1. Permanent Magnets 101
Stan Trout
March 12, 2015

2. What Spontaneous Materials Does
• Technical Training
• Material Selection
• Industry Reports
• Process Development
• Troubleshooting
• Magnetic Design Review
• Due Diligence
• Expert Witness
• Advice to Investors
• Magnetic Measurements

3. Permanent Magnets 101
• Welcome
• Background
• Magnetic Theory
– Hysteresis
– Three materials
– Three vectors
– Units and Conversions
• Ferromagnetism
– Spin
– Exchange
– Anisotropy
– Curie temperature
• Permanent magnets
– Processing
• Summary

4. Hysteresis
• A delayed response to a stimulus
• In this case, the stimulus is an applied magnetic
field and the response is the magnetization or
flux density
• The shape of the hysteresis loop tells us what
kind of material we have

6. Two Types of Materials
• Soft Magnetic
• Hard Magnetic (Permanent Magnets)

7. Soft Materials
• Low Hci
• High Saturation (Ms)
• High Permeability (µ)
• Applications
– Transformers
– Inductors
• Materials
– Low Frequency (1 kHz)
• Fe, Si-Fe, Ni-Fe, Fe-B
– High Frequency
• Zn-ferrite
H
M

8. Permanent Magnets
• High Hci
• High Br
• Squareness Hk
• Applications
– Motors, generators
– Actuators
– Speakers
• Materials
– Alnico, Ferrite
– SmCo, NdFeB
H
M

9. The Three Vectors
• B, Magnetic flux density or Induction.
• H, Magnetic field. (from current)
• M, Magnetization. (a material property)
• Vectors are not independent, but related.
• Induction is the combination of
magnetization and magnetic field.

10. Flux Density or Induction, B
• Concentration of total magnetic flux in a region
• Lines of magnetic flux passing through a given
area, lines per area
• Units: lines/cm2 or Gauss (G)
Webers/m2 or Tesla (T)
• Magnetic Flux
Φ=B A cosθ

11. Magnetic Field, H
• A magnetic field
created by current
flowing in a wire.
• Units: Oersted (Oe),
Ampere-turn/meter
(A/m)
Source: Cullity

12. Magnetization, M
• The magnetic state of a material
• The sum of all the individual magnetic moments per
unit volume
• Moments arise from unpaired electron spins, usually
in the 3d or 4f electron shells
• Units: Gauss (G) for 4M
emu/cm3 for M
Tesla (T) for 0M and J
M
Emu, source: Wikipedia

13. How are B, H and M related?
Induction, B is a combination of H and M.
B=H + 4M CGS units
B= 0 (H+M) SI units
0=4  10-7 Tesla-m/A
0M=J, Polarization

14. CGS and SI Units
Source: IEEE Magnetics Society

15. Ferromagnetism
• Spinning electrons contributes to the
magnetization
• Strong exchange interaction, keeps spins
parallel
• Spontaneous magnetization
• Interaction reduced by increasing
temperature, destroyed at Curie
Temperature (Tc)

16. Ferromagnetic Elements
What elements are ferromagnetic at room temperature?

17. Electron Spin
• Spinning and orbiting electrons produces tiny
magnetization called magnetic moments
• Small contribution from orbiting
• Bohr Magneton, μB=9.274 x 10-24 A-m2 about one spin
• Normally electron spins are “quenched” (cancel out)
– Why most elements are not ferromagnetic
• Net moments likely when inner electron shells are
unfilled
– Transition metals (3d)
– Rare earths (4f)

18. Exchange Interaction
• An interaction between neighboring atoms
• Cooperative effect, favors ordering of spins
in several configurations
– Ferro-
– Antiferro-
– Ferri-
• Exchange competes with thermal energy
Source: Intro to Solid State Physics, Kittel

19. Anisotropy
• Many properties vary
with direction of
measurement
• Not just magnetic
• Often hard to see in
polycrystalline
materials
• High anisotropy
enables high coercivity
Source: Cullity

