Superparamagnetic behaviour

Since the end-members of the phase are Fe 03 and Fe Ti " 03, the intermediate compositions must contain mixed valence states of iron. As x decreases from 1-0 to about 0-5, the basic six-line spectrum of a-Fc203 is retained, although it is noticeably broadened by the proliferation of site symmetries. At x =0 -33 only a very broad spectrum envelope is seen, and this has been claimed to indicate the presence of ferrimagnetic clusters and general superparamagnetic behaviour [105]. 

The characteristic measurement time (t ,) associated to a particular technique will determine whether or not the superparamagnetic behaviour of a sample will be observed during an experiment. In the case of AC magnetometry t , = lO -lO s for low frequency experiments and T = 10 -10 s for classical experiments. For a given system of nanoparticles, a superparamagnetic behaviour will be observed if t ,t. 

PAPERXTITLE>Elaboration of PLLA-based superparamagnetic nanoparticles Characterization, magnetic behaviour study and in vitro relaxivity evaluation. [175]... 

Superparamagnetism is a specific type of magnetic behaviour observed with fine magnetically ordered particles dispersed in a diamagnetic matrix [1], Understanding of this phenomenon has been a very important contribution to fundamental theories of magnetism and laid the foundation... 

Superparamagnetism leads to the collapse of the sextet to give a quadrupole doublet at a temperature referred to as the blocking temperature, Tg, which is below the magnetic-ordering temperature. For particles of a given structure, Tg often varies linearly with particle volume and this behaviour can be used to characterise small particles in catalyst systems. 

The typically very small crystals of the different components in the aerosol sample give rise to the so-called supeiparamagnetic (spin) behaviour (cf, Bronger et al. [32] and references therein) resulting in the pronounced resonance doublet for Fe " compounds in the room temperature spectrum. The low temperature spectrum (8 K) revealed that this doublet component mainly originates in superparamagnetic microcrytals of goe-thite and hematite. 

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