Cullity B.D. Introduction to Magnetic Materials. Second Edition (2008)

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Fig. 14.13 Demagnetization curves of typical rare-earth permanent magnets.

14.7EXCHANGE-SPRING MAGNETS

It was suggested in 1991 by E. Kneller and R. Hawig [IEEE Trans. Mag., 27 (1991) p. 3588] that, if the spaces between the high-coercivity particles in a fine-particle permanent magnet deriving its coercivity from crystal anisotropy could be filled with a highmagnetization, low-coercivity material such as iron or a cobalt-iron alloy, and if the high-moment alloy were exchanged-coupled to the permanent magnet particles, the result should be a material with a significantly higher net magnetization but the same coercive field, and therefore a higher energy product, than the original fine-particle material. One condition would be that the soft magnetic layers would have to be thin enough so that no domain walls could form. The original paper called this an exchange-spring magnet; the term exchange-coupled magnet has also been used. There has been considerable interest in this idea, but no commercial products have yet resulted.

14.8NITRIDE MAGNETS

In 1991, J. M. D. Cooey and H. Sun [J. Mag. Magn., Mater., 87 (1991) p. L251] reported that nitrogen can be added to Sm2Co17, causing an expansion of the lattice, an increase in magnetization and anisotropy, and also a higher Curie temperature. There is a stable compound corresponding to the composition Sm2Co17N3. The improvement in properties is substantial, and a burst of research activity, both basic and applied, resulted. However, no commercial magnets have yet appeared. One of the problems is that the materials are not stable at conventional sintering temperatures.

14.9DUCTILE PERMANENT MAGNETS

Most permanent-magnet materials are so extremely brittle that they can be put into usable form only by casting or by pressing and sintering a powder. But there are a few exceptional alloys which are magnetically hard and yet ductile enough to be hot or cold worked into

wire, sheet, and other forms. Of these, Cunife (60% Cu, 20% Ni, 20% Fe) and Remalloy or Comol (68% Fe, 17% Mo, 12% Co) are no longer made.

Vicalloy II (52% Co, 38% Fe, 10% V), when rapidly cooled from 11808C, is ductile. After producing the desired shape, it is given a precipitation heat treatment of several hours at 6008C, after which it is hard and brittle, with a maximum energy product of about 1.5 MGOe. Another version of Vicalloy, with more vanadium and requiring severe deformation to develop its magnetic properties, was used for audio recording in the early days of that industry.

Chromindur (61.5% Fe, 28% Cr, 10.5% Co) has properties similar to Vicalloy, but contains less cobalt. Its precipitate forms by spinodal decomposition, which requires a slow, carefully controlled cooling from 6808C.

Arnokrome (26–30% Cr, 7–10% Co, balance Fe) is a proprietary alloy whose permanent magnet properties can be controlled over a fairly wide range by varying the heat treatment.

The maximum energy product of these alloys is a few MGOe, much inferior to other permanent magnet materials. A major use for materials such as these is for activating and deactivating magnetic sensors used as antishoplifting tags. The need is for a material that can be made in thin strip form, and can be magnetized and demagnetized in the fields available from permanent magnet assemblies.

14.9.1Cobalt Platinum

The equiatomic alloy CoPt is the most expensive magnetic material ever made commercially. Like the alloys listed above, when heat treated to produce a single-phase solid solution, it is ductile. It is then heat treated to develop a two-phase, partially ordered structure, which can have an energy product approaching 10 MGOe. Until the development of the rare-earth permanent magnets, CoPt was the best permanent magnet available, and was used in miniature devices such as hearing aids and watches, and in some military and space applications where cost was not a major consideration. It now has only a few special-purpose uses, some due to its good corrosion resistance.

14.10ARTIFICIAL SINGLE DOMAIN PARTICLE MAGNETS (LODEX)

Field-treated alnico with excellent properties had been made some ten years before publication of the classic paper of Stoner and Wohlfarth in 1948 on single-domain particles or the direct observation of domain walls and wall motion, i.e., years before any one had any clear idea of how the magnetization of a body actually changed. In fact, far from theory guiding practice, it was the other way around, because it is fair to say that the development of alnico stimulated theorists to search for an explanation of its high coercivity.

By about 1950, however, there was sufficient understanding of fine-particle theory to allow magneticians to proceed, for the first time, with the development of a synthetic magnet material. What was required was an assembly of elongated single-domain particles of, for example, iron, dispersed in a suitable matrix. The particles themselves could have negligible crystal anisotropy, because their shape alone would lead to high coercivity (Table 9.1). It is not easy to make such particles. The initial development work was done in France but did not lead to a commercially successful material, chiefly because the particles produced were not sufficiently elongated. Finally, in 1955, L. I. Mendelsohn, F. E. Luborsky, and T. O. Paine [J. Appl. Phys., 26 (1955) p. 1274] of the U.S. General

496 HARD MAGNETIC MATERIALS

Fig. 14.16 Effects of increased temperature on the induction at the operating point. (a) Reversible;

(b) irreversible.

