Magnetism

Magnetism is a class of physical phenomena that are mediated by magnetic fields, due to the movement of electric charges, from which derives the property of some materials (natural or artificial magnets) to attract and hold iron and ferromagnetic materials. 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 first observations on magnetism are owed to the Greeks, who established that an iron mineral, magnetite, has the property of attracting iron filings, especially in certain points of its surface. The observation remained for a long time completely isolated and served, especially in the Middle Ages and Renaissance, to accredit animistic and magical conceptions of the world. Starting from the XI century they tried to explain the behavior of the magnetic needle and especially the phenomenon of magnetic declination, discovered in the late fifteenth century, which seemed connected to the important problem of determining longitude. A summary treatment of the knowledge of this period on magnetism, with some interesting new elements, is found in the seventh book of “Magia naturalis” (Magiae naturalis sive de miraculis rerum naturalium) by Della Porta, 1589, where he speaks of the shielding effect of metal plates, the barbs of filings formed around the poles of the magnet, the demagnetization resulting from strong heating, etc.

At the beginning of XVII century the English W. Gilbert began to study magnetic phenomena with a quite homogeneous experimental method, conducting experiments with a magnetic needle and a terrella (spherical magnet), and came to note a profound analogy with the Earth, “the great magnet”. Although in his De magnete (1600) the explanation given to these phenomena is still in animistic terms, the way to a rigorous and controlled treatment is now open. Subsequently analogies were found with electrical phenomena, but the differences appeared immediately clear: the polarities of the magnet cannot be separated by successive breaking of the magnet itself (experience of the broken magnet). In the XVIIIth century the problem of controlling quantitatively the forces that the poles of magnets exert between them was posed: C. A. Coulomb recognizes them, in analogy with what he had done for electrical forces, as Newtonian forces (Coulomb’s magnetostatic law). The problem of the relationship with electricity, strong of this new similarity, is strongly proposed: in the following century magnetism is increasingly analyzed in close connection with electrical phenomena. From this moment on its history is identified with that of electromagnetism.

Magnetic phenomena are explained at the level of elementary atomic currents hypothesized by A.-M. Ampère in 1828. These currents are the object of study of solid state physics that finds an adequate setting only in quantum physics. Classical physics is able to give only some qualitative descriptions of the corresponding phenomena and this limitation makes impossible an exhaustive treatment of magnetism at elementary level; moreover, the complexity of many magnetic phenomena has not yet allowed to find an adequate theoretical framework.

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.

Terrestrial magnetism

The phenomena related to terrestrial magnetism, already known in ancient times, found practical application only in the Middle Ages with the construction of the first compasses; in a Chinese manuscript of the twelfth century is already noted in this regard that the direction indicated by the magnet is shifted by a few degrees with respect to the geographic north. This deviation, called magnetic declination, was confirmed by Christopher Columbus in his travels across the Atlantic; later Hartmann observed (1544) the inclination of magnetic needles with respect to the horizontal plane. The naturalists of the time explained the terrestrial magnetism as a result of a strong concentration of magnetic minerals located in an unspecified location of the N; the first scientific theory on terrestrial magnetism was enunciated only in 1600 by W. Gilbert in De magnete.

This hypothesis was resumed and improved by K. F. Gauss with whom begins the physical-mathematical study of the Earth’s magnetic field. To these researches brought decisive contributions E. Gunter, W. E. Weber, E. Halley, and later A. Schmidt, S. J. Barnett, J. A. Bartels, L. Bauer. The presence of the Earth’s magnetic field is revealed by many phenomena directly observable; among these the most notable are the orienting action that magnetized bodies undergo, the magnetism induced in ferromagnetic materials, the capture by the Earth of electrically charged particles from outer space.

The elements that define the Earth’s magnetic field for each point on the Earth’s surface are the field strength vector, the magnetic declination, the magnetic inclination and the components of the field vector in the horizontal and vertical planes. To define the field at a point three elements are sufficient: usually are considered the inclination, the declination and the horizontal or vertical component; or are used the three components of the field vector according to an orthogonal Cartesian reference system with origin in the point under examination.

The analysis of the values of the magnetic elements conducted with appropriate mathematical methods, introduced by Gauss, has allowed to establish that the origin of the Earth’s magnetic field is almost entirely (96%) internal. The remaining value, called residual field, is due to the contribution of magnetic anomalies, of electrical exchanges between atmosphere and earth surface, and of swarms of charged particles coming from space, especially from the Sun. Considering only the value of the preponderant field due to internal causes, it can be considered as generated by a magnetic dipole located at the center of the Earth and inclined 11º30′ with respect to the Earth’s axis. The points where the Earth’s diameter coinciding with the direction of the dipole meets the Earth’s surface are called geomagnetic poles: geomagnetic axis is the above mentioned Earth’s diameter and geomagnetic equator is the maximum circle perpendicular to this axis and with center in that of the dipole.

