Читать книгу Magnetic Nanoparticles in Human Health and Medicine - Группа авторов - Страница 24

Bulk ferro‐ and ferrimagnetic materials are generally characterized by the presence of magnetic hysteresis, more or less wide, when they are magnetized in an external magnetic field. Hysteresis can take different forms specific to soft or hard magnetic materials. They have specific saturation magnetizations and coercive fields that are known.

However, in the case of magnetic nanoparticles, they change significantly, even if the nature of the magnetic material is the same, as an effect of the small size of the nanoparticles.

The main basic characteristics of magnetic nanoparticles, in the dimensional range considered relevant for them, respectively 1–100 nm, are the following:

1 At sizes larger than the critical diameter (Dc), the nanoparticles have (i1) a structure of magnetic domains, similar to bulk magnetic material, but having a small number of domains; (i2) surface effects are not very pronounced; (i3) the behavior in the external field is with hysteresis, more or less pronounced, depending on the nature of the magnetic material (soft or hard).

2 At very small sizes, below the threshold diameter (Dth) (generally below 10 nm), depending on the nature of the material, magnetic nanoparticles behave radically differently, (ii1) lacking the structure of magnetic domains, having only one domain spontaneously magnetized to saturation; (ii2) the surface effects are strong, leading to a pronounced decrease in the saturation magnetization of the nanoparticles in ordinary fields; (ii3) the effect of superparamagnetism is manifested (lack of hysteresis), the nanoparticles being magnetized according to the Langevin function, like paramagnetic atoms.

Between the two states (i) and (ii), for D_th < D < D_c, the phenomenon of magnetic relaxation (Nèel relaxation) appears. Thus, for D closer to D_th, but with D > D_th, the magnetization of unidominal magnetic nanoparticles will present a deviation from the Langevin function, more or less pronounced, depending on the nature of the nanoparticle material (reflected in the magnetic anisotropy), the size (volume) of the nanoparticle and the temperature at which it is found, or even the appearance of a small, narrow hysteresis, at larger sizes in the considered interval. In the case of magnetic dispersions for biomedical and technical applications, there will be in addition a Brown time relaxation, processes characterized quantitatively by a resulting relaxation time (Nèel–Brown). For D closer to D_c, but with D < D_c, the magnetization of single‐domain nanoparticle is stable, and nanoparticle is magnetized according to the Stoner–Wohlfarth model: with rectangular hysteresis when the magnetization is done along the direction of easy magnetization of the nanoparticle, and linear until magnetic saturation when the magnetization of the nanoparticle is done in a direction perpendicular to that of easy magnetization.

In dynamic conditions, in harmonic alternating magnetic field, the magnetic relaxation process must be viewed in relation to the measurement time, in which the process is observed (quantitatively, the relaxation time related to the measurement time). The situation changes radically depending on the ratio in which the two times are found: the magnetic relaxation time and the measurement time. Thus, for τ ≪ t_m, there will be a pure magnetic relaxation process, and for τ ≫ t_m, there will be a blocking of the magnetic moments of the nanop. Between the two cases, there will be a magnetic drag.

In the alternating magnetic field (harmonic), magnetic relaxation processes lead to a heating of nanoparticles, an effect with applications in biomedicine such as in the alternative therapy of tumors by magnetic hyperthermia. Also, the rapid response of nanoparticles to the application of an external magnetic field makes them easy to manipulate, with application in target medication and drug delivery. In addition, in biomedicine, the magnetic nanoparticles are also used as contrast agents in magnetic resonance imaging (MRI) based on two essential characteristics of nanoparticles: their magnetism and small size.