Neodymium Iron Boron Magnets - General Information

Some important properties used to compare permanent magnets are: remanence (Mr), which measures the strength of the magnetic field; coercivity(Hci), the material's resistance to becoming demagnetized; energy product (BHmax), the density of magnetic energy; and Curie temperature (TC), the temperature at which the material loses its magnetism.

Neodymium magnets have higher remanence, much higher coercivity and energy product, but often lower Curie temperature than other types. Neodymium is alloyed with terbium and dysprosium in order to preserve its magnetic properties at high temperatures. The table below compares the magnetic performance of neodymium magnets with other types of permanent magnets.

Magnet

Mr (T)

Hci (kA/m)

BHmax (kJ/m3)

TC (°C)

Nd2Fe14B (sintered) 1.0–1.4 750–2000 200–440 310–400
Nd2Fe14B (bonded) 0.6–0.7 600–1200 60–100 310–400
SmCo5 (sintered) 0.8–1.1 600–2000 120–200 720
Sm(Co, Fe, Cu, Zr)7 (sintered) 0.9–1.15 450–1300 150–240 800
Alnico (sintered) 0.6–1.4 275 10–88 700–860
Sr-ferrite (sintered) 0.2–0.4 100–300 10–40 450

Comparison of physical properties of sintered neodymium and Sm-Co magnets

PropertyNeodymiumSm-Co
Remanence (T) 1–1.3 0.82–1.16
Coercivity (MA/m) 0.875–1.99 0.493–1.59
Permeability 1.05 1.05
Temperature coefficient of remanence (%/K) −0.12 −0.03
Temperature coefficient of coercivity (%/K) −0.55..–0.65 −0.15..–0.30
Curie temperature (°C) 320 800
Density (g/cm3) 7.3–7.5 8.2–8.4
CTE, magnetizing direction (1/K) 5.2×10−6 5.2×10−6
CTE, normal to magnetizing direction (1/K) −0.8×10−6 11×10−6
Flexural strength (N/mm2) 250 150
Compressive strength (N/mm2) 1100 800
Tensile strength (N/mm2) 75 35
Vickers hardness (HV) 550–650 500–550
Electrical resistivity (Ω·cm) (110–170)×10−6 86×10−6

The cost of Neodymium magnet material on weight basis is 10 to 20 times more than Ferrite magnets. But if the cost comparison is done on per energy product basis, this cost is comparable to that of Ferrite magnets due to the high energy products of neodymium.

Key Benefits

Very high strength
Relatively low cost (by weight about 20 times Ferrite magnets, by "Dollars per BHmax" about 1.5 times Ferrite magnets).
Relatively easy to machine, compared to Alnico and SmCo magnets.

Key Challenges

Properties deteriorate rapidly at temperatures in excess of about 150°C (depending on grade and permeance coefficient magnet is operating at).
Most grades of NdFeB magnets need to be protected against oxidation - by coating or plating the magnets.

Quick Facts

Density - 0.275 lbs per cubic inch
Saturation magnetizing field required - about 35kOe
Manufacturing methods - sintering (most common), injection molding, compression bonding, or calendaring.
Shapes available - blocks, bars, discs, rings, arc segments, etc.
Grades available - from about 3330 to 5311. (First 2 digits represent BHmax, and second two digits represent Intrinsic Coercivity, Hci.)
Sizes - off tool the largest die pressed blocks are about 4" cube, while isostatically pressed blocks can be much longer in the orientation direction (up to 9 feet).
Machining - Neodymium magnets should be machined by grinding using diamond wheels, However, of the hard magnet materials, Neo magnets are the least difficult to machine. We have successfully machined very small magnets - down to 0.012" diameter with a center hole of 0.003" diameter, 0.040" long.

Surface Treatments

Painting, coating, or plating is generally recommended for NdFeB, although recently certain grades have been made that exhibit higher resistance to oxidation. Plating NdFeB is a difficult process, and commercial plating houses unfamiliar with the specialized plating techniques required are unlikely to be able to achieve plating with good adhesion on Neo magnets. Nickel, Zinc, or Tin plating, plating provides good corrosion resistance for NdFeB magnets, though longer lead times or higher volumes may be required for these. We are also able to cadmium chromate or aluminum chromate plate NdFeB using ion vacuum deposition (IVD) techniques. A variety of organic coatings have also been successfully developed for NdFeB, exhibiting good corrosion resistance characteristics. For especially harsh environments, it may be advisable to use a combination of coating techniques, or to encapsulate the material in a sealed housing.

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