Diamagnetic materials are repelled by a magnetic field ; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagnetic materials are attracted by a magnetic field. Diamagnetism is a quantum mechanical effect that occurs in all materials; when it is the only contribution to the magnetism, the material is called diamagnetic. In paramagnetic and ferromagnetic substances, the weak diamagnetic force is overcome by the attractive force of magnetic dipoles in the material.
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The complete description of magnetic moment for a free atom incorporates the atomic angular moment, electron spin, and diamagnetic response. Essentially, diamagnetic behavior is the change in orbital angular momentum induced by an external magnetic field . All materials exhibit a diamagnetic response, and it may be understood as the attempt to expel the applied magnetic field. Typically, the diamagnetic susceptibility for a material is negative and on the order of 10 -6 , overwhelmed by other magnetic behavior such as in antiferromagnetism , if present .
Paul Langevin proposed a classical-based model of diamagnetism. Because this is a classical theory, it is an approximation, but its results give good agreement with experimental results . Considering an electron moving in a loop, the induced magnetic moment is the product of the current in the loop and the area it encloses, or.
Taking this as a model for a single orbiting electron, if exposed to the presence of an external magnetic field, the resulting change in the electron acceleration would induce a change in the magnetic moment. The acceleration can be quantified as. Where the acceleration is equal to force per unit mass, which is the electric field strength times an electric charge per electron mass. Now apply Lenz's law and see that an emf is created to counteract the change in flux of the loop per unit length.
The diamagnetic response of a material has a measurable contribution to the materials' magnetization only if there are no other magnetic effects present, such as Ferrimagnetism whose susceptibility is much larger in most cases . For this reason, we classify only materials whose net magnetization is diamagnetic, as a diamagnet.
This requires that compound to have empty or closed valence shell. The inert noble gases have filled valence shells and thus respond diamagnetically. Substances like silicon, germanium, most covalent solids and polymers also exhibit diamagnetic behavior . Diamagnetism arises in metals when the paramagentic behavior is sufficiently small. For example, examine beryllium. It has no contribution from ferro, ferri, or antiferromagnetism, so we check its paramagnetic contribution.
A single atom of beryllium has paired 1s and 2s electrons. However, in a crystal lattice, the 2s electron populate the bottom of the empty 2p band because of band overlap see: Band Theory of Metals and Insulators. This makes the density of states at the Fermi level very low, thus the paramagnetic susceptibility is much smaller than any diamagnetic contribution .
Landau set the framework for diamagnetic calculations of atoms in a lattice, see  for further reading. Because diamagnetism is essentially the expelling of magnetic fields within a material, strong diamagnetic materials can be levitated, or if they are sufficiently strong and enough area, can levitate magnets.
Langevin Theory of Diamagnetism Paul Langevin proposed a classical-based model of diamagnetism. Diamagnetic Materials The diamagnetic response of a material has a measurable contribution to the materials' magnetization only if there are no other magnetic effects present, such as Ferrimagnetism whose susceptibility is much larger in most cases .
Applications Because diamagnetism is essentially the expelling of magnetic fields within a material, strong diamagnetic materials can be levitated, or if they are sufficiently strong and enough area, can levitate magnets. They are perfect diamagnets. Questions Bismuth is heated from K to K, what is the change in diamagnetic susceptibility? Can you calculate the diamagnetic susceptibility of single crystal HCP titanium using the classical Langevin model?
Explain why or why not.. What is the ratio of magnetization to applied field for a YBCO crystal behaving as a superconductor at 77 K? Why would doping a piece of silicon change its bulk magnetic susceptibility? Now what happens if we vary the temperature? Answers 0.
From equation 1 we see that the diamagnetic susceptibility has no dependence on temperature, so heating a material will not change its diamagnetic susceptibility. The derivation of Langevin's susceptibility relies on the assumption that the material has a classically bound electron rotating around an atom to create the magnetic moment , however, metals do not have localized electrons.
Therefore, the substitutability cannot be determined using this theory. The susceptibility depends on the number of contributing electrons surrounding an atom, Z. Doping silicon introduces atoms that have different valencies and thus changes the overall susceptibility contribution of atoms in the material.
By heating the material through different temperature regimes the electrons bound to their atoms can be freed, ionizing the donor atoms, such as in n-type silicon. The freed electrons populate the material at different temperatures see: Extrinsic Semiconductors making the susceptibility vary with temperature. References N. Cambridge University Press, S.
Kasap, Principles of Electronic Materials and Devices. McGraw-Hill, R. Hummel, Electronic Properties of Materials. Springer New York, Askerov, S. Figarova, M. Makhmudov, and V.
Relations between the Weber-Langevin theory and that of Pauli. The first theory gives a band for the Zeeman effect; the second, which is based on Larmor precession, gives sharp lines, as is known. The susceptibilities, K 1 and K 2 , are different except when the orbits are normal to the intensity H of the magnetic field. When they are parallel to H , K 1 vanishes and K 2 is half that for the normal orbits, an extreme case.
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