Magnetism

Magnetism is a class of physical phenomena that are mediated by magnetic fields. In particular, for stationary phenomena (not variable over time) we speak of magnetostatics (which has some formal analogies with electrostatics when the densities of electric current are replaced by the distributions of electric charge). For time-dependent phenomena, however, the electric and magnetic fields influence each other and it is necessary to resort to a unified description of the two fields obtained in 1864 by the British scientist James Clerk Maxwell within the theory of classical electromagnetism or classical electrodynamics.

The Bohr–van Leeuwen theorem, discovered in the 1910s, showed that classical physics theories are unable to account for any form of magnetism. Magnetism is now regarded as a purely quantum mechanical effect. One of the fundamental properties of an electron (besides that it carries charge) is that it has a magnetic dipole moment, i.e., it behaves like a tiny magnet, producing a magnetic field. This dipole moment comes from the more fundamental property of the electron that it has quantum mechanical spin. Due to its quantum nature, the spin of the electron can be in one of only two states; with the magnetic field either pointing “up” or “down” (for any choice of up and down). The spin of the electrons in atoms is the main source of magnetism, although there is also a contribution from the orbital angular momentum of the electron about the nucleus. When these magnetic dipoles in a piece of matter are aligned, (point in the same direction) their individually tiny magnetic fields add together to create a much larger macroscopic field.

However, materials made of atoms with filled electron shells have a total dipole moment of zero: because the electrons all exist in pairs with opposite spin, every electron’s magnetic moment is canceled by the opposite moment of the second electron in the pair.

Types of magnetism

Diamagnetism

Diamagnetism is a form of magnetism that some materials show in the presence of a magnetic field. Diamagnetic materials are characterized by the fact that magnetization has the opposite direction with respect to the magnetic field; therefore these materials are weakly “rejected.” This is a very weak quantum effect, which is voided if the material has other magnetic properties such as ferromagnetism or paramagnetism. In the non-scientific field, diamagnetic materials are often simply called non-magnetic.

Paramagnetism

Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. Paramagnetic materials are characterized at the atomic level by magnetic dipoles that align with the applied magnetic field, being weakly attracted to it.

Ferromagnetism

Ferromagnetism is the property of some materials, called ferromagnetic materials, to magnetize very intensely under the action of an external magnetic field and to remain magnetized for a long time when the field is canceled, thus becoming magnets. This property is maintained only below a certain temperature, called Curie temperature, above which the material behaves like a paramagnetic material. For iron, for example, this temperature is around 770 °C.

The magnetization of a ferromagnetic material can occur naturally or artificially, subjecting the material to a magnetic field. Natural ferromagnetic materials are, for example, magnetite, iron, cobalt, nickel and some transition metals. In ferromagnetic materials, the relative magnetic permeability of the material is not constant with the variation of the field, as instead occurs in diamagnetic materials and paramagnetic materials: the relationship between the magnetic induction field and the magnetic field is therefore not linear, nor even unique.

Ferromagnetic properties have a quantum origin and depend on the electronic structure of the materials and their crystalline structure. Ferromagnetism arises due to two effects from quantum mechanics: spin and the Pauli exclusion principle. Only atoms with partially filled shells (i.e., unpaired spins) can have a net magnetic moment, so ferromagnetism occurs only in materials with partially filled shells. Because of Hund’s rules, the first few electrons in a shell tend to have the same spin, thereby increasing the total dipole moment. These unpaired dipoles (often called simply “spins” even though they also generally include orbital angular momentum) tend to align in parallel to an external magnetic field, an effect called paramagnetism. Ferromagnetism involves an additional phenomenon, however: in a few substances, the dipoles tend to align spontaneously, giving rise to a spontaneous magnetization, even when there is no applied field.

Antiferromagnetism

Antiferromagnetism is a characteristic property of some materials such as manganese, chromium, hematite, oxides MnO2, FeO, CoO, etc. (called antiferromagnetic materials); in these materials, contrary to what happens for ferromagnetic materials (in which the configuration of minimum energy occurs for parallel spins), the interaction between the atoms is such as to create a configuration of minimum energy when the spins are antiparallel. In other words, materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring spins (on different sublattices) pointing in opposite directions.

