A Survey of Magnetic Materials in Relation to Structure

A G N E T I C materials may be classified according to properties into two groups: (1) soft magnetic materials, and (2) hard or permanent magnet materials. The differences in properties of the two groups are illustrated in Fig. 1, which shows typical magnetization cycles for the two types. The dashed line is a normal magnetization curve while the closed full line is known as the hysteresis loop. A soft magnetic material is, in general, characterized by a steeply ascending magnetization curve; that is, large values of flux density are produced by small magnetizing forces. For certain applications where the flux density is low, the initial portion of the curve is important. For intermediate flux density applications the steeply ascending portion is of paramount interest while for higher densities, the upper portion is of prime importance. Another term which is widely used in discussing soft magnetic materials is permeability. The permeability at a given flux density is the slope of the straight line joining that point on the magnetization curve with the origin. Algebraically, it is the ratio of the flux density, B, to the magnetizing force, H. It varies with the flux density, and for soft magnetic materials at low magnetizing forces, is a large quantity. The permeabilities usually reported in discussing magnetic properties are the initial permeability, mo, and the maximum permeability, /j m3Ii . The initial permeability is the initial slope of the magnetization curve. The * Published in Metals and Alloys, first part in December 1934 issue, second and concluding part in J a n u a r y 1935 issue.