20. Magnetic Domains
Source: Cullity
Which
arrangement
has the lowest
energy?

21. Domain Wall
Source: Cullity

22. Curie Temperature
Tc

23. Demagnetization Curves
H (kA/m)
-1200 -1000 -800 -600 -400 -200 0
B&J(T)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
H (kOe)
-14 -12 -10 -8 -6 -4 -2 0
(BH)m a x
Br
4M&B(kG)
0
2
4
6
8
10
12
14
HcHc i
Intrinsic Curve, 4πM or J
Induction Curve, B

24. Comparison of Materials
Source: VAC Magnet Catalog
2 grams
6 grams
90 grams
150 grams

25. Four Families of
Permanent Magnets
Ferrite Alnico SmCo NdFeB
Property Ceramic 8 Alnico 5 1-5 2-17 Bonded Sintered
Br (kG) 4.0 12.5 9.0 10.4 6.9 13.4
 (%/C) -0.18 -0.02 -0.045 -0.035 -0.105 -0.12
(BH)max MGOe 3.8 5.5 20 26 10 43
Hci (kOe) 3.3 0.64 30 25 9 15
 (%/C) +0.4 -0.015 -0.3 -0.3 -0.4 -0.6
Hs (kOe) 10 3 20 30 35 35
Tc (C) 460 890 727 825 360 310
Notes:
The quantity  is the reversible temperature coefficient of Br. (20 ºC to 100 ºC minimum)
The quantity  is the reversible temperature coefficient of Hci. (20 ºC to 100 ºC minimum)
The field required to saturate the magnet is Hs.

26. Permanent Magnets 1930 to 2010

27. Requirements
for a Permanent Magnet Material
1. High Curie Temperature
2. Large Saturation Magnetization
3. Highly Anisotropic
4. Physically and Magnetically Stable
5. Sufficiently Abundant Raw Materials
6. Simple, Profitable and Environmentally Safe to
Manufacture
7. Easily magnetized to saturation
Conditions 1 through 3 are from W. E. Wallace, Rare Earth Intermetallics, Academic Press, New York (1973)
p.159.
Condition 7 is from Reinhold Strnat, private communication

28. Powder Metallurgy Process
• Commonly used for
SmCo and NdFeB
• Similar process is
used for ferrites

29. Rare Earth Processing
• Mine
– Bastnasite or other
• Solubilize
– Put RE ions in H2O
• Separate
– Solvent extraction
– Disadvantage of chemical similarity
– Re ion’s tendency to one liquid over another
– Inefficient, many steps to high purity
– Pregnant solution
• Precipitate
– Oxalate or carbonate
• Calcine
– Heat in air, 1000 °C
– Convert into oxide

30. Metals and Alloys
• Alloy
– Vacuum furnace
– Strip casting
Source: Less Common Metals
Source: US Patent 7,004,228

31. Crushing
• Goal: single crystal particles
• H2 rapidly diffuses into alloy and
expands it
• Coarse mill
– 40 mesh
• Jet mill
– fine particles
– uniform particles
– 3 µm size
– Non-contaminating
Source: The Jet Pulverizer Company

32. Pressing
• Shape powder
• Magnetically orient
particles
• Apply pressure
– Lock-in orientation
– 1 x 108 Pa die press
– 4 x 108 Pa isopress
– Start densification
Sources:
Avure & VAC

33. Sintering & Heat Treating
• Sinter
– 1070°C, 1-2 hours, vac.
– Quench
– Density
– Shrink and warp
• Heat Treat
– 600°C, 1-5 hours
– Quench
– Not standardized
– Develops Hci and Hk
Both steps are done in a Vacuum
Furnace with gas quenching capabilities
Source: Solar Manufacturing

34. Permanent Magnets 101
• Welcome
• Background
• Magnetic Theory
– Hysteresis
– Three materials
– Three vectors
– Units and Conversions
• Ferromagnetism
– Spin
– Exchange
– Anisotropy
– Curie temperature
• Permanent magnets
– Processing
• Summary