Due to defects in the atomic structure, the antiparallel configuration is never perfectly respected, and therefore a small residual magnetic moment is generated; this phenomenon is called parasitic ferromagnetism.

Substances that exhibit the phenomenon of antiferromagnetism, therefore, do not have magnetic properties since, although there is a very high degree of polarization, the two opposite polarizations neutralize each other. However, above a certain value of temperature, called Neel temperature, the alignment of the dipoles suddenly disappears and the substances in question have paramagnetic properties.

Ferrimagnetism

Ferrimagnetism is a type of permanent magnetism that occurs in some crystals when the magnetic moments of nearby ions tend to align antiparallel: it is, therefore, a type of antiferromagnetism; this situation occurs mainly in compounds known as ferrites. The term ferrimagnetism was originally proposed by Néel to describe the magnetic ordering phenomena in ferrites, in which iron (Fe) ions appear in two different ionic states and hence bear different magnetic moments with mutual antiferromagnetic coupling.

The simplest magnetic ferrite variety may be represented by the chemical formula MOFe2O3, where M is a divalent metal ion. The crystalline structure shows two possible arrangements of metal ions: in one the metal ion is in the tetrahedral position, which is surrounded by four oxygen ions; in the other, the metal ion is in octahedral position, that is surrounded by six oxygen ions.

ferrimagnetic material is one in which the magnetic moments of the atoms in different sub-lattices are antiparallel, as in the antiferromagnetic materials; however, in ferrimagnetic materials, since the antiparallel moments are not the same in modulus, a magnetic moment results which is not null and therefore the material has a spontaneous magnetization, high resistivity, and anisotropic properties.

All ferrimagnetic materials contain various types of magnetic ions, or various crystallographic positions for the magnetic ion, or a combination of the two cases. The magnetic behavior of single crystals of ferrimagnetic materials can be attributed to the parallel alignment; the magnetic force of these materials kept by the dilution effect of the atoms in the antiparallel arrangement is generally less than that of purely ferromagnetic solids such as metallic iron.

Ferrimagnetic materials, such as ferromagnetic ones, hold a spontaneous magnetization below the Curie temperature and show no magnetic order (are paramagnetic) above this temperature. However, there is sometimes a temperature below the Curie temperature, at which the two opposing moments are equal, resulting in a net magnetic moment of zero; this is called the magnetization compensation point. This compensation point is observed easily in garnets and rare-earth–transition-metal alloys (RE-TM). Furthermore, ferrimagnets may also have an angular momentum compensation point, at which the net angular momentum vanishes. This compensation point is a crucial point for achieving high-speed magnetization reversal in magnetic memory devices.

Ferrimagnetic materials are widely used in non-volatile memory devices such as hard drives, which utilize their ability to easily switch the spins of electrons and be magnetized. When a ferrimagnet inside a coil of conducting wire is rotated, the current is generated, so they are also widely used in power motors and generators. Because ferrimagnets are electrically insulating, they are also widely used in high-frequency devices because no eddy currents are induced under AC fields.

Superparamagnetism

Metamagnetism

Molecule-based magnets

Single-molecule magnet

Spin glass

Magnet

magnet is a material or object that produces a magnetic field. The name derives from the Greek μαγνήτης λίθος (magnétes líthos), that is “Magnesia stone,” from the name of a place in Asia Minor, known since ancient times for the huge deposits of magnetite.

The magnet, made in the shape of a horseshoe, has the two magnetic poles close together. This shape creates a strong magnetic field between the poles, allowing the magnet to pick up a heavy piece of iron.

A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. The materials that can be magnetized are also those strongly attracted to a magnet, and are called ferromagnetic (or ferrimagnetic); these include iron, nickel, cobalt, some rare earth alloys and some natural minerals such as magnetite. Even if ferromagnetic (and ferrimagnetic) materials are the only ones attracted by a magnet so intensely as to be commonly considered “magnetic”, all substances weakly respond to a magnetic field, through one of the numerous types of magnetism.

References

  1. C. D. Stanciu, A. V. Kimel, F. Hansteen, A. Tsukamoto, A. Itoh, A. Kirilyuk, and Th. Rasing, Ultrafast spin dynamics across compensation points in ferrimagnetic GdFeCo: The role of angular momentum compensation, Phys. Rev. B 73, 220402(R) (2006).
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