Magnetic shielding

Isolators themselves do not shield magnetic fields because they have no free electrons that can react to magnetic or electric fields. However, metals such as copper can actually attenuate alternating magnetic fields. The reason for this is Lenz's rule, according to which a time-varying magnetic field induces circulating currents that are opposite to their cause. Thus, an opposing field is generated which attenuates the external magnetic field. However, the magnetic shielding of normal metals works only at high frequencies, since the metal have a certain electrical resistance. At low frequencies or static magnetic fields, there is no sufficient driving force that can sustain the circulating currents.

Diamagnets have a negative susceptibility, $\chi < 0$ , and tend to displace magnetic field lines from within. Thus, a diamagnetic material can in principle attenuate magnetic fields in its interior, but the effect is very low for normal diamagnets such as graphite ( $\chi> -10^{-4}$ ). Exceptions are only superconductors at low temperatures, which show a perfect diamagnetism ( $\chi = - 1$ ).

Ferromagnets have a magnetic hysteresis and can be magnetized. This means that ferromagnets, even without external fields ( $H = 0$ ), can have a remanent magnetization $M_r$ which depend on their history. However, the hysteresis curve $M(H)$ of ferromagnets differs greatly depending on the material. Hard ferromagnets like neodymium magnets show an almost square hysteresis curve with a high remanence $M_r$ and high coercive field strength $H_c$ . The magnetization of such a substance is very resistant and changes only under the influence of extremely large external fields. Such materials are used as permanent magnets, for example in electric motors.

On the other hand, there are also soft ferromagnets, which show virtually no significant hysteresis. The magnetization curve shows a linear slope for small external fields, as in the case of paramagnets, but corresponds to an extremely high susceptibility $\chi = \frac{dM}{dH}$ . Certain nickel-iron alloys can achieve values of $\chi > 10^5$ . Thus, the material reacts extrem sensitively to external magnetic fields and amplifies the magnetic flux density many times over. As a consequence, magnetic field lines are concentrated inside a soft ferromagnetic core. Such a material is therefore suitable for the magnetic core of a transformer.

If you now make an entire box from a soft ferromagnet and place it in an outer magnetic field, the magnetic field lines run practically only over the outer edge of the box. The inside of the box is thus practically field-free.