Rare Earth Magnets

18 

Rare Earth Magnets 

History 

Development SmCo: Middle of the 1960s 

Use: End of the 1960s/beginning of the 1970s 

Development NdFeB: Beginning of the 1980s 

Use: Middle of the 1980s 

Production begins at Magnetfabrik Schramberg 

1986 

Raw material availability 

The essential constituents of SmCo are samarium 

und cobalt; those of NdFeB are neodymium and iron. 

Samarium and neodymium occur abundantly as 

ores. They are classified as rare earth metals in the 

periodic table of elements. Cobalt also occurs in sufficient 

abundance as a natural raw material. 

Raw material sources 

Neodymium, samarium and cobalt are mined in various 

countries around the globe. 

Type of material 

Metallic 

Manufacturing process 

Pressing and sintering in inert gas 

Application areas and particular properties 

Rare earth magnets exhibit very high energy density. 

They are indicated wherever maximum force and 

magnetic flux density are required in small spaces. 

Their high energy density makes it possible to use 

miniature magnets, for example in sensor technology, 

and more compact modules, for example in 

motor engineering.

20 Rare earth magnets 

The Path from Raw 

Material to Magnet 

Alloying 

Outgoing goods inspection 

Magnetising, marking, coating 

to customer specifications 

Rare earth magnets consist mainly of intermetallic 

compounds of rare earth metals (samarium, 

neodymium) and transition metals (such as cobalt, 

iron). In contrast to hard ferrite magnets, milling, 

pressing, and sintering is carried out in an inert gas 

atmosphere. The magnets are pressed either in an 

oil bath (isostatically) or in a die (axially or diametrically). 

After sintering they may be further processed, 

for example by grinding with diamond discs. 

Surface finishing 

(grinding/sawing) 

Sintering

Rare earth magnets 21 

Inspection of incoming raw material 

Breaking up/sifting 

Milling 

Mixing 

Isostatic pressing 

Axial and transverse field pressing 

with magnetic field


Magnetic Ratings Compared 

1400 

1300 

NdFeB 300/125 h 

NdFeB 270/125 h 

NdFeB 250/125 w 

NdFeB 250/175 h 

Remanence B r 

[mT] (mean values) 

1200 

1100 

1000 

NdFeB 230/175 w 

Sm2Co17 195/160 h 

Sm2Co17 180/160 w 

NdFeB 230/220 h 

NdFeB 210/220 h 

NdFeB 200/220 w 

NdFeB 180/220 w 

NdFeB 210/250 h 

NdFeB 180/250 w 

SmCo5 160/175 h 

900 

SmCo5 140/175 w 

800 

1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 3100 3200 3300 3400 

Coercivity H cJ 

[kA/m] (mean values) 

Forming techniques 

Ring, disc, segment, and rectangular magnets are the most common 

shapes for permanent magnets produced by pressing techniques. 

More unusual shapes can also be produced. It is better to press the 

magnets into the desired shapes, since subsequent shape changes 

(adding drill holes, chamfers, notches, indentations, and similar) are 

labour-intensive and require diamond tools. Note that these shape 

features can be produced only in the pressing direction. Since for 

anisotropic magnets, the direction of magnetisation is the same as 

the pressing direction, drill holes, chamfers, notches, indentations, 

and similar can be produced only in the direction of magnetisation. 

>> Further details are found under Typical Magnet Shapes in the technical information 

on page 60. 

Mechanical properties 

As typical sintered metals, rare earth magnets are brittle under impact 

and bending loads. Sm2Co17 is the most brittle. Processing such as 

grinding and cutting requires diamond tools because of their specific 

hardness. Processing with spark erosion and water jet cutting is 

also possible. 

Magnetic ratings 

The possible maximum operating temperatures of NdFeB magnets 

vary between 130 °C and 220 °C. For SmCo magnets, they are between 

250 °C und 350 °C. The complete magnetic ratings measured as in 

IEC 60404-8-5 are presented in graphs and tables on pages 24-27. 

Variations in shape and size may lead to deviations in these ratings. 

Axially, transverse field (diametrically) and isostatically pressed 

magnets 

Rare earth magnets are either cut from isostatically pressed raw magnets 

or pressed in a transverse field (h material) or in the axial field 

(w material). The different production methods affect the magnetic 

properties. The h materials have somewhat higher remanence (B r 

). 

