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Magnetic Materials

Magnetism has captivated humans since remote times. Magnet’s ability to attract ferrometals fascinated ancient Greeks, Chinese and Amer-Indians. The history of magnetism is rich and intrinsically attached to the main course of major discoveries in Science. [1]

Lodestone (ferrite, Fe3O4) was described around 600 BC by Greek philosophers. The first reference to a Compass was made in 1175 by Alexander Neckem an English monk of St. Albans. 200 years after Petrus Peregrinus de Marincourt, a French Crusader, describes a floating compass and a compass with a pivot point. In the 16th century, William Gilbert (1544-1603), the Court Physician to Queen Elizabeth I, proved that many other substances are electric (from the Greek word for amber, elektron) and that they have two electrical effects. Gilbert also studied magnetism and in 1600 wrote “De magnete” which gave the first rational explanation to the mysterious ability of the  compass needle to point north-south: the Earth itself was magnetic. “De Magnete” opened the era of modern physics and astronomy and started a century marked by the great achievements of Galileo, Kepler, Newton and others. Gilbert recorded three ways to magnetize a steel needle: by touch with a loadstone; by cold drawing in a North-South direction; and by exposure for a long time to the Earth’s magnetic field while in a North-South orientation. The compass guided the braves voyagers to discover new continents; America for instance.

In 1743 a Swiss mathematician, Daniel Bernoulli introduced the magnet ikon; the horseshoe magnet. The shape of the magnets is the ingenious solution for which the magnet will not destroy itself in its own demagnetizing field. Still nowadays as the most recognized image related to magnetism. After this, a remarkable sequence of events occur in the following years to complete the history of magnetism. Scientist as Galvani, Franklin, Oersted, Ampere, Faraday, Volta, Seebeck, Ohm, Fourier, Watt, Edison, Morse, Heaviside, Tesla, Curie, Bohr, Planck, Maxwell and other contributing to reveal the mysteries of magnetism.

However, not was until the advent of Quantum Mechanics (QM) that the first description for ferromagnets was made. Thanks to the foundations of QM It was possible understand that the magnetic properties of solids arise essentially from the magnetic moment of their atomic electrons. The first was made by Langevin-Weiss molecular field theory, where interactions between electrons are responsible for ferromagnetism on 3d electrons.  And in 1929 was shown by Werner Heisenberg to be electrostatic in nature, originated quantum mechanically from Pauli-exclusion principle. Heisenberg formulated a simple Hamiltonian to map the interaction of two neighboring atoms whose total electron spin energy in units of Planck’s constant. At that time of the Solvay congress in 1930, Paul Dirac said: The underlying physical phenomena necessary for a mathematical explanation of a large part of physics and all chemistry are now understood in principle, the only difficult being that the exact application of these laws leads to equations much too complicated to be soluble” (P. Dirac Proc. Roy. Soc. A123, 714, 1929)

In the next years, Louis Neel point out the origin of anti-ferromagnetism or ferrimagnetism around 1948, depending on the topology of the crystal, thus based in the work of Heissenberg. After this series of major understandings, much of the progress was made by empirically and models.  After the first world war, steel magnets appeared. Following by the AlNICOS, discovered by Japanese scientist that were detrimental in the second war world. It was not until late 60s (at the same time Pink Floyd released his first album) in a U.S. Air Force Materials Laboratory, that rare-earth magnets were discovered. A high energy product (18 MGOe) of the Samarium-Cobalt (SmCo5) compound allowed much of the adventure in space on the 70s!.

A breakthrough in permanent magnetism was announced by two groups working independently in 1985. The Nd-Fe-B alloy doubled the energy product of previous magnets. The new material soon established itself as the leading high-performance magnet, and facilitated small motors, and coils actuator needed for hard-disk, thereby enabling the realization of laptop computers. No better magnet than NdFemB has been discovered since. Japanese scientists have optimized the material up to a point were experimental values approach the theoretical ones. The highest energy product is close to 512 kJ/m3 (50MGOe).

Already 30 years of this breakthrough and optimists scientist, engineers and chemist strongly believe that is possible to further increase the energy product of magnets. Imagine, if one can find a material with an enhanced energy product and without rare-earth elements on ti, certainly, will put a brick in the history of magnetism. Theoretically and experimentally in Solid State Physics this a real challenge that represents the frontier of nowadays Science.

Magnetism and his complexity.

Magnetism is considered one of the most intriguing and challenging phenomena in nature. Is vast, and spread in many field of physics; covering from biology -sensing of birds, Geology, Informatics, Nanotechnology and even in Astronomy.

Strictly speaking, all matter is magnetic. It’s just that some materials are much more magnetic than others. The main distinction is that in some materials there is no collective interaction of atomic magnetic moments, whereas in other materials there is a very strong interaction between atomic moments. The origin of magnetism, then lies in the orbital and spin motions of electrons and how the electrons interact with one another. There are essentially two main models to describe magnetism in solids. One is the localized electron and the other is the picture of itinerant electron. These two models are opposed but at some extent complementary to each other. The best way to introduce the different types of magnetism is to describe how materials respond to magnetic fields.

According to this, the magnetic behaviour of materials can be classified into the following five major groups: 1. Diamagnetism 2. Paramagnetism 3. Ferromagnetism 4. Ferrimagnetism 5. Anti-ferromagnetism.

Magnet technology has made enormous advances in recent years- without the reduction in size that have come with these advances many modern devices would be impracticable.

 

 

[1] Lee, E. W. Magnetism, An Introductory Survey, Dover Publications Inc. (1970)