228 12.Magnetic Properties of Materials FIGURE 12.3.Induction of a current in a loop-shaped piece of wire by moving a bar magnet toward the wire loop.The current in the loop causes a magnetic field that is directed opposite to the magnetic field of the bar magnet (Lenz law). The current thus induced causes,in turn,a magnetic moment that is opposite to the one of the bar magnet(Figure 12.3).(This has to be so in order for mechanical work to be expended in pro- ducing the current,i.e.,to conserve energy;otherwise,a perpet- ual motion would be created!)Diamagnetism may then be ex- plained by postulating that the external magnetic field induces a change in the magnitude of the atomic currents,i.e.,the external field accelerates or decelerates the orbiting electrons,so that their magnetic moment is in the opposite direction to the external mag- netic field.In other words,the responses of the orbiting electrons counteract the external field [Figure 12.2(c)]. Superconductors have extraordinary diamagnetic properties They completely expel the magnetic flux lines from their interior when in the superconducting state (Meissner effect).In other words,a superconductor behaves in a magnetic field as if B would be zero inside the material [Figure 12.2(d)].Thus,with Eq.(12.5) one obtains: H=-M, (12.9) which means that the magnetization is equal and opposite to the external magnetic field strength.The result is a perfect diamag- net.The susceptibility, 也 (12.6) H in superconductors is therefore-1 compared to about-10-5 in the normal state (see Table 12.1).This strong diamagnetism can be used for frictionless bearings,that is,for support of loads by a repelling magnetic force.The often-demonstrated levitation effect in which a magnet hovers above a superconducting material also can be ex- plained by these strong diamagnetic properties of superconductors.The current thus induced causes, in turn, a magnetic moment that is opposite to the one of the bar magnet (Figure 12.3). (This has to be so in order for mechanical work to be expended in pro￾ducing the current, i.e., to conserve energy; otherwise, a perpet￾ual motion would be created!) Diamagnetism may then be ex￾plained by postulating that the external magnetic field induces a change in the magnitude of the atomic currents, i.e., the external field accelerates or decelerates the orbiting electrons, so that their magnetic moment is in the opposite direction to the external mag￾netic field. In other words, the responses of the orbiting electrons counteract the external field [Figure 12.2(c)]. Superconductors have extraordinary diamagnetic properties. They completely expel the magnetic flux lines from their interior when in the superconducting state (Meissner effect). In other words, a superconductor behaves in a magnetic field as if B would be zero inside the material [Figure 12.2(d)]. Thus, with Eq. (12.5) one obtains: H
M, (12.9) which means that the magnetization is equal and opposite to the external magnetic field strength. The result is a perfect diamag￾net. The susceptibility, $
M H , (12.6) in superconductors is therefore 1 compared to about 105 in the normal state (see Table 12.1). This strong diamagnetism can be used for frictionless bearings, that is, for support of loads by a repelling magnetic force. The often-demonstrated levitation effect in which a magnet hovers above a superconducting material also can be ex￾plained by these strong diamagnetic properties of superconductors. FIGURE 12.3. Induction of a current in a loop-shaped piece of wire by moving a bar magnet toward the wire loop. The current in the loop causes a magnetic field that is directed opposite to the magnetic field of the bar magnet (Lenz law). 228 12 • Magnetic Properties of Materials m i N S