Magnetic Hysteresis

The relation B=μH holds good for paramagnetic and diamagnetic substances and therefore their B-H graph is a straight line passing through the origin.

B-H graph of para and diamagnetic substances

In ferromagnetic substances, the relation B=μH does not hold i.e. the magnetic induction vector B of the ferromagnetic material does not increase linearly with magnetizing field vector H. A graph that shows the variation of B with H for ferromagnetic materials is called magnetic hysteresis curve.

Magnetic Hysteresis Loop

When a ferromagnetic substance is placed in a magnetizing field, the substance gets magnetized by magnetic induction. The point O means that at the beginning, the substance is completely unmagnetized (B=0) in absence of magnetic field vector. When H is increased from zero, the magnetization and hence, the total field B in the material increases along the curve OA.

A point BS at A is reached when B does not increase anymore with increasing H. This condition is known as magnetic saturation. This condition occurs when all the magnetic domains in the material align in the same direction with H.

When the field H is decreased slowly, the field B does not retrace its original path, but instead, it follows the curve AR. This curve AR shows that when the value of H reduces to zero, the value of B does not become zero but has a value of OR=BR called residual magnetism or remanance or retentivity. Thus, the value of magnetic inductor vector (or intensity of magnetization) for which magnetizing field vector is zero is called residual magnetism. This is the condition for permanency of a magnet i.e. the material becomes a permanent magnet.

When H is reversed, the magnetic moments in the material reorient and B reduces along RC till it becomes zero at C. Thus, when the value of H becomes equal to OC=HC, the value of B becomes zero. OC=HC represents the reverse field (demagnetizing field) required to reduce the residual magnetism to zero (to wipe out magnetism from a ferromagnetic substance) which is called coercivity. Thus, the reverse magnetizing field required to completely demagnetize the ferromagnetic substance is called coercivity or coercive force. It tells how difficult it is to destroy magnetization in the material.

On further increasing H in the same reverse direction beyond OC, B also increases in the reverse direction along CD till the substance becomes saturated at D. As H is decreased, B also decreases and when the value of H becomes zero, the value of B becomes equal to OE. After then, if H is increased in the original direction, B further decreases and becomes zero when the value of H becomes OF. Again if the value of H is increased beyond OF, B goes on increasing until it reaches magnetic saturation at A.

In this way, a closed curve ARCDEFA is obtained for one complete cycle of magnetization. This closed curve is known as magnetic hysteresis loop. Hysteresis means “delayed or coming late“. During the process of applying a magnetizing field by varying its magnitude and direction continuosly, the magnetic induction vector or the intensity of magnetization does not become zero on making the magnetizing field zero but does so a little late. This lagging of magnetic induction vector or intensity of magnetization behind the magnetic field vector in a ferromagnetic material taken through a complete cycle of magnetization is called magnetic hysteresis.

The cycle of operation for a hysteresis loop is called hysteresis cycle. On repeating the process, the same closed curve is obtained again and again but never the portion OA.

Since the magnetization during an increase and a decrease of the field is not the same, there is always a loss of magnetic potential energy in the form of heat and so the temperature of the ferromagnetic material rises. This loss of energy per unit volume of the material per cycle of the magnetizing field is called hysteresis loss. Hysteresis loss is equal to the area of the hysteresis loop if B and H are in their proper units i.e. hysteresis loss per unit volume per cycle. This loss is due to the orientation of the atomic moments in one direction and reorientation in opposite direction in a magnetization cycle.

The shape and size of hysteresis loop are characteristics of each material. A broad hysteresis loop with high values of retentivity and coercive force are called magnetically “hard”. The larger value of HC means that greater work must be done to reduce magnetization in the material. Such materials are suitable for making permanent magnets for speakers and moving coil-meters. This property is observed in steel. The materials like soft iron has narrow hysteresis loop with small value of HC and are called magnetically “soft”. The smaller area of loop means that heat dissipation is minimum. So, soft iron is useful in transformer core as there is less loss of energy and high efficiency. Such materials are also useful in electromagnets, magnetic taps and compact diskette.

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