Grades[edit]
Neodymium magnets are graded according to their maximum energy product, which relates to the magnetic flux output per unit volume. Higher values indicate stronger magnets. For sintered NdFeB magnets, there is a widely recognized international classification. Their values range from 28 up to 52. The first letter N before the values is short for neodymium, meaning sintered NdFeB magnets. Letters following the values indicate intrinsic coercivity and maximum operating temperatures (positively correlated with the Curie temperature), which range from default (up to 80 °C or 176 °F) to TH (230 °C or 446 °F).
Grades of sintered NdFeB magnets:
N30 – N55
N30M – N50M
N30H – N50H
N30SH – N48SH
N30UH – N42UH
N28EH – N40EH
N28TH – N35TH
Magnetic properties[edit]
Some important properties used to compare permanent magnets are:
Remanence (Br), which measures the strength of the magnetic field.
Coercivity (Hci), the material's resistance to becoming demagnetized.
Maximum energy product (BHmax), the density of magnetic energy,[18] characterized by the maximum value of magnetic flux density(B) times magnetic field strength (H).
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 of magnets. Special neodymium magnet alloys that include terbium and dysprosium have been developed that have higher Curie temperature, allowing them to tolerate higher temperatures.[19] The table below compares the magnetic performance of neodymium magnets with other types of permanent magnets.
| Magnet | Br (T) |
Hci (kA/m) |
BHmax (kJ/m3) |
TC | |
|---|---|---|---|---|---|
| (°C) | (°F) | ||||
| Nd2Fe14B, sintered | 1.0–1.4 | 750–2000 | 200–440 | 310–400 | 590–752 |
| Nd2Fe14B, bonded | 0.6–0.7 | 600–1200 | 60–100 | 310–400 | 590–752 |
| SmCo5, sintered | 0.8–1.1 | 600–2000 | 120–200 | 720 | 1328 |
| Sm(Co, Fe, Cu, Zr)7, sintered | 0.9–1.15 | 450–1300 | 150–240 | 800 | 1472 |
| Alnico, sintered | 0.6–1.4 | 275 | 10–88 | 700–860 | 1292–1580 |
| Sr-ferrite, sintered | 0.2–0.78 | 100–300 | 10–40 | 450 | 842 |
Physical and mechanical properties[edit]
Photomicrograph of NdFeB. The jagged edged regions are the metal crystals, and the stripes within are the magnetic domains.
| Property | Neodymium | Sm-Co |
|---|---|---|
| Remanence (T) | 1–1.5 | 0.8–1.16 |
| Coercivity (MA/m) | 0.875–2.79 | 0.493–2.79 |
| Recoil permeability | 1.05 | 1.05–1.1 |
| Temperature coefficient of remanence (%/K) | −(0.12–0.09) | −(0.05–0.03) |
| Temperature coefficient of coercivity (%/K) | −(0.65–0.40) | −(0.30–0.15) |
| Curie temperature (°C) | 310–370 | 700–850 |
| Density (g/cm3) | 7.3–7.7 | 8.2–8.5 |
| Thermal expansion coefficient, parallel to magnetization (1/K) | (3–4)×10−6 | (5–9)×10−6 |
| Thermal expansion coefficient, perpendicular to magnetization (1/K) | (1–3)×10−6 | (10–13)×10−6 |
| Flexural strength (N/mm2) | 200–400 | 150–180 |
| Compressive strength (N/mm2) | 1000–1100 | 800–1000 |
| Tensile strength (N/mm2) | 80–90 | 35–40 |
| Vickers hardness (HV) | 500–650 | 400–650 |
| Electrical resistivity (Ω·cm) | (110–170)×10−6 | (50–90)×10−6 |
