Below a certain critical size, particles of ferromagnetically or ferrimagnetically ordered phases are formed by a single magnetic domain, and at a certain finite temperature, the so-called superparamagnetic behaviour is observed. In this regime, the thermal activation overcomes the anisotropy energy barrier and fluctuations of particle magnetic moment occur, which is termed as the Néel relaxation. If magnetic particles are not immobilized (e.g. particles in a solvent forming a colloidally stable suspension), the relaxation of magnetic moments occurs also by rotational movements of whole particles, i.e. by the Brown relaxation. In an AC magnetic field, the magnetization dynamics is affected by both Néel and Brown relaxation processes and the trajectory of magnetic moments becomes complicated.
The introduction above brings us to novel applications of magnetic nanoparticles, such as magnetic particle imaging (MPI), where the non-linear response of superparamagnetic nanoparticles to a sinusoidal AC magnetic field, leading to higher harmonics in the measured signal, is employed for their visualization. In principle, a similar approach could be used for reading of neural activity (dynamical electric fields) deep in the brain by using magneto-electric nanoparticles, in which magnetostrictive cores and ferroelectric shells are combined to achieve strong magneto-electric coupling at room temperature. Our recent experimental effort has been focused also on the application of superparamagnetic nanoparticles as cores of complex particles with gold nanoshells, whose surface functionalization by suitable organic molecules enables sensing of pH or selected molecular species based on chemical equilibria and surface-enhanced Raman spectroscopy. The last point is finally leading to world of molecules.
We will mention some analogies between magnetic nanoparticles and molecules of certain coordination compounds, namely single-molecule magnets, which exhibit hysteresis below blocking temperature similarly to magnetic naoparticles. The talk will briefly describe our recent work on magnetically interesting molecular systems and will critically consider its future outlook.
