Superparamagnetic relaxation phenomena

Morup, S. "Paramagnetic and Superparamagnetic Relaxation Phenomena studied by Mossbauer Spectroscopy", Polyteknisk Forlag, Lyngby 1981. 

Meisel W (1994) Hyperfine Interact 92 1213 Mitra S (1992) Applied Mossbauer spectroscopy, physics and chemistry of earth, Vol 18. Pergamon, Oxford, New York, Seoul, Tokyo Moon PB (1950) Ptoc Phys Soc 63 1189 Morup S (1981) Paramagnetic and superparamagnetic relaxation phenomena studied by Mossbauer spectroscopy. Polyteknisk Forlag, Lyngby... 

A first illustration is given by the efficient coating of bisphosphonates onto iron oxide nanoparticles as ferrofluids. From transmission electron microscopy, the core size of as-prepared nanoparticles is estimated at about 5 nm while the hyperfine structure is consistent with the presence of superparamagnetic relaxation phenomena, that to confirm the size and the non aggregated assembly of nanoparticles. As is illustrated in Fig. 4.11, the comparison of the 77 K hyperfine structures... 

In the second regime, where typically k T/KV OA, fluctuations of the magnetic spin direction through 180°, as shown by the dashed arrows of Figure 4.27, become predominant, giving the phenomenon of superparamagnetic relaxation. The fluctuation time t for these spin reversals is given by... 

The influence of the superparamagnetic phenomenon has been clearly evidenced for Ni particles embedded in an SiO matrix. " Here, the conduction occurs via tunneling electrons. Without field, below T, the magnetic moments m of the particles are bloeked at random due to the disordered arrangement of particles, and above T, m relaxes. Then the correlations between spins are weak. Under field, the correlations increase and are maximum near Tg (Fig. F.9.2). For these materials, the magnetoresistance at few thousand Oersted is of the order of 1% and shows a tendency to saturate. 

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