Welcome to the Magnetic Materials Theory Page.

 

Magnet Selection

 

A magnet must operate reliably with the total effective air gap in the working environment. It must fit the available space. It must be mountable, affordable, and available.

 

Figures of Merit

 

The figures of merit commonly applied to magnetic materials are:

  • Residual induction (Br) in gauss (G). How strong is the magnetic field?

  • Coercive force (Hc) in oversteds (Oe). How well will the magnet resist external demagnetizing forces?

  • Maximum energy product (BHmax) in gauss-oersteds  106. A strong magnet that is also very resistant to demagnetizing forces has a high maximum energy product. Generally, the larger the energy product, the better, stronger, and more expensive the magnet.

  • Temperature coefficient. The rate of change of the operate or release switch points over the full operating temperature range, measured in gauss per degree Celsius. How much will the strength of the magnet change as temperature changes?

Magnetic Materials

 

Neodymium (Ne-Fe B). The new neodymium-iron-boron alloys fill the need for a high maximum-energy product, moderately priced magnet material. The magnets are produced by either a powdered-metal technique called orient-press-sinter or a new process incorporating jet casting and conventional forming techniques. Current work is being directed toward reducing production costs, increasing operating temperature ranges, and decreasing temperature coefficients. Problems relating to oxidation of the material can be overcome through the use of modern coatings technology. Maximum energy products range from 7 to 15 MGOe depending on the process used to produce the material.

 

Rare-earth cobalt is an alloy of a rare-earth metal, such as samarium, with cobalt (abbreviated as RE cobalt). These magnets are the best in all categories, but are also the most expensive by about the same margins. Too hard for machining, they must be ground if shaping is necessary. Maximum energy product, perhaps the best single measure of magnet quality, is approximately 16  106.

 

Alnico is a class of alloys containing aluminum, nickel, cobalt, iron, and additives that can be varied to give a wide range of properties. These magnets are strong and fairly expensive, but less so than RE cobalt. Alnico magnets can be cast, or sintered by pressing metal powders in a die and heating them. Sintered Alnico is well suited to mass production of small, intricately shaped magnets. It has more uniform flux density, and is mechanically superior. Cast Alnico magnets are generally somewhat stronger. The non-oriented or isotropic Alnico alloys (1, 2, 3, 4) are less expensive and magnetically weaker than the oriented alloys (5, 6, 5-7, 8, 9). Alnico is too hard and brittle to be shaped except by grinding. Maximum energy product ranges from 1.3  106 to 10  106.

 

Ceramic magnets contain barium or strontium ferrite (or another element from that group) in a matrix of ceramic material that is compacted and sintered. They are poor conductors of heat and electricity, are chemically inert, and have-high values of coercive force. As with Alnico, ceramic magnets can be fabricated with partial or complete orientation for additional magnetic strength. Less expensive than Alnico, they also are too hard and brittle to shape except by grinding. Maximum-energy product ranges from 1  106 to 3.5  106.

 

Cunife is a ductile copper base alloy with nickel and iron. It can be stamped, swaged, drawn, or rolled into final shape. Maximum energy product is approximately 1.4  106.

 

Iron-chromium magnets have magnetic properties similar to Alnico 5, but are soft enough to undergo machining operations before the final aging treatment hardens them. Maximum energy product is approximately 5.25  106.

 

Plastic and rubber magnets consist of barium or strontium ferrite in a plastic matrix material. They are very inexpensive and can be formed in numerous ways including stamping, molding, and machining, depending upon the particular matrix material. Because the rubber used is synthetic, and synthetic rubber is also plastic, the distinction between the two materials is imprecise. In common practice, if a plastic magnet is flexible, it is called a rubber magnet. Maximum energy product ranges from 0.2  106 to 1.2  106.

 

Choosing Magnet Strength

 

A magnet must have sufficient flux density to reach the Hall switch maximum operate-point specification at the required air gap. Good design practice suggests the addition of another 50 G to 100 G for insurance and a check for sufficient flux at the expected temperature extremes.

 

For example, if the Hall device datasheet specifies a 350 G maximum operate point at 25C, after adding a pad of 100 G, we have 450 G at 25C. If operation to 70C is required, the specification should be 450 G + 45 G = 495 G. (For calculations, we use 0.7 G/C operate point coefficient and 1 G/C release point coefficient.) Because the temperature coefficient of most magnets is negative, this factor would also require some extra flux at room temperature to ensure high-temperature operation.

 

Coercive Force

 

Coercive force becomes important if the operating environment will subject the magnet to a strong demagnetizing field, such as that encountered near the rotor of an AC motor. For such applications, a permanent magnet with high coercive force (ceramic, Alnico 8, or, best of all, RE cobalt) is clearly indicated.

