Nanosystems and magnetism

Abstract

Magnetic Nanoparticles (MNPs) generally consist of two components a magnetic material, most often iron, nickel and cobalt, (ferromagnetic one) and the other a chemical component having wide functionality, reactivity and stability.1–5 The typical size of such nanoparticles lies between 1–100 nanometer and may display superparamagnetism.6,7 In Figure 1, we schematically show the multifunctional character of various nanoparticles. In a common paramagnentic material, spins are not subjected to any exchange interaction and they do not show any hysterisis or domain like a ferromagnet. In the presence of an external magnetic field, the spins tend to align to it generating a weak attractive interaction. However, in a superparamagnetic material, spins are substituted by small ferromagnetic domains characterized by positive exchange interaction. In the presence of an external magnetic field, these domains tend to align to it generating a strong attractive interaction. Thus, superparamagnetism is another characteristic form of magnetism that does appear in small ferromagnetic or ferrimagnetic nanoparticles. Besides their magnetic response is significantly higher than paramagnetism. Moreover, magnetization in such smaller sized nanopartciles can randomly ip direction under the influence of temperature. Another significant characteristic feature is that it occurs below the Curie temperature of the material. Note that generally any ferromagnet or ferrimagnet material transforms to a paramagnet only above the unique Curie temperature dependent on the strength of exchange interaction and the underlying lattice structure. This particular magnetism occurs in those nanoparticles composed of single domain. Further due to the magnetic anisotropy of the nanoparticles, the relevant magnetic moment possesses two stable orientations antiparallel to each other separated by an energy barrier (KV). The competition between this energy barrier and thermal energy (KV~25kBT ) gives rise to a characteristic relaxation time ( T=T0 exp(KV=kBT )) in this nanometerial. The exchange bias between ferromagnetic/ferrmagnetic and antiferromagnetic interface is the key parameter in controlling the magnetization and other related phenomena in these systems.8–10 In fact, the particles can invert their magnetization by tunneling without the help of thermal energy. Under the application of an external magnetic field, these materials develop magnetization and as a function of the external field, the magnetization looks like reversible S-shaped increasing curve (L(x) = coth(x) -1/x ). The AC susceptibility measurements of these nanoparticles can identify the various time scales and frequency dependent susceptibility. Discussion