The coercivity (H cJ 

) is identical. In general, the types pressed in an axial 

field fulfil user requirements and for quantity orders, they can be 

produced at lower cost.


Background information 

Temperature behaviour 

Temperature changes also affect the magnetic behaviour of rare earth 

magnets.The remanence and the coercivity decrease with rising temperature 

and increase with falling temperature (for temperature coefficients, 

see tables). The processes are reversible. Magnets with low 

operating point and/or opposing magnetic fields can, however, suffer 

persistent loss of magnetisation caused by reduction of 

coercivity at high temperatures. 

Chemical properties/corrosion resistance 

Rare earth magnets have the properties of metals; for instance, they 

appear shiny directly after surface processing. Acid ambient conditions 

will lead to the dissolution of the magnets, while they are somewhat 

resistant to alkaline media. 

SmCo5 and Sm2Co17 magnets 

Since they consist only of stable intermetallic phase material, at low 

temperatures these magnets are relatively resistant to moisture, solvents, 

alkaline solutions, lubricants, and neutral noxious gasses. Acids 

and salt solutions, in contrast, attack the magnets. Sm2Co17, as 

opposed to SmCo5, contains iron and can exhibit red rust. Samarium 

cobalt magnets are usable without protection for most applications. 

However, the chemical resistance can be further improved by metallic 

or plastic coatings. 

NdFeB magnets 

The microstructure of sintered NdFeB materials is characterised by 

a predominant Nd 2 

Fe 14 

B magnetic phase and an intermetallic grain 

boundary phase. In traditional NdFeB materials, the grain boundary 

phase consists of free neodymium. Like most rare earth metals, 

neodymium is extremely susceptible to corrosion, and spontaneously 

generates neodymium oxide powder or neodymium hydroxide powder, 

expanding in volume. 

In our NdFeB materials, this free neodymium is replaced as far as possible 

by a stable intermetallic phase (corrosion-stabilised). The corrosion 

susceptibility of the material is thus significantly reduced. 

Although the traditional, nonstabilised NdFeB materials are destroyed 

by pulverisation in a short time, the corrosion-stabilised NdFeB materials 

exhibit extraordinarily good corrosion resistance. The behaviour 

of the material in a humid environment can be investigated in an 

autoclave (see comparison of corrosion behaviour in an autoclave). 

Since substituting a stable intermetallic phase for the free neodymium 

affects the magnetic properties, it is easier to protect NdFeB materials 

with high coercivity from corrosion than those with high remanence. 

NdFeB is in principle relatively resistant to most solvents, but salts 

and acids are very corrosive to it. Hydrogen embrittles the material. 

NdFeB contains iron and can exhibit red rust. The reaction is spontaneous, 

gives off heat, and strongly increases the volume. The magnetic 

properties are lost. 

Corrosion-stabilised NdFeB magnets are usable without protection 

for many applications. Their chemical resistance can be further 

improved by metallic or plastic coatings. 

Comparison of corrosion behaviour in an autoclave 

In the autoclave, rare earth magnets are tested at +121 °C, 2.05 bar 

absolute pressure, and 100 % relative humidity. 

Surface loss in mg/cm 2 

0,01 

0,10 

1,00 

10,00 

100,00 

1000,00 

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 

Days 

Curve 1: Conventional NdFeB disintegrates completely within a few 

days. 

Curve 2: For corrosion stabilised NdFeB magnets, the slope of the 

corrosion curve stabilises after the initial, minimal surface corrosion. 

Further material loss is not observed. The material is passivating 

and in the long term exhibits a behaviour similar to Sm2Co17, which 

is known as a corrosion resistant material. 

Curve 3: Sm2Co17 shows only minimal surface corrosion in the autoclave. 

>> Further details are found under the heading Coatings in the technical information 

on page 66-67. 

Curve 2: Corrosion stable, high coercivity 

NdFeB 180/220 w – NdFeB 210/220 h 

Curve 1: Conventional NdFeB 

Curve 3: Sm2Co17 195/160 h


Material Data 

SmCo5 140/175 w 

SmCo5 

Magnetic values as in DIN IEC 60404-8-1 

140/175 w 

anisotropic 

160/175 h 

anisotropic 

-40 °C 

20 °C 

Energy product 

(B·H) max. 

typ. kJ/m 3 

min. kJ/m 3 

155 170 

140 160 

100 °C 200 °C 

Remanence 

B r 

typ. 

min. 

mT 

mT 

880 925 

850 900 

revers. Temp. 

coeff. of B r 

approx. 1) 

%/K 

-0.042 -0.042 

H cB typ. 

kA/m 

690 710 

Coercivity 

H c 

H cB min. 

