Magnetic material is mainly classified as permanent ferromagnetic material and soft ferromagnetic material. Permanent ferromagnetic material is also called hard ferromagnetic material; the material can keep the strong magnetic force after applying an external magnetic field. The characteristic is high coercivity (Hc). Typical coercivity is Hc>103A/m, BHmax is high. Soft ferromagnetic material will be magnetized after applying an external magnetic field but it will lose the magnetics force easily. Characteristic is low coercivity, typical Hc<103A/m.
Characteristic and classification of Permanent (Hard) ferromagnetic materials
4 main characteristics of Permanent ferromagnetic material
1. High BHmax BHmax is the measurement of the storage and usable magnetic flux density
2. High Hc measures the resistance of a ferromagnetic material to becoming demagnetized
3. High Br and high Mr – they are the measurement of magnetic field inside air gap of the permanent magnet
4. High stability the stability against external magnetic interference, humidity and environmental changes.
Characteristics and classification of Soft ferromagnetic materials
Characteristics of soft ferromagnetic materials
1. Low Hc that indicates soft ferromagnetic material is easily magnetized or demagnetized by external magnetic field. That means core loss is low too
2. High Bs and high Ms that can achieve high permeability (u) and low Hc, this can also increase magnetic flux density.
3. Low magnetic loss and low electric loss – this requires low Hc and high electrical resistivity
4. High Stability – High stability against changes of humidity, vibration and environment
Classification of soft ferromagnetic materials
1. Ferrite a series if iron oxide and other oxide compound (or called ceramic materials), characteristic is low saturation induction (below 0.5T), high permeability, high electrical resistivity, and used in high frequency application. Typical there are Mn-Zn cores, Ni-Zn cores and Mg-Zn cores.
2. Metallic soft ferromagnetic materials – it has higher saturation induction and lower Hc when compared with ferrite material. Mainly has iron series like industrial pure iron, iron powder cores; ironnickel series like MPP cores and High Flux cores, Iron silicon series like sendust core, Fe-SiTM and silicon steel sheet.
3. Amorphous and Nano crystal materials – A newly developed material in late 20th century
Main characteristic of metallic powder core
Metallic powder cores are made of ferromagnetic powder, coated by insulation material, then mix with organic or inorganic bonding material and press into a soft ferromagnetic material. The metallic particles are small and coated by insulation material so it can prevent eddy current and the material can be used in higher frequency. On the other hand, the effect of isolated particles causes the material a low permeability and inherent distribution air gap characteristics. It is suitable for energy storing inductor application. Due to the small size of powder particles, it can prevent skin effect; the change of permeability versus frequency is also stable. The electromagnetic characteristic of powder core is decided by the powder’s permeability, size and shape of the powder, the filling coefficient, insulation content, forming pressure and heat treatment technology.
Due to the distributed air-gap, magnetic powder core has soft saturation characteristic. The permeability will drop in a predictable way when DC bias increase. This is a big advantage against the hard saturation in ferrite materials.