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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DC magnetization and AC susceptometer analysis
DC analysis
Magnetization isotherms can be used to extract information relating to the particle size distribution, magnetic anisotropy and particle magnetic moments for non-interacting particle systems. Field dependent magnetization curves M(H,T) are recorded at temperatures above the zero-field blocking temperature where the magnetization is reversible. If the thermal energy is large enough in comparison to the anisotropy energy, the analysis is based on the Langevin function and yields information on the particle size distribution. At low temperature, but still at temperatures above the zero-field blocking temperature, magnetic anisotropy must be included in the analysis and the analysis thus yields information relating both to the particle size distribution and the magnetic anisotropy. At even lower temperature, below the zero-field blocking temperature, the magnetization curves will reveal hysteresis loops.
In some magnetic nanoparticle systems the hysteresis loops are displaced by an exchange bias field. The exchange bias effect for iron-oxide nanoparticles stems from magnetic disorder associated with defective crystal structures, appearing either as a crystallographically defective core volume or as a defective surface layer. In either case, magnetic disorder within the nanoparticles results in smaller particle magnetic moments and will thus have a negative impact on any application using the nanoparticles. In some cases, it is even possible to obtain evidence for a second phase by measuring the temperature dependence of the high field magnetization, in particular in cases where the secondary phase exhibits antiferromagnetic spin order (e.g. wüstite) with a Néel temperature below room temperature.
AC analysis
Results from AC-susceptibility and ZFC magnetic relaxation measurements can be used to determine the distribution of energy barriers in samples consisting of nano-sized single-domain magnetic particles where the interaction between particles is negligible. From the full analysis, values of the Arrhenius law pre-factor in the expression for the Néel relaxation time, magnetic anisotropy constant and particle magnetic moments may also be derived. The AC-susceptibility measurements are performed at constant frequency f of the AC magnetic field as a function of temperature and are repeated for different frequencies of the magnetic field.
The ZFC relaxation measurements are performed at constant temperature as a function of observation time t and are repeated for different temperatures. By using either the out-of-phase component of the AC-susceptibility χ''(f,T) or the relaxation rate S(t,T) of the ZFC magnetization MZFC(t,T), defined as S = 1/H δMZFC/δlnt, the desired properties of the magnetic nanoparticle system can be extracted (t is the time, T the temperature, H the magnetic field and f is the frequency). Moreover, in AC-susceptibility measurements, by including also the in-phase component of the AC-susceptibility χ'(f,T) in the analysis it is possible to distinguish between inter-potential- and intra-potential-well contributions to the magnetic response.
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