H cJ typ. 

kA/m 

kA/m 

640 680 

2000 2000 

SmCo5 160/175 h 

H cJ min. 

kA/m 

1750 1750 

revers. Temp. 

coeff. of H cJ 

relative permanent 

permeability 

µ rec. 

Curie 

temperature 

approx. %/K 

approx. 

approx. °C 

-0.25 -0.25 

1.03 1.03 

720 720 

-40 °C 

20 °C 

100 °C 200 °C 

max. operating 

temperature 

approx. °C 

250 250 

Magnetising field 

strength 

min. 

kA/m 

>3000 >3000 

Density 

approx. g/cm 3 

8.3 8.3 

Vickers 

hardness 

Elasticity 

modulus 

Compressive 

strength 

Flexural 

strength 

Expansion 

coefficient 

spec. elec. 

resistance 

spec. 

heat capacity 

Thermal 

conductivity 

HV 

approx. 10 3 N/mm 2 

approx. N/mm 2 


p.p.d. 2) 

i.p.d. 3) 

approx. 

approx. 

approx. 

approx. 

10 -6 /K 

10 -6 Ωm 

J/(kg·K) 

W/mK 

540-560 540-560 

100-200 100-200 

900 900 

120 120 

12.5 

7 

12.5 

7 

0.5 0.5 

370 370 

12 12


Material Data 

Sm2Co17 180/160 w 

-40 °C 

180/160 w 

anisotropic 

195/160 h 

anisotropic 

Sm2Co17 

20 °C 100 °C 200 °C 

200 220 


kJ/m 3 typ. 


180 195 

kJ/m 3 

min. 

(B·H) max. 

1040 1100 

mT 

typ. 

Remanence 

980 1040 

mT 

min. 

B r 

-0.032 -0.032 

%/K approx. 1) 

revers. Temp. 

coeff. of B r 

750 800 

kA/m 

H cB typ. 

Sm2Co17 195/160 h 

700 720 

kA/m 

H cB min. 

Coercivity 

1800 1800 

kA/m 

H cJ typ. 

H c 

1600 1600 

kA/m 

H cJ min. 

-40 °C 

20 °C 

100 °C 

-0.19 -0.19 

1.04 1.04 

%/K 

approx. 

approx. 

revers. Temp. 



permeability 

µ rec. 

200 °C 

800 800 

350 350 

°C approx. 

°C approx. 

Curie 

temperature 


temperature 

4300 4300 

kA/m 

min. 


strength 

8.3 8.3 

g/cm 3 

approx. 

Density 

600 600 

HV 

Vickers 

hardness 

150 150 

10 3 N/mm 2 approx. 

Elasticity 

modulus 

800 800 

N/mm 2 

approx. 

Compressive 

strength 

150 150 

N/mm 2 

approx. 

Flexural 

strength 

11 

8 

11 

8 

0.75-0.9 0.75-0.9 

340 340 

10-13 10-13 

approx. 

10 -6 /K 

p.p.d. 2) 

i.p.d. 3) 

10 -6 Ωm approx. 

J/(kg·K) 

W/mK 

approx. 

approx. 

Expansion 

coefficient 

spec. elec. 

resistance 

spec. 

heat capacity 

Thermal 

conductivity 

1) In the temperature range from 20° C to 200° C 

2) p.p.d = perpendicular to preferred direction 

3) i.p.d = in preferred direction 

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Material Data 

180/250 w 

anisotropic 

180/220 w 

anisotropic 

200/220 w 

anisotropic 

180/250 NdFeB 180/250 w w 

NdFeB * 



(B·H) max. 

typ. kJ/m 3 

min. kJ/m 3 

20 °C 

150 °C 

20 °C 

150 °C 

20 °C 

150 °C 

210 160 210 155 230 180 

180 130 180 130 200 150 

-40 °C 

20 °C 

100 °C 

150 °C 

200 °C 

Remanence 

B r 

typ. 

min. 

mT 

mT 

1050 920 1040 920 1110 970 

1000 880 980 860 1050 940 

revers. Temp. 

coeff. of B r 

approx. 1) 

%/K 

-0.08 -0.08 -0.08 

H cB typ. 

kA/m 

790 680 790 680 850 740 

Coercivity 

H c 

H cB min. 

