Fundamentals of Permanent
Magnets
Magnetic Orientation
Isotropic Anisotropic
(non-oriented) (oriented)
•Magnetic strength is the same in all directions
•Can be magnetized in any direction
•Almost all bonded magnets
•Magnetic strength is greatest in the oriented direction
•Can be magnetized only in the oriented direction
•Almost all sintered magnets
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Main Magnetic Properties
Residual induction - B
r Coercive force - H
c Intrinsic coercive force - H
ci Maximum Energy Density – (BH)
max Maximum Working Temperature - Tw
Reversible Temperature Coefficient - -%Br/C
Curie Temperature - Tc
Main Types of Magnet Materials
Alnico
Ferrite
Samarium Cobalt
Neodymium Iron Boron
Bonded Grades of the Above Types
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Typical Supplier Data Sheet
Hysteresis / Demagnetization Curve
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I
I
Hci
Br, Bi
-H Hc
-B
+H +B
(BiH)max
Temperature Effects
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25.0 22.5 20.0 17.5 15.0 12.5 10.0 7.50 5.00 2.50 H (KOe) 15.0
13.5
12.0
10.5
9.00
7.50
6.00
4.50
3.00
1.50 B (Kg)
Alnico Ferrite SmCo NdFeB(SH)
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Magnet Characteristics
Materials Typical Shapes Pro Con
Cast Alnico
AlNiCo Rods, Bars, U shape and other cast type
High Br High working T
Good T coef.
Very Low Hc High cost
High L/D Requires Cast Sintered Alnico
AlNiCo Powder pressed to shape Complex shapes High Br, T
Requires Tool High cost Low market Ceramic/Ferrite
SrFe2O3 Blocks, Rings, Arcs, Discs Most flux for $ High usage Low corrosion
Low Br Requires tool Simple shapes Samarium
Cobalt SmCo
Blocks, Rings, Discs Arcs, Segments
No corrosion Very low T coef
Stable, No tool
Very expensive Simple shapes High Co content Neodymium
NdFeB Blocks, Rings, Discs Arcs, Segments
Highest magnetic properties No tooling
Corrodes Low working T Difficult to Mag Bonded Grades
All materials
Difficult geometries Can be insert molded
or over-molded
Complex shapes Various resins
High tooling Low magnetics
High volumes
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Cast Alnico Magnets
Ferrites / Ceramics (SrFe 2 O 3 )
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Neodymium, Neo, NdFeB
Injection and Compression Molded
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