MMPA STANDARD No. 0100-00
STANDARD SPECIFICATIONS FOR PERMANENT MAGNET MATERIALS
MAGNETIC MATERIALS PRODUCERS ASSOCIATION

RARE EARTH MAGNETS

1.0 CHEMICAL COMPOSITION
Rare earth magnet materials currently fall into three families of materials. They are rare-earth cobalt 5, the rare earth 2 transition metal 17 group and rare earth iron alloys.
1.1 l-5 Alloys (Rare-Earth Cobalt 5): These alloys are usually binary or ternary alloys with the approximate atomic ratio of one rare earth atom to five cobaltatoms. The rare earth element is most commonly
samarium but can also be other light rare earth such as, but not limited to, praseodymium, cerium, neodymium or a combination, or a mixture known as misch metal. Heavy rare earths such as gadolinium,
dysprosium and erbium can substitute for the light rare earth elements to give the magnetic material a lower temperature coefficient of remanence. The rare earth elements typically are 34 to 39 weight percent of the alloy.
1.2 2-17 Alloys (Rare-Earth 2 Transition Element 17): These alloys are an age hardening type with a com- position ratio of 2 rare earth atoms to 13-17 atoms of transition metals. The rare earth atoms can be
any of those found in the l-5 alloys. The transition metal (TM) content is a cobalt rich combination of cobalt, iron and copper. Small amounts of zirconium, hafnium or other elements are added to enhance the heat treatment response. The rare earth content of 2-17 materials is typically 23 to 28 weight percent of the alloy.
1.3 Rare Earth Iron Alloys: These alloys have a composition of two rare earth atoms to 14 iron atoms with one boron atom. There may be a substitution of other rare earth and/or minor additions of other elements. Cobalt is substituted for the iron at 3 to 15 % to improve high temperature performance. The rare earth content of RE-Fe magnet alloys is typically 30 to 35 weight percent.

2.0 MANUFACTURING METHODS
The rare earth magnet alloys are usually formed by powder metallurgical processes. The magnetic performance of all grades is optimized by applying a magnetic field during the pressing operation, thus producing a preferred direction of magnetization. Pressing and aligning techniques can substantially vary the degree of orientation and the residual induction (Br) of the finished magnet.
The direction of the magnetic field during die pressing can be either parallel or perpendicular to the pressing direction. Magnets can also be formed by isostatic pressing. After pressing, the magnets are sintered, heat treated and ground to the final dimensions. Rare earth magnets are inherently brittle and cannot be machined with con-ventional metal cutting processes such as drilling, turning or milling. The magnets can be readily ground with abrasive wheels if liberal amounts of coolant are used. The coolant serves to minimize heat cracking, chipping and also eliminates the risk of fires caused by sparks contacting the easily oxidized grinding dust.

3.0 MAGNETIC PROPERTIES
The magnetic properties and chemical compositions of the commercial grades of rare earth magnet materials are given in Table IV-l. Since many combinations of elements and orientations are possible, many additional grades are available from various producers.

4.0 DIMENSIONS AND TOLERANCES
Allowable tolerances for sintered rare earth magnets are given in Table IV-2.

5.0 MECHANICAL CHARACTERISTICS
The following general specifications are for mechanical characteristics and visual imperfections.
5.1 Surface Conditions
5.1 .1 All magnet surfaces shall be free of foreign materials which would tend to hold or collect extraneous particles on the magnet surface in the unmagnetized condition.
5.2 Chips
5.2.1 Magnets shall be free of loose chips. They shall be free of imperfections which will result in loose chips or particles under normal conditions of handling, shipping, assembly and service.
5.2.2 A chipped edge or surface shall be acceptable if no more than 10 percent of the surface is removed, provided that no loose particles remain at the edge or surface, and further provided the magnet under examination meets the magnetic specification agreed upon between the producer and user.

5.3 Other Physical Imperfections
5.3.1 Imperfections such as minor hairline cracks, porosity, voids, and others, all of the type commonly found in sintered metallic magnets, shall be judged acceptable if the following conditions are met:
5.3.1.1 The magnet meets the minimum magnetic performance criteria agreed upon.
5.3.1.2 The imperfections do not create loose particles or other conditions which will interfere with proper functioning of the end device.
5.3.1.3 Cracks shall be acceptable provided they do not extend across more than 50 percent of any pole surface.
5.4 Other Conditions
5.4.1 Non-destructive inspection methods such as the use of penetrants, microscopy, magnetic particle analysis, ultrasonic inspection, or x-ray shall not be acceptable able methods for judging the quality of
sintered rare earth magnets except as provided in Section 5.4.2.
5.4.2 In cases where the magnet is expected to withstand abnormal conditions or stresses, such conditions must be previously specified and a mutually acceptable service test devised to assure that the
magnet shall not fail under the specified service conditions. Such tests should duplicate service conditions with appropriate safety factors.

6.0 PHYSICAL AND THERMAL PROPERTIES
Typical physical and thermal properties for rare earth magnets are given in Table IV-3.

7.0 PROCESS CONTROL
Most manufacturers use statistical process control to monitor key parameters at each process step. Control plans are individually negotiated with customers to meet specific quality requirements.

8.0 INSPECTION & TESTING
In the absence of a control plan, rare earth magnets will be inspected for all specific characteristics using a statistically valid sampling plan. Such plans may be derived from, Quality Planning And Analysis: From Product Development Through Use, J.M. Juran and F. M. Gryna, 3rd Edition, McGraw Hill (1993), Chapter 19. ISBN 0-07-033183-9.