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Magnetic Properties Material
Materials may be classified by their response to externally applied magnetic fields as diamagnetic, paramagnetic, or ferromagnetic. These magnetic responses differ greatly in strength.
The orbital motion of electrons creates tiny atomic current loops, which produce magnetic fields. When an external magnetic field is applied to a material, these current loops will tend to align in such a way as to oppose the applied field. This may be viewed as an atomic version of Lenz’s law: induced magnetic fields tend to oppose the change which created them. Materials in which this effect is the only magnetic response are called diamagnetic.
All materials are inherently diamagnetic, but if the atoms have some net magnetic moment, these stronger effects are always dominant.
Any conductor will show a strong diamagnetic effect in the presence of changing magnetic fields because circulating currents will be generated in the conductor to oppose the magnetic field changes. A superconductor will be a perfect diamagnetic since there is no resistance to the forming of the current loops.
Some materials exhibit a magnetization which is proportional to the applied magnetic field in which the material is placed. These materials are said to be paramagnetic and follow Curie’s law.
All atoms have inherent sources of magnetism because electron spin contributes a magnetic moment and electron orbits act as current loops which produce a magnetic field. In most materials the magnetic moments of the electrons cancel, but in materials which are classified as paramagnetic, the cancelation is incomplete.
Ferromagnetic is a materials which exhibit a long-range ordering phenomenon at the atomic level which causes the unpaired electron spins to line up parallel with each other in a region (called a domain). Sizes of domains range from a 0.1 mm to a few mm, volumes of domains is about 10-12 to 10-8 m3 and contain 1017 to 1021 atoms (3). The boundaries between the various domains having different orientations are called domain walls (3). The long-range order which creates magnetic domains in ferromagnetic materials arises from a quantum mechanical interaction at the atomic level. This interaction is remarkable in that it locks the magnetic moments of neighboring atoms into a rigid parallel order over a large number of atoms in spite of the thermal agitation which tends to randomize any atomic-level order (2). All ferromagnetic have a maximum temperature where the ferromagnetic property disappears as a result of thermal agitation. This temperature is called the Curie temperature.
The examples of ferromagnetic materials: Iron, nickel, cobalt and some of the rare earths (gadolinium, dysprosium). Samarium and neodymium in alloys with cobalt have been used to fabricate very strong rare-earth magnets.
In the ferromagnetism materials, a small externally imposed magnetic field (as an example from a solenoid), can cause the magnetic domains to line up with each other. In this condition, the material is said to be magnetized. For an example in iron, a magnetic field of about 1 T can be produced in annealed iron with an external field of about 0.0002 T, a multiplication of the external field by a factor of 5000. The Curie temperature of iron is about 1043 K (2).
2. Ohanian, Hans, Physics, 2nd Ed. Expanded, Norton, 1985
3. Halliday, Resnick – Fundamentals of Physics.