FERROMAGNETISM

Ferro-magnetism is very commonly used type of magnetism. Ferromagnetism is found on the Bar magnets, compass needles, and the coating on magnetic hard disks. It is a fine property of iron, cobalt, and compounds of these few other metals which includes nickel, chromium and magnesium. All these stated materials shows ferromagnetic properties because the spins of the electrons cancel out much less than in other materials. Groups of spinning electrons align readily with an external magnetic field, thus forming magnetic domains. This alignment is permanent so that the material becomes magnetized, & remains so, even when we remove the external field.

By placing a small piece of ferromagnetic material in a magnetic field, it gets magnetized. The stronger the field, the more domains align themselves with it & the the material become magnetized more strongly. This principle is the basis of the hard disk drive that works in a computer. The relationship between the strength of the external field (H) and the magnetic flux density in the material(B) is demonstrated in the graph shown below.

out and further increase of H does not cause any increase in B. The material is saturated.If H is steadily reduced, B will fall too. But when H is reduced to “0”, B still sands greater than zero. The material now has become magnetised.Some of the magnetic field is still present (it means,some of the domains are permanently aligned). This remaining field is the remanence.

If the field is then reversed & made to increase in the reverse direction, B falls and finally becomes saturated in the reverse direction. From that state,returning the field through zero and into the original direction will restore the original saturation. The effect is that changes in B lag behind changes in H. This lagging behind is known as hysteresis.