Soft and Hard magnetic materials and Stoner-Wohlfarth theory
Stoner-Wohlfarth Model
Figure 1 show a tyipical magnetic hysteresis loop of ferromagnetic materials under an external magnetic field
Figure 2 show the magnetic hysteresis loops for both soft and hard magnetic materials
hard magnetic material show low intitial permeability and high coercive force
soft magnetic materials exhibit high initial permeability and low coercivity
use for disk media or permanent magnet
used for transformer or magnetic head
schematic of SW particle
the energy density given by
the hysteresis arises in the field range where two minima are present
switching occurs when
the loop shape of hysteresis is perfectly square when alpha=0
the coercivity is equal to the anisotropy field
an array of non-interacting magnetic particles with a random distribution of anisotropy axes is a crude model for a real polycrystalline magnet and hysteresis loop as shown
the deviations from a linear plot of Mri versus Mrd known as Henkel plot, was pointed by Wohlfarth, and the related is
Soft Magnetic Materials
easy to magnetize and demagnetize and desirable for electromagnets
favor rapid switching of magnetism to applied ac field and have low retentivity
low coercivity and hysteresis losses because the leeser area of the hysteresis loop
high permeability and magnetic saturation
Si-Steel/Soft iron/ Fe-Si alloy is used up to power frequency (50 to 60 Hz), used in power transformer, increases maximum permissible d.c. resistivity by adding 5% Si and reduce the area of hysteresis loop
Fe-Ni alloy used for high-frequency application for 36% Ni, 50% Ni is used for magnetic memory, 77% Ni used for precision voltage and current transformer
Hard Magnetic Materials
also called permanent magnet material which retain their magnetization and difficult to demagnetize
have high retentivity, coercivity, permeability, magnetic saturation and Curie point or temperature
high hysteresis losses because of the larger area of the hysteresis loop
carbon steel used as magnet for latching relays or compass needle
tungsten steel used as magnets in D.C. motors
AlNiCo magnetic properties are very stable with time and temperature