H cJ typ. 

kA/m 

kA/m 

720 610 720 600 790 680 

2800 1300 2500 900 2500 1000 

180/220 NdFeB 180/220 w w 

H cJ min. 

kA/m 

2500 1050 2200 770 2200 800 

revers. Temp. 



permeability 

µ rec. 

approx. 2) 

approx. 

%/K 

-0.5 -0.5 -0.5 

1.1 1.1 1.1 

-40 °C 

20 °C 

100 °C 

150 °C 

200 °C 

Curie 

temperature 


temperature 


strength 

approx. °C 350 350 350 

approx. °C 220 190 190 

min. kA/m ~2000 ~2000 ~2000 

Density 


7.6 7.6 7.6 

Vickers 

hardness 

HV 560-580 560-580 560-580 

Elasticity 

modulus 

Compressive 

strength 



150 150 150 

1000 1000 1000 

200/220 NdFeB 200/220 w w 

Flexural 

strength 


250 250 250 

Expansion 

coefficient 

p.p.d. 3) 

i.p.d. 4) 

approx. 

10 -6 /K 

-1 

5 

-1 

5 

-1 

5 

-40 °C 

spec. elec. 

resistance 

spec. 

heat capacity 

Thermal 

conductivity 

approx. 

approx. 

approx. 

10 -6 Ωm 

J/(kg·K) 

W/mK 

1.6 1.6 1.6 

440 440 440 

8 8 8 

20 °C 

100 °C 

150 °C 

200 °C


Material Data 

NdFeB 210/250 h 

210/250 h 

anisotropic 

210/220 h 

anisotropic 

-40 °C 

20 °C 

100 °C 

150 °C 200 °C 

20 °C 

230/220 h 

anisotropic 

150 °C 

20 °C 

150 °C 

20 °C 

150 °C 

NdFeB * 


240 190 240 190 255 200 

kJ/m 3 

typ. 


210 160 210 160 230 175 

kJ/m 3 

min. 

(B·H) max. 

1110 980 1115 980 1160 1020 

mT 

typ. 

Remanence 

1050 940 1050 940 1100 970 

mT 

min. 

B r 

-0.08 -0.08 -0.08 


revers. Temp. 

coeff. of B r 

860 750 860 750 890 780 

kA/m 

H cB typ. 

NdFeB 210/220 h 

800 690 800 690 840 730 

kA/m 

H cB min. 

Coercivity 

2800 1300 2500 900 2500 1000 

kA/m 

H cJ typ. 

H c 

-40 °C 

2500 1050 2200 770 2200 800 

kA/m 

H cJ min. 

20 °C 

100 °C 

150 °C 

200 °C 

-0.5 -0.5 -0.5 

1.1 1.1 1.1 

350 350 350 


approx. 

°C approx. 

revers. Temp. 



permeability 

µ rec. 

Curie 

temperature 

220 190 190 

°C approx. 


temperature 

~2000 ~2000 ~2000 

kA/m 

min. 


strength 

7.6 7.6 7.6 

g/cm 3 

approx. 

Density 

560-580 560-580 560-580 

HV 

Vickers 

hardness 

150 150 150 


Elasticity 

modulus 

NdFeB 230/220 h 

1000 1000 1000 

N/mm 2 

approx. 

Compressive 

strength 

250 250 250 

N/mm 2 

approx. 

Flexural 

strength 

-40 °C 

20 °C 

-1 

5 

-1 

5 

-1 

5 

approx. 

10 -6 /K 

p.p.d. 3) 

i.p.d. 4) 

Expansion 

coefficient 

100 °C 

150 °C 

200 °C 

1.6 1.6 1.6 

440 440 440 

8 8 8 


J/(kg·K) 

W/mK 

approx. 

approx. 

spec. elec. 

resistance 

spec. 

heat capacity 

Thermal 

conductivity 

1) In the temperature range from 20° C to 100° C 

2) At higher temperatures, the temperature coefficient has smaller values 



*Licenced from NEOMAX Co. Ltd. 



