropy constant and the particle volume and directly proportional to the temperature. Hence, studies of the temperature dependence of the splitting of 6-line spectra can be used to find the anisotropy constant times the particle volume. Upon further increasing the temperature, the particle magnetic moments may be able to fluctuate between different easy directions (superparamagnetism) on a time scale comparable to 1 ns. In this case, the 6-line spectrum will collapse to a 2-line spectrum. The temperature dependence of the spectra can be used to determine the distribution of anisotropy energies for the particle ensemble (and to be compared with ac susceptibility and ZFC/FC measurements). Moreover, measurements performed as function of an applied magnetic field at a temperature where all particles show superparamagnetic behaviour can be used to determine the average magnetic moment of a magnetic nanoparticle ensemble. The technique can also provide unique information on spin structures and particle phase composition, e.g., distinguish Fe3O4 from g-Fe2O3, which is not accessible by other methods. Finally, the technique is also sensitive to magnetic interactions between nanoparticles, which may affect the dynamic magnetic behaviour.
In the nanoMag project, the Mössbauer technique will be used to characterise on the order of ten selected nanoparticle /nanobead samples with respect to a selection of the above parameters. The technique will be used to determine Fe-phases of the particles, anisotropy energies, superparamagnetic relaxation properties, size distributions and average magnetic moments of the particles.
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