Superparamagnetic phases

At temperatures Tfield dependence of magnetization has the form of hysteresis loop, as it is shown in Fig. 2.38 of Chap. 2 for the 6 nm particles. Down to the lowest temperatures (around T = 5 K) all the results can be well described without consideration of the particles interaction. It should be emphasized, that superparamagnetic phase exists in the noninteracting nanoparticles ensemble, while their interaction could lead to spin glass state or ferromagnetic phase depending on the particles concentration. As the characteristic feature of hysteresis loop is presence of a remnant magnetization at // = 0, the authors [111] had considered its behavior with the help of the expression... 

K Curie-Weiss behavior between 230 and 95 K assumed to be due to superparamagnetic phase ... 

The temperature dependence of these interactions is of particular interest for the interpretation of Mossbauer data. In the case of superparamagnetic phases the grain sizes of nanoparticles can be obtained by measurement at liquid He temperature. 

The crystal stmcture of the intermediate is not well understood. The final iron phase is termed superparamagnetic because the particle size is too small to support ferromagnetic domains. At low rates, the discharge occurs in two steps separated by a small voltage difference. At high rates, however, the two steps become one, indicating that the first step is rate limiting, ie, the second step (eq. 34) occurs immediately after formation of the intermediate (eq. 33). 

AC susceptibility measurements are frequently used to identify thermodynamic phase transitions and to characterize spin-glass behavior and superparamagnets such as the single-molecule magnets discussed below. 

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