Magnetic permalloy

Soft magnetic materials are characterized by high permeabiUty and low coercivity. There are sis principal groups of commercially important soft magnetic materials iron and low carbon steels, iron—siUcon alloys, iron—aluminum and iron—aluminum—silicon alloys, nickel—iron alloys, iron-cobalt alloys, and ferrites. In addition, iron-boron-based amorphous soft magnetic alloys are commercially available. Some have properties similar to the best grades of the permalloys whereas others exhibit core losses substantially below those of the oriented siUcon steels. Table 1 summarizes the properties of some of these materials. 

Another area with a large research activity is also related to computer technology. It is electrodeposition of magnetic alloys for thin-film recording heads and magnetic storage media. Here new magnetic materials are needed that have properties superior to those of electrodeposited NiEe (Permalloy). These activities are reviewed by Andricacos and Romankiw (25) and Romankiw (32). 

Magnetically soft Fe-Ni alloys can have their properties altered by heat treatment. The compound NisFe undergoes an order-disorder transformation at about 500°C. Since the susceptibility of the ordered phase is only about half that of the disordered phase, a higher susceptibility is realized when the alloy is quenched from 600°C, a process that retains the high-temperature, disordered structure. Heat treatment of Fe-Ni alloys in a magnetic field further enhances their magnetic characteristics (see Figure 6.61), and the square hysteresis loop of 65 Permalloy so processed is desirable in many applications. A related alloy called Supermalloy (see Table 6.19) can have an initial susceptibility of approximately one million. 

Figure 6.61 Hysteresis loop for Permalloy 65 heat treated with and without an applied magnetic field. From K. M. Ralls, T. H. Courtney, and J. Wuhf, Introduction to Materials Science and Engineering. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission John Wiley Sons, Inc.

A square-loop hysteresis curve for permalloy can be obtained by heat treating in a magnetic field. 

Wirth, S. von Molnar, S. Magnetic interactions in nanometer-scale particle arrays grown onto permalloy films. J. Appl. Phys. 87, 7010-7012 (2000). 

R. P. Cowbum, A. O. Adeyeye, and J. A. C. Bland, Magnetic Domain Formation in Lithographically Defined Antidot Permalloy Arrays , Appl. Phys. Lett. 70, 2309 (1997). 

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