 

Price and Peak Energy Product

 

The common permanent magnet materials and their magnetic properties are summarized in table 4. The Cost column shows the relationship between the price paid for a magnet and its peak energy product.

 

Properties of Magnetic Materials

Material

Maximum Energy Product (G-Oe)

Residual Induction (G)

Coercive Force (Oe)

Temperature Coefficient %/C

Cost

Comments

RE cobalt

16106

8.1103

7.9103

-0.05

Highest

Strongest, smallest, resists demagnetizing best

Alnico 1, 2, 3, 4

1.3 to 1.7106

5.5 to 7.5103

0.42 to 0.72103

-0.02 to -0.03

Medium

Non-oriented

Alnico 5, 6, 5-7

4.0 to 7.5106

10.5 to 13.5103

0.64 to 0.78103

-0.02 to -0.03

Medium to high

Oriented

Alnico 8

5.0 to 6.0106

7 to 9.2103

1.5 to 1.9103

-0.01 to 0.01

Medium to high

Oriented, high coercive force, best temperature coefficient

Alnico 9

10106

10.5103

1.6103

-0.02

High

Oriented, highest energy product

Ceramic 1

1.0106

2.2103

1.8103

-0.02

Low

Non-oriented, high coercive force, hard, brittle, nonconductor

Ceramic 2, 3, 4, 6

1.8 to 2.6106

2.9 to 3.3103

2.3 to 2.8103

-0.02

Low-medium

Partially oriented, very high coercive force, hard, brittle, nonconductor

Ceramic 5, 7, 8

2.8 to 3.5106

3.5 to 3.8103

2.5 to 3.3103

-0.02

Medium

Fully oriented, very high coercive force, hard, brittle, nonconductor

Cunife

1.4106

5.5103

0.53103

-

Medium

Ductile, can cold form and machine

Fe-Cr

5.25106

13.5103

0.60103

-

Medium

Can machine prior to final aging treatment

Plastic

0.2 to 1.2103

1.4 to 3103

0.45 to 1.4103

-0.02

Lowest

Can be molded, stamped, machined

Rubber

0.35 to 1.1106

1.3 to 2.3103

1 to 1.8103

-0.02

Lowest

Flexible

Neodymium

7 to 15106

6.4 to 11.75103

5.3 to 6.5103

-0.157 to -0.192

Medium-high

Non-oriented

 

Materials most commonly used are various Alnicos, Ceramic 1, and barium ferrite in a rubber or plastic matrix material (see above table). Manufacturers usually have stock sizes with a choice of the number of pole pairs. Custom configurations are also available at a higher cost.

 

Alnico is a name given to a number of aluminum nickel-cobalt alloys that have a fairly wide range of magnetic properties. In general, Alnico ring magnets have the highest flux densities, the smallest changes in field strength with changes in temperature, and the highest cost. They are generally too hard to shape except by grinding and are fairly brittle, which complicates the mounting of bearings or arbor.

 

Ceramic 1 ring magnets (trade names Indox, Lodex) have somewhat lower flux densities (field strength) than the Alnicos, and their field strength changes more with temperature. However, they are considerably lower in cost and are highly resistant to demagnetization by external magnetic fields. The ceramic material is resistant to most chemicals and has high electrical resistivity. Like Alnico, they can withstand temperatures well above that of Hall switches and other semiconductors, and must be ground if reshaping or trimming is necessary. They may require a support arbor to reduce mechanical stress.

 

The rubber and plastic barium ferrite ring magnets are roughly comparable to Ceramic 1 in cost, flux density, and temperature coefficient, but are soft enough to shape using conventional methods. It is also possible to mold or press them onto a shaft for some applications. They do have temperature range limitations, from 70C to 150C, depending on the particular material, and their field strength changes more with temperature than Alnico or Ceramic 1.

 

Regardless of material, ring magnets have limitations on the accuracy of pole placement and uniformity of pole strength which, in turn, limit the precision of the output waveform. Evaluations have shown that pole placement in rubber, plastic, and ceramic magnets usually falls within 2 or 3 of target, but 5 errors have been measured. Variations of flux density from pole to pole will commonly be 5%, although variations of up to 30% have been observed.

 

Figure 24 is a graph of magnetic flux density as a function of angular position for a typical 4 pole-pair ceramic ring magnet, 25.4 mm in diameter, with a total effective air gap, TEAG, of 1.7 mm (1.3 mm clearance plus 0.4 mm package contribution). It shows quite clearly both the errors in pole placement and variations of strength from pole to pole.