Super paramagnetism

Properties of FeCr,o solid samples have been studied by X-ray diffraction, 57Fe Mossbauer spectroscopy and magnetic measurements to stimulate the interaction of Fe with fullerene. FeCr,o samples have been prepared by decomposition of the 1,3-dipolar cycloadduct of the fullerene and ferrocene nitrile oxide. The components exhibit super paramagnetic properties originating from an interaction between FeCr,o complexes within the nano-particles. Each nano-particle consists of hundreds to thousands complexes (546). 

The magnetic moments of Fe, Co, and Ni clusters with sizes up to 700 atoms have been measured by Billas et al.1,2 Those measurements were made under conditions where the clusters exhibit super-paramagnetic behavior for low cluster temperatures (vibrational temperature Tv b = 78 K for Ni and Co clusters and 120 K for Fe clusters). Their results are shown in Figure 1. As... 

It is important to note that in addition to microporous solids, other chemical systems have been used to template the growth of nanomaterials. For example, emulsions have been used to pattern both the pores in titania [14] and the packing of latex particles [46]. Reversed micelles have also been used as patterning agents. Examples include the syntheses of super-paramagnetic ferrite nanoparticles [15] and BaC03 nanowires [47]. Finally, carbon nanotubules have also been used as templates [16,48,49]. A variety of nanomaterials including metal oxides [16,48,49] and GaN have been synthesized inside such tubules [50]. 

G. Application of the relaxometric study of super-paramagnetic particles II. Ferritin, an iron-storage protein... 

Super-paramagnetic (spm) crystals are used as contrast agents for Magnetic Resonance Imaging (MRI). Improving their efficiency requires a... 

Fig. 1. Liver MR images before and after administration of a super-paramagnetic contrast agent.

Ferrofluid NMR studies can also be used in order to determine geometrical and physical properties of the super-paramagnetic crystals, like their specific magnetization or radius. They also give valuable information on the aggregation level and on anisotropy. 

As illustrated in Fig. 3, the proton relaxation in super-paramagnetic colloids occurs because of the fluctuations of the dipolar magnetic coupling between the nanocrystal magnetization and the proton spin. The relaxation rate increases with the fluctuation correlation time and with the magnitude of this fluctuation. Different processes cause the fluctuation of the magnetic interaction. 

Fig. 3. Illustration of the origin of proton nuclear magnetic relaxation induced by a super-paramagnetic crystal. The water molecule (symbolized by a bee) experiences a magnetic field which fluctuates because of the translational diffusion and because of Neel relaxation. The bottom curve represents a typical time evolution of this field.

Relaxation induced by super-paramagnetic crystals is moreover complicated by another feature the influence of the electron magnetic moment is modulated by Neel relaxation, which depends on the crystal anisotropy (see Fig. 4). 

For large super-paramagnetic crystals or for crystals with a very high anisotropy constant (12), the anisotropy energy is larger than the thermal energy, which maintains the direction of the crystal magnetic moment very... 

When the anisotropy energy is large enough it prevents any precession of the magnetic moment of super-paramagnetic crystals. The magnetic fluctuations then arise from the jumps of the moment between different easy directions. The precession prohibition is introduced into the Freed equations in order to meet that requirement every time the electron Larmor precession frequency appears in the equations, it is set to zero 12). 

Exchange energy within super-paramagnetic crystals is sufficiently large to deal with the ensemble of electronic spins as with one large superspin S,... 

G. Application OF THE Relaxometric Study of Super-Paramagnetic Particles... 

The first step in the characterization of a new super-paramagnetic colloid is obviously the evaluation of its relaxometric properties, which determine its potential efficiency for MRI (27,28). Relating these valuable relaxometric data to morphological and physical properties of the particles may be carried out thanks to a proton relaxivity theory. 

These recent results are thus likely to have put an end to a previously controversial question ferritin is antiferromagnetic at room temperature (because T < T ) and the size of the ferrihydrite grains is small enough to observe super-paramagnetism with a Neel relaxation time of about 0.1 ns. 

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