# Magnetic Properties of Nanocomposites
## 2 A Survey of Magnetic Measurements and Typical Magnetic Parameters of Nanocomposite Systems  
### Magnetic Measurements in Nanocomposite Materials  
Typical magnetic nanocomposites of relevance for applications exhibit magnetization values ranging between 1 and 10 emu/g and coercive fields ranging from about zero to some hundreds of Oe. The low magnetization values are explained considering that the nanoparticulate magnetic fraction is typically much smaller than the diamagnetic host fraction. The above reported values imply that small or thin samples (exhibiting, for example, magnetic moments of much less than \(10^{-2}\) emu) must be measured using high-sensitivity magnetometers; in this case, extraction methods are not advisable, whereas vibrating sample magnetometers (maximum sensitivity around \(10^{-6}\) emu (\(10^{-9}\) Am\({}^{2}\))) or Superconducting Quantum Interference Device (SQUID) magnetometers (maximum sensitivity around \(10^{-8}\) emu (\(10^{-11}\) Am\({}^{2}\))) offer the best performances and provide results virtually free from experimental uncertainties. Static magnetic measurements give information on the behavior of major hysteresis loops as functions of temperature (whence typical hysteretic parameters such as coercive field and magnetic remanence are easily obtained) and on magnetic susceptibility (both initial and maximum). A central role in static magnetic measurements is played by field-cooled (FC) and zero-field-cooled (ZFC) magnetization curves because of the information content about nanoparticle size, size distribution, effective anisotropy, and interactions that can be attained by careful analysis of the shape of these curves [15]. The FC/ZFC protocol involves, in principle, measuring the initial susceptibility or the low-field magnetization of a sample after demagnetization at a temperature higher than the blocking temperature (where all particle moments are randomly oriented), and subsequent cooling without applied field down to the lowest temperature (typically, a few K); a small field is then applied to measure the sample's response during the temperature rise up to the starting temperature. The complementary FC curve is obtained by repeating the same process after cooling the sample under a small applied field. FC/ZFC curves are particularly interesting because it is easy to estimate useful magnetic properties (e.g., the blocking temperature, the effective anisotropy, and--to a lesser extent--the strength of interaction between particles) of a nanoparticle system just by looking at the graphs of temperature dependence of low-field magnetization. In ZFC conditions at low temperature, the magnetization of each particle is aligned along a direction that minimizes the total energy given the anisotropy and the applied field. As the temperature increases, thermal fluctuations allow the magnetization to rotate towards the applied field direction, thus resulting in an increase of the net magnetization up to a maximum value reached at temperature \(T_{max}\). As the temperature is further increased, thermal agitation starts to compete with the aligning effect of the applied field; as a consequence, the net magnetization starts reducing. The competition between the aligning field and the disordering effects of temperature determines, on the time scale of experiment, the _blocking temperature_\(T_{B}\), which is closely related to \(T_{max}\) (and often taken equal to \(T_{max}\)). In the FC condition, the magnetic moments are again blocked at a low temperature, but now they are no longer randomly oriented in space, being partially aligned along the field direction. As in the case of the ZFC curve, the magnetization decreases as a result of thermal activation. The FC and the ZFC curves merge at the _irreversibility temperature_\(T_{irr}\), that is, when all the magnetic moments have become unblocked. Above \(T_{irr}\), the _superparamagnetic regime_ is achieved (see Section 2.2) and the two merged curves reach the temperature at which the experiment has begun.