- Magnetic Properties Definition: The magnetic properties of engineering materials determine their suitability for magnetic applications.
- Permeability: Permeability is how easily a material supports magnetic flux, crucial for electrical machine cores.
- Retentivity: Retentivity is a material’s ability to retain magnetization after the external magnetic field is removed.
- Coercive Force: Coercive force is the required magnetic field to eliminate residual magnetism in a material.
- Reluctance: Reluctance resists magnetic flux buildup in a material and is measured in Ampere-turns per Weber.
Engineers compare the magnetic properties of materials under defined field, frequency, temperature and magnetic-history conditions. Those results help match a material to a transformer, rotating machine, sensor, inductor or permanent-magnet application. This page covers four related magnetic properties of engineering materials: permeability, retentivity, coercive force and reluctance.
- Permeability
- Retentivity or Magnetic Hysteresis
- Coercive force
- Reluctance
Permeability
Permeability describes the relationship between magnetic flux density and magnetic field strength in a magnetic material. A higher permeability can produce more magnetic flux for a given magnetic-circuit geometry and magnetizing current. Permeability differs from magnetic susceptibility, which relates magnetization M to magnetic field strength H.
For a linear, isotropic material, permeability is the ratio of magnetic flux density B to magnetic field strength H. Ferromagnetic materials are nonlinear and hysteretic, so their quoted permeability must identify the measurement condition.
The linear relation is μ = B/H.
B is magnetic flux density in teslas (T), equivalent to Wb/m2.
H is magnetic field strength in amperes per metre (A/m).
The SI unit of magnetic permeability is the henry per metre (H/m).
Absolute and relative permeability are related by μ = μ0 μr.
Here, µ0 is the permeability of vacuum. Its value is approximately μ0 = 4π × 10-7 H/m under the revised SI, while µr is the dimensionless relative permeability. For vacuum, µr = 1; air is close to 1 under ordinary conditions.
Soft magnetic cores usually need high permeability to reduce the magnetizing ampere-turns, but designers must also check saturation flux density, core loss, coercivity, frequency and temperature.
Retentivity
A ferromagnetic material can retain magnetization after an applied field returns to zero. Retentivity describes this ability, while remanence or remanent flux density gives a measured value for a specified magnetic history. On a saturated hysteresis loop, Br marks the flux density remaining at H = 0. A partial loop or a different prior field can produce a different remanent value.
Coercive Force
After magnetization, an opposing magnetic field is needed to bring magnetization or flux density back to zero. Coercivity is the magnitude of that field under a specified hysteresis test, measured in amperes per metre rather than ampere-turns. The zero crossing used must be stated because intrinsic coercivity and normal coercivity are different quantities. On the shown hysteresis loop, -Hc represents the opposing coercive field.
Magnetically hard materials have high coercivity and suit permanent magnets. Magnetically soft materials have low coercivity and a narrow hysteresis loop, which reduces reversal loss in transformer and machine cores. Remanence is a separate property and need not track coercivity in a fixed proportion.
Reluctance
Reluctance describes a complete magnetic circuit’s opposition to magnetic flux. It depends on path length l, cross-sectional area A and permeability μ, so it is not an intrinsic material property. Its unit is ampere-turns per weber (At/Wb), equivalent to inverse henries.
For a uniform section, reluctance is ℜ = l/(μA). Series sections contribute separate reluctances, and air gaps often dominate the total.
A hard magnetic material is generally selected for permanent magnets, not low-loss machine cores. A soft magnetic material with high permeability and low coercivity is normally used for an alternating-flux core.
