If a magnetic substance is magnetized in a strong magnetic field, it retains a considerable portion of magnetism after the magnetic force has been withdrawn. The phenomenon of lagging of magnetization or induction flux density behind the magnetizing force is known as magnetic hysteresis.
Let a core of specimen of iron be wound with a number of turns of a wire and current be passed through the solenoid. A magnetic field of intensity H Proportional to the current flowing through the solenoid is produced. Let magnetizing force H is increased from zero to a certain maximum value and then gradually reduced to zero. If the values of flux density B in the core corresponding to various values of magnetizing force H are determined and B-H curves are drawn for increasing and decreasing values of magnetizing force H then it will be observed that B-H curve obtained for decreasing values of H lies above that obtained for increasing values of H.
While decreasing the magnetizing force H, when H is brought to zero the induction density B, is represented by OC and is called as residual magnetism. The power of retaining the residual magnetism is called the retentivity of the material.
Now if the direction of flow of current is reversed, the magnetizing force H is reversed. Let the current be increased in the negative direction until the induction density B becomes zero. At this instant i.e. when B =0, the demagnetizing force H=OD, which is required to neutralize the residual magnetism, and is known as coercive force. If the demagnetizing force H is further increased to the previous maximum value and again gradually decreased to zero, reversed and further increased in the original or positive direction to the maximum value, a closed loop ACDEFGA is obtained which is usually known as hysteresis loop or magnetic cycle.
It is to be noted from the hysteresis loop that B lags behind H. The two never attain zero value simultaneously.
Hysteresis is especially pronounced in materials of high residual magnetism, such as hardened steel. In most cases, hysteresis is a liability as it causes dissipation of heat, waste of energy, and humming due to change in polarity and rotation of element magnets in the material.
Hysteresis loops for hard steel, wrought iron, and cast steel and for alloyed sheet steel are shown in fig 2.
Loop(a) is for hard steel. Due to its high retentivity power and large coercive force, this material is well suited for permanent magnets. Since the area of the hysteresis loop for hard steel is large, hard steel is not suitable for rapid reversals of magnetization. Certain alloys are extremely suitable for permanent magnets.
Loop(b) is for wrought iron and cast steel which rises steeply. Hence these materials have high magnetic permeability and good retentivity, therefore, these materials are suitable for cores of electromagnets.
Loop(c) is for iron, low carbon steel, silicon alloys, permalloy or Mumetal sheets. Since the permeability of these materials is very high and hysteresis losses are very low, these materials are most suitable for transformer cores and armatures, which are subjected to rapid reversals of magnetization. Silicon alloys and permalloy (78.5% Ni; 21% iron with small quantities of copper, molybdenum, chromium, cobalt and manganese etc.) are better for use as compared to iron and low carbon steel.
When a ferromagnetic material is magnetized, its dimensions change slightly, and the sample being magnetized either expands or contracts in the direction of magnetization. This magnetically induced reversible elastic strain ( ) is called magnetostriction and is of the order of 10-6. The energy due to the mechanical stresses created by magnetostriction is called magnetostrictive energy. For iron, the magnetostriction is positive at low fields and negative at high fields.
The cause of magnetostriction is attributed to the change in the bond length between the atoms in a ferromagnetic metal when their electron-spin dipole moments are rotated into alignment during magnetization. The fields of the dipoles may attract or repel each other, leading to the contraction or expansion of the metal during magnetization.
Let us now consider the effect of magnetostriction on the equilibrium configuration of the domain structure of cubic crystalline materials, such as shown in Figs.(a) and (b). Because of the cubic symmetry of the crystals, the formation of triangular-shaped domains, called domains of closure, at the ends of the crystal eliminates the magnetostatic energy associated with an external magnetic field and hence lowers the energy of the material. It might appear that very large domain such as those shown in figs. (a) and (b) would be the lowest energy and most stable configuration since there is minimum wall energy. However, this is not the case since magnetostrictive stresses introduced during magnetization tend to be larger domains. Smaller magnetic domains, such as those in the fig.(c) reduce magnetostrictive stresses but increase domain wall area and energy. Thus, the equilibrium domain configuration is reached when the sum of the magnetostrictive and domain wall energies is a minimum.
In summary, the domain structure formed in ferromagnetic materials is determined by the various contributions of exchange, magnetostatic, magnetocrystalline anisotropic, domain wall, and magnetostrictive energies to its total magnetic energy. The equilibrium or most stable configuration is that for which the total magnetic energy is the lowest.