Material Data 

230/175 w 

anisotropic 

250/125 w 

anisotropic 

230/175 NdFeB 230/175 w w 

-40 °C 

NdFeB * 



(B·H) max. 

typ. kJ/m 3 

min. kJ/m 3 

20 °C 

150 °C 

20 °C 

100 °C 

260 190 280 220 

230 165 250 190 

20 °C 

100 °C 

150 °C 

Remanence 

B r 

typ. 

min. 

mT 

mT 

1190 1020 1230 1100 

1130 970 1170 1060 

revers. Temp. 

coeff. of B r 

approx. 1) 

%/K 

-0.09 -0.10 

H cB typ. 

kA/m 

890 620 890 750 

Coercivity 

H c 

H cB min. 

H cJ typ. 

kA/m 

kA/m 

840 480 840 650 

1900 650 1400 800 

250/125 NdFeB 250/125 w w 

revers. Temp. 



permeability 

µ rec. 

H cJ min. 

approx. 2) 

approx. 

kA/m 

%/K 

1750 500 1250 700 

-0.6 -0.6 

1.1 1.1 

-40 °C 

20 °C 

100 °C 

Curie 

temperature 

approx. °C 

340 330 


temperature 

approx. °C 

160 130 


strength 

min. 

kA/m 

~2400 ~2400 

Density 


7.6 7.5 

Vickers 

hardness 

Elasticity 

modulus 

Compressive 

strength 

Flexural 

strength 

Expansion 

coefficient 

spec. elec. 

resistance 

spec. 

heat capacity 

Thermal 

conductivity 

HV 




p.p.d. 3) 

i.p.d. 4) 

approx. 

approx. 

approx. 

approx. 

10 -6 /K 

10 -6 Ωm 

J/(kg·K) 

W/mK 

560-580 560-580 

150 150 

1000 1000 

250 250 

-1 

5 

-1 

5 

1.6 1.6 

440 440 

8 8


Material Data 

NdFeB 250/175 h 

-40 °C 

20 °C 

250/175 h 

anisotropic 

270/125 h 

anisotropic 

300/125 h 

anisotropic 

100 °C 

150 °C 

20 °C 

150 °C 

20 °C 

100 °C 

20 °C 

100 °C 

NdFeB * 


295 200 300 230 330 260 

kJ/m 3 

typ. 


250 175 270 200 300 240 

kJ/m 3 

min. 

(B·H) max. 

1240 1050 1280 1150 1320 1185 

mT 

typ. 

Remanence 

1180 1000 1220 1110 1260 1160 

mT 

min. 

B r 

-0.09 -0.10 -0.10 


revers. Temp. 

coeff. of B r 

920 620 920 700 950 650 

kA/m 

H cB typ. 

NdFeB 270/125 h 

860 480 870 600 900 540 

kA/m 

H cB min. 

Coercivity 

1900 650 1400 780 1400 700 

kA/m 

H cJ typ. 

H c 

-40 °C 

20 °C 

100 °C 

1750 500 1250 680 1250 550 

-0.6 -0.6 -0.6 

1.1 1.1 1.1 

kA/m H cJ min. 


approx. 

revers. Temp. 



permeability 

µ rec. 

340 330 330 

°C approx. 

Curie 

temperature 

160 130 130 

°C approx. 


temperature 

~2400 ~2400 ~2400 

kA/m 

min. 


strength 

7.6 7.5 7.5 

g/cm 3 

approx. 

Density 

560-580 560-580 560-580 

HV 

Vickers 

hardness 

150 150 150 


Elasticity 

modulus 

NdFeB 300/125 h 

1000 1000 1000 

N/mm 2 

approx. 

Compressive 

strength 

-40 °C 

20 °C 

100 °C 

250 250 250 

-1 

5 

-1 

5 

-1 

5 

N/mm 2 

approx. 

10 -6 /K 

approx. 

p.p.d. 3) 

i.p.d. 4) 

Flexural 

strength 

Expansion 

coefficient 

1.6 1.6 1.6 


spec. elec. 

resistance 

440 440 440 

J/(kg·K) 

approx. 

spec. 

heat capacity 

8 8 8 

W/mK 

approx. 

Thermal 

conductivity 

1) In the temperature range from 20 °C to 100 °C 

2) At higher temperatures, the temperature coefficient has smaller values 



*Licenced from NEOMAX Co. Ltd.

Rare Earth Magnets

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