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Magnetism, Electromagnetism, Magnetic Materials and Applications

Rare-earth Magnets_Tetrataenite_102622A
[Rare-earth Magnets: Tetrataenite - a ‘cosmic magnet’ that takes millions of years to develop naturally in meteorites - Cambridge University]


- Overview

A magnet is a material or object that produces a magnetic field. This magnetic field is invisible, but is responsible for the most striking property of magnets: a force that pulls on other ferromagnetic materials (such as iron, steel, nickel, cobalt, etc.) and attracts or repels other magnets. Permanent magnets are objects made of materials that are magnetized and produce their own persistent magnetic field. 

Magnetic and electromagnetic fields play important roles and are fundamental to electronic and electromechanical systems. Motors, generators, speakers, microphones, computer storage devices like hard drives and many other devices are based upon magnetic, electromagnetic principles. 

Magnetic alloys are the single most important material underpinning the new green economy. 


- Magnets and Magnet Fields

A magnetic field is the area around a magnet, magnetic object, or an electric charge in which magnetic force is exerted. 

The invisible region around a magnetic object that can pull another magnetic object toward it or push another magnetic object away from it is called a magnetic field. 

Magnetic fields are created by moving charges. When negatively charged electrons move in a certain way, a magnetic field is created. These fields can be generated inside the atoms of magnetic objects or inside wires (electromagnetism).

The strength of a magnetic field is called the magnetic flux density and is measured in Tesla (International System of Measurements or SI). There are many other units and terms in the field of electromagnetism, including Weber, Maxwell, Gauss, etc.


University of Oxford_061522B
[University of Oxford]

- Electromagnetism

Electromagnetism has been studied since ancient times. Many ancient civilizations, including the Greeks and Mayans, created extensive theories to explain the attraction between lightning, static electricity, and magnetized iron ore. However, it was not until the late 18th century that scientists began to establish the mathematical basis for understanding the nature of electromagnetic interactions. 

In the 18th and 19th centuries, famous scientists and mathematicians such as Coulomb, Gauss, and Faraday formulated the laws of the same name that helped explain the formation and interaction of electromagnetic fields. This process culminated in the 1860s with the discovery of Maxwell's equations, a set of four partial differential equations that provide a complete description of the classical electromagnetic field. 

In addition to providing a solid mathematical basis for the relationship between electricity and magnetism that scientists have explored for centuries, Maxwell's equations also predicted the existence of self-sustaining electromagnetic waves. Maxwell hypothesized that such waves constituted visible light, which was later proved to be correct. 

In fact, gamma rays, X-rays, ultraviolet light, visible light, infrared radiation, microwaves, and radio waves are all identified as electromagnetic radiation, just in different frequency ranges.

In modern times, scientists continue to refine the laws of electromagnetism to take into account the influence of modern physics, including quantum mechanics and relativity. In fact, the theoretical implications of electromagnetism, especially the establishment of the speed of light based on the properties of the "medium" of propagation (magnetic permeability and permittivity), helped inspire Einstein's 1905 theory of special relativity. 

At the same time, the field of quantum electrodynamics (QED) modified Maxwell's equations to make them consistent with the quantized nature of matter. In QED, electromagnetic fields are represented as discrete particles called photons, which are also physical quanta of light. 

Today, there are many unsolved problems in the field of electromagnetism, such as the existence of magnetic monopoles and the mechanisms by which certain organisms perceive electric and magnetic fields.


- Magnetic Materials

Materials that can be magnetized, that is, materials that are strongly attracted by magnets, are called ferromagnetic materials (or ferrimagnetic materials). These include the elements iron, nickel and cobalt and their alloys, some rare earth metal alloys, and some naturally occurring minerals such as magnetite. 

Traditionally, only those materials that exhibit ferromagnetism (or ferrimagnetism) have been called "magnetic". Only nine elements are ferromagnetic. All are metals, of which three (Fe, Co, Ni) are iron group metals and the other six (Gd, Tb, Dy, Ho, Er, Tm) are rare earth metals.

The transition metals Fe and Co are essential elements for the preparation of alloys and compounds with large Curie temperatures (TC) and large spontaneous magnetization (Ms). Some intermetallic compounds with rare earth metals are characterized by very high values of magnetocrystalline anisotropy and magnetostriction.


- The Fabrication of Magnetic Materials

The fabrication of magnetic materials is a long-standing topic, and today magnetic materials are used in a variety of applications, including magnetic recording, magnetic sensor technology, magnetic levitation, magnetic cooling, spintronics, and more. However, modern challenges of ecological, resource-friendly, and environmental concerns place more stringent requirements on the study of magnetic materials. 

Much effort has been put into producing environmentally friendly materials to reduce energy consumption in use, improve cost-effectiveness, reduce the weight of equipment, and consume fewer resources such as expensive rare earth materials or rare lithium. In addition, material recycling issues must also be considered.



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