Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 149

For most atoms (or ions), the magnetic effects of the electrons, including their spin and orbital motions, cancel out and, consequently, the atom is not magnetic. This is true for atoms of the inert gasses and atoms of elements such as zinc that have completely filled electron shells. The Pauli exclusion principle requires that two electrons in any energy level of an atom must have opposite spins and, thus, the resultant spin magnetic moment of paired electrons is zero. Materials composed of atoms in which all the electrons are paired have no intrinsic magnetic moment and are described as diamagnetic. A weak magnetic dipole may be induced in these materials when they are placed in a magnetic field but the induced dipole points in a direction opposite to that of the field.

On the other hand, for other atoms or ions such as those of some transition elements, the magnetic effects of the electrons do not cancel, since one or more unpaired electrons are present, and the atom as a whole has a magnetic moment. In the absence of an applied magnetic field, the orientations of the atomic dipoles are random and, thus, the material as a whole has no net magnetic moment. However, when placed in an applied magnetic field, the atomic dipoles line up in the direction of the applied field. This type of magnetic behavior is referred to as paramagnetism.

Diamagnetic materials have χ_m values in the range approximately −10⁻⁶ to −10⁻³, whereas paramagnetic materials have χ_m values in the range ~10⁻⁶ to 10⁻¹. The susceptibility of diamagnetic materials is negative whereas it is positive for paramagnetic materials. This is because in an applied magnetic field, the induced atomic dipoles in a diamagnetic material point in a direction opposite to the magnetic field, making the field within the material lower than that of a vacuum. On the other hand, the atomic dipoles present in a paramagnetic material line up in the direction of the applied field, in much the same way as a compass needle lines up in the earth’s magnetic field. This has the effect of increasing the magnetization of the material.