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Ferromagnetism A small number of crystalline substances exhibit strong magnetic effects called All ferromagnetic materials are made up of microscopic regions called domains, regions within which all magnetic moments are aligned. These domains have volumes ' 12 to 10 ' 8 m 3 and contain 10 17 to 10 21 atoms. The boundaries between the various domains having different orientations are called domain walls. In an unmagnetized sample, the magnetic moments in the domains are randomly oriented B 0 , the size of those domains with magnetic moments aligned with the field grows, which results in a magnetized sample, as in A typical experimental arrangement that is used to measure the magnetic proper- ties of a ferromagnetic material consists of a torus made of the material wound with N B in the torus is measured by increasing the current in the toroid from zero to I. As the B corresponding to any value of the current in the primary coil can be obtained. The magnetic field B is measured first in the absence of the torus and then with the torus in place. The magnetic properties of the torus material are then obtained from a Now consider a torus made of unmagnetized iron. If the current in the primary coil is increased from zero to some value I, the magnitude of the magnetic field SECTION 3 0.8 • Magnetism in Matter 949 R G S ε (c) B 0 (b) B 0 (a) Figure 30.29 (a) Random orienta- tion of atomic magnetic dipoles in the domains of an unmagnetized substance. (b) When an external field B 0 is applied, the domains with components of magnetic moment in the same direction as B 0 grow larger, giving the sample a net magnetization. (c) As the field is made even stronger, the domains with magnetic moment vectors not aligned with the external field become very small. Figure 30.30 A toroidal winding arrangement used to measure the magnetic properties of a material. The torus is made of the material under study, and the circuit containing the galvanometer measures the magnetic flux. |