Quantum mechanics-The key to understanding magnetism

Abstract

The existence of magnetic materials has been known almost since prehistoric times, but only in the 20th century has it been understood how and why the magnetic susceptibility is influenced by chemical composition or crystallographic structure. In the 19th century the pioneer work of Oersted, Ampere, Faraday and Joseph Henry revealed the intimate connection between electricity and magnetism. Maxwell's classical field equations paved the way for the wireless telegraph and the radio. At the turn of the present century Zeeman and Lorentz received the second Nobel Prize in physics for respectively observing and explaining in terms of classical theory the so-called normal Zeeman effect. The other outstanding early attempt to understand magnetism at the atomic level was provided by the semi-empirical theories of Langevin and Weiss. To account for paramagnetism, Langevin (1) in 1905 assumed in a purely ad hoc fashion that an atomic or molecular magnet carried a permanent moment µ, whose spatial distribution was determined by the Boltzmann factor. It seems today almost incredible that this elegantly simple idea had not occurred earlier to some other physicist inasmuch as Boltzmann had developed his celebrated statistics over a quarter of a century earlier. With the Langevin model, the average magnetization resulting from N elementary magnetic dipoles of strength µ in a field H is given by the expression (1) At ordinary temperatures and field strengths, the argument x of the Langevin function can be treated as small compared with unity. Then L(x) = :x, and Eq. (1) becomes perature, a relation observed experimentally for oxygen ten years earlier by Pierre Curie (2) and hence termed Curie's law.To explain diamagnetism, Langevin took into account the Larmor precession of the electrons about the magnetic field, and the resulting formula for the diamagnetic susceptibility is

Vibration Induced Electric Dipole in a Weakly Bound Molecular Complex

“…(4)] was not null. In this interpretation, the susceptibility is independent of the temperature, because h 2 i T is roughly proportional to T. This is different from the case discussed by Van Vleck where the susceptibility is independent of the temperature when the energy distances E between the ground state and the excited states are large as compared to kT [14,17]. Corrections to the susceptibility due to the distortion of the molecule have already been observed for simple polar molecules but, in most systems, these distortions induce small corrections to the dipole and the dominant term in the susceptibility remains proportional to 1=T [13,18].…”

Theoretical study of the magnetic ordering in rare-earth compounds with face-centered-cubic structure

“…2 Their work suggested that the 4f electrons responsible for the magnetism of the rare earth are sequestered in the interior of the atom, and so experience only a small crystalline field. 3 Thus, they explained why Hund's rule with L-S coupling could give values of the magnetic moments of rare-earth ions very close to experimental observations, at least in the high-temperature region. However, understanding the magnetic ordering of the rare earths is still a challenging problem to the magnetists.…”

Evaluating the Curie-Weiss Temperature of a Magnetic System Composed of Nonequivalent Magnetic Ions in Terms of Spin Exchange Constants

“…Magnetic properties of a system containing magnetic ions are related to its electronic structure, [1][2][3][4] and are commonly probed by measuring the temperature-dependence of its magnetic susceptibility χ(T) at a given magnetic field, the field-dependence of its magnetization M(H) at a very low temperature, or the magnetic specific heat as a function of temperature. When fitted with a Curie-Weiss law, the paramagnetic region of the χ(T) vs. T plot leads to the CurieWeiss temperature θ and the effective magnetic moment μ eff .…”