The change in induction B during this reversible excursion is small, and the magnet is back at its original operating point. But if the true field changes from Hd to H2 and back, the operating point moves from a to c to d, causing a large permanent drop in induction. However, once the magnet is at d, temporary external fields in either direction merely move it reversibly back and forth along the recoil line through d. The resulting changes in B are small and temporary.

This suggests a method for stabilizing a magnet against the effects of external fields. After magnetization the magnet is subjected to a temporary demagnetizing field that moves its operating point from a to d. This treatment increases stability at the cost of decreased induction and is called “knockdown.” An ac field is usually preferred over dc for stabilization, because it will cycle the material many times over the recoil line.

A material with a demagnetization curve like that in Fig. 14.15b is stable against demagnetizing fields, because its recoil line has the same slope as the demagnetization curve. Permanent demagnetization can occur only if the negative field approaches the value of the intrinsic coercive field Hci, which is generally unlikely. The high-coercivity ferrite magnets and all the rare-earth magnets have linear demagnetizing curves like Fig. 14.15b.

14.12.2Temperature Changes

These can produce three effects. One is reversible and two are irreversible, in different ways. The property of interest is the flux Bd at the operating point, because this determines the flux in the air gap.

Reversible Changes. An increase in temperature will normally lower Bd, because both Ms and K generally decrease with temperature. If the value of Bd returns to its original value when the temperature returns to its original value, as in Fig. 14.16a, the change is by definition reversible. Manufacturers specify a maximum operating temperature for each of their permanent magnet materials; this is temperature below which only reversible changes occur. A temperature coefficient of magnetization is also specified, which applies in the temperature interval between room temperature and the maximum operating temperature. This quantity may be important if the permanent magnet will be used in a hightemperature environment: under the hood of an automobile or near a jet engine.

Table 14.1 gives some typical values for reversible temperature coefficients of common permanent magnet materials. The values for FeNdB are relatively high because the Curie temperature of this material is near 3008C, and the values for alnico are low because its Curie temperature is high.

Irreversible Changes. If Bd does not return to its original value after a temperature cycle, as in Fig. 14.16b, the change is by definition irreversible. Two cases arise: either the magnetization can be restored to its original value by remagnetization, or it cannot. In the latter case, some permanent change in the structure of the material must have occurred and the magnet can be restored only by a complete new heat treating cycle. In the former case, it is usually found that the loss in Bd at constant temperature occurs rapidly at first, then at a decreasing rate, so that holding the material at or slightly above the maximum expected operating temperature for a few days will lead to a decrease in Bd but will largely eliminate any further changes with time at the same or lower temperature.

14.13SUMMARY OF MAGNETICALLY HARD MATERIALS

It may be useful at this point to look back over all the magnetically hard materials and attempt to classify them into broad groups and make some generalizations. Of the permanent magnet materials in general use today (alnico, hexagonal ferrite, rare-earth compounds), all can be considered fine-particle magnets, although in several cases the fine particles are not single-domain particles. Not only are the particles often above the size limit for single domains, but direct observation shows the presence of domain walls within the particles. It therefore appears that the Stoner–Wohlfarth model of single-domain particles, which initiated a long and highly successful series of developments in permanent magnet materials, does not actually apply to the resulting materials. The necessity for small particles is attributed to the reduced number of domain nucleation sites in a small particle, plus the fact that in magnetically isolated particles a freely moving domain wall can reverse the magnetization direction of only a single particle.

In alnico, the precipitate particles are believed to be single domain, with their coercivity arising from the shape anisotropy of the particles. In the other common permanent magnet materials, the coercivity arises from crystal anisotropy plus the barriers to domain

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Head Positioners. The read/write heads of a computer hard disk drive are moved from track to track by a head positioner which incorporates a special permanent magnet configuration. As shown in Fig. 14.19a, two arc-shaped permanent magnets create a field between them that varies smoothly from a maximum positive value at one end of the arc to a maximum negative value at the other end. A current-carrying loop mounted on one end of a pivoted arm is located in this magnetic field. The equilibrium position of the loop depends on the current in the loop. The equilibrium condition can be stated in various ways: when the force on leg A of the loop is equal and opposite to the force on leg B, or when the total magnetic flux through the loop (the sum of the magnet flux and the loop flux) is minimum. The result is that the equilibrium position of the loop varies directly with the current through the loop, allowing the read/write heads on the opposite end of the pivoted arm to be accurately moved from track to track.

Fig. 14.19 Hard drive head positioner. (a) Magnet configuration; (b) current loop in magnet field, with head at the other end of the pivoted arm.