Magnetic materials

A bar magnet strongly attracts an iron piece, but other materials are weakly attracted and some are actually repelled. We may use this response of material to the field of a bar magnet to broadly classify magnetic materials. All magnetic are classified into three categories. These are: diamagnetic, paramagnetic and ferromagnetic materials. The classification depends on the magnetic dipole moment of atoms of the material and on the interactions among the atoms. When the different magnetic materials are placed in a uniform magnetic field, the field lines are changed as shown in figure.

1.       Diamagnetic material: the diamagnetic materials are those substances which are feebly magnetized in the direction opposite to the applied field. So, they are weakly repelled by magnets as shown in figure. Examples of diamagnetic material are bismuth, copper, water, alcohol, mercury etc. the magnetic moment of atoms of a diamagnetic material is zero. But they acquire induced dipole moments when the material is placed in an external magnetic field. These moments, however, are opposite in direction to the applied field. So, the magnetization in a diamagnetic material always opposes the applied field. They are repelled by magnets. The diamagnetic materials move from a stronger to a weaker field. These materials are independent of temperature.

2.       Paramagnetic material: the paramagnetic materials have atoms that have permanent magnetic moments. These moments interact weakly with each other and randomly orient in different directions. When an external magnetic field is applied to the material, its atomic moments tend to line up with the field. The magnetic field inside it is the sum of the applied field and the induced field due to magnetization. These are found in solid liquid and gas. A paramagnetic rod, freely suspended in a magnetic field. The paramagnetic materials are temperature dependent and follow curie law.

3.       Ferromagnetic material: the ferromagnetic materials are highly magnetized in a magnetic field. The examples of ferromagnetic materials are iron, nickel and cobalt, and their alloys such as alnico. Godolinium and dysprosium are ferromagnetic at low temperature, and compounds such as CrO₂ used in a magnetic tap recording are also ferromagnetic materials though though neither chromium nor oxygen is ferromagnetic. They are highly attracted by magnets. The magnetic susceptibility is positive and very high, and varies with applied field.