The pole located in the northern hemisphere, conventionally indicated with B (boreal), has negative polarity and is located at 78º30’N, 69ºW, while the other, indicated with A (austral), is positive, with position 78º30’S, 111ºE. A better approximation, with the actual distribution of the field, is obtained by imagining the dipole not at the center of the Earth but somewhat shifted even with the same direction and magnetic moment. The effect due to the residual field means that the two points where the magnetic inclination is 90º do not coincide with the geomagnetic poles; these points define the magnetic poles: the one indicated by the N pole of the compass needle is located in the Canadian Arctic Archipelago, while the S pole is located in Antarctica; in 1968 their positions were respectively 76ºN, 101ºW and 67ºS, 142ºE. The intensity of the magnetic field, always assuming a uniformly magnetized Earth, grows regularly from a minimum of 0.28 Oersted to a maximum of 0.71 Oersted at the poles. Because of anomalies the course of the field is actually very irregular: equator and magnetic meridians are twisted lines. The field also undergoes oscillations in time, varying from place to place, in relation to cosmic and solar factors and internal causes related to the origin of the magnetic field itself. Variations in time can be completely irregular or manifest in a periodic way.

Long-period (secular) variations appear to be due to changes in the Earth’s rotational velocity that cause drift movements of the Earth’s fluid core mass. Short-period variations (diurnal, annual) are functions of latitude and the height of the Sun on the horizon: at middle latitudes, for example, the magnetic field vector describes an ellipse during the day following the movement of the Sun.

Other periodic oscillations are those that follow the sunspot cycle. Directly connected with solar activity are also the irregular variations, those produced by magnetic storms in the upper atmosphere. In relation to secular variations of the field there is the problem of the displacement of the poles, a phenomenon that is well documented by observations carried out in the twentieth century, but that certainly has occurred even in ancient times as evidenced by the fossil magnetism of certain rocks, even the polarity of the field has been reversed several times during geological eras.

The nature and origin of the Earth’s magnetic field are still poorly known. From paleomagnetic evidence it is known that the Earth’s magnetic field exists since at least three billion years. It is mainly dipolar, with fairly constant characteristics: the magnetic axis in the course of time has undergone a maximum oscillation of 11º, the intensity is little variable, although many important phenomena of polarity reversal have been observed. This main magnetic field is accompanied by a minor magnetic field, not dipolar, much more variable. All these features explain why it is so difficult to find a mechanism to explain all the observed geomagnetic phenomena.

However, the observation that the magnetic field of the Earth can not be traced back to the presence of geological layers with permanent magnetism (since no known material is), and that it shows the repetition of “reversals” of polarity in cycles of 1000-2000 years (negligible entity compared to the typical duration of geological eras), leads to believe that its source is located in depth and that the cause of its existence is the establishment of electric currents of adequate intensity and trajectory. On intuition of E. C. Bullard (1948), has been successful among geologists and, more generally, among planetologists, the so-called “self-excited dynamo” model.

If a magnetic field, even a weak one, pre-exists with direction concordant with the axis, it is able to arouse an electromotive force between the periphery and the axis of the disk and to produce a current displacement. If the axis and the rim of the disk were electrically connected according to the scheme, the circuit would become home to its own induced magnetic field that – depending on the polarity – would be added to or subtracted from the external one. This mechanism could play a real function inside celestial bodies massive enough and mineralogically differentiated enough to have, as the Earth, an internal nucleus in the molten state, and to be equipped with axial rotation motion.

Satisfied these conditions, it would be the melting materials to act as conducting substance, while their convective motions would establish, together with the general rotation, the displacements responsible for the onset of the magnetic field and its variations. In fact, as space probes have ascertained, planets with large masses and with rapid rotation have been shown to possess fields of significant intensity. A mechanism of the self-excited dynamo type could also provide an explanation for the eleven-year cycle of the Sun’s activity, as well as for several properties of so-called “magnetic stars”.

For the trend of magnetic declination and inclination in addition to the corresponding entries, see isogonic line and isoclinic line; for the examination of variations of magnetic elements, see isopor, for their cartographic representation.

Types of magnetism

  • Diamagnetism
  • Paramagnetism and superparamagnetism
  • Ferromagnetism
  • Antiferromagnetism
  • Ferrimagnetism
  • Metamagnetism
  • Molecule-based magnets and single-molecule magnet
  • Spin glass

Related keywords

  • Magnet

Bibliography

  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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