Magnetic materials

Naturally occurring magnetic materials have been known and used for centuries. Materials that can be temporarily magnetized by an electrical current are widely used in applications ranging from simple electrical appliances and motors to sophisticated computer systems.

Definition

Substances that respond to a magnetic field are called magnetic materials. The most common magnetic materials are iron (Fe), cobalt (Co), nickel (Ni), and their alloys. These three elements belong to Group VIIIB of the periodic table. Four varieties of magnetism are recognized: ferromagnetism, ferrimagnetism, diamagnetism, and paramagnetism. Iron, cobalt, nickel, gadolinium (Gd), and chromium dioxide (CrO₂) are examples of ferromagnetic materials. Ferroferric oxide (Fe₃O₄) is a ferrimagnetic material. Feeble magnetism is exhibited in certain alloys and elements. A substance that is magnetized in the opposite direction of the external magnetic field is called a diamagnetic material. Some examples are gold, silver, copper, and quartz. A substance that is magnetized in the same direction as the external magnetic field is called a paramagnetic material. Certain types of special alloys are paramagnetic.

Overview

Magnets attract materials or objects made of iron (and steel), cobalt, and nickel. A magnet’s power is strongest at its two ends, called poles. One is called the north pole and the other the south pole. A compass is, in principle, a magnet pivoted at its center which orients itself in the direction of the Earth’s magnetic field. A compass has long been one of the most important navigational instruments onboard ships and airplanes.

The largest deposits of the mineral magnetite (Fe₃O₄), magnetic iron ore, are found in northern Sweden. Sizable deposits of magnetite are also found in Australia, Italy, Switzerland, Norway, the Ural Mountains in Russia, and several other regions. In the United States, magnetite is found in Arkansas, New Jersey, and Utah. The Precambrian rocks of the Adirondacks contain large beds of magnetite.

The ancient Chinese discovered that a freely suspended lodestone (naturally occurring polarized magnetite) would always orient itself in the same geographical direction. This observation led to the development of the compass. In the West, historical records of magnetic materials date back to the ancient Greeks. By 500 B.C.E., the Greeks had discovered that certain rocks were attracted to iron nails on ships and boats. In 1600, William Gilbert, an English doctor, published De Magnete, in which he identified the Earth itself as a giant magnet.

A number of fundamental advances in the practical applications of magnetism occurred in the early nineteenth century. In 1820, the Danish scientist Hans Christian ersted discovered that a magnetic needle could be deflected by a current in a wire. In 1823, English scientist William Sturgeon wound an insulated copper wire around an iron bar and discovered that the iron bar became a strong magnet. Thus the electromagnet was born. In 1821, Michael Faraday demonstrated the first electric motor, the “magnetic rotation of a conductor and magnet.” In 1828, Joseph Henry produced silk-covered wires and developed more powerful electromagnets.

Magnetic materials have a tremendous range of uses, from huge industrial electromagnets to the use of “magnetic bubbles” in highly advanced computer systems. Magnetic materials are classified into three major categories: hard, soft, and memory-quality materials. Hard magnetic materials have applications as permanent magnets in small motors, small direct-current generators (dynamos), measuring instruments, and speaker systems. Soft magnetic materials—those that are influenced by external fields—are widely used in transformers, generators, motors, and alternators of all sizes and rating capacities. Almost all appliances used in homes and industry, from shavers to washing machines to relays, contain electromagnets with soft magnetic materials. The materials used most often are iron, silicon-iron combinations, nickel-iron alloys, and ferrites. Memory-quality magnetic materials are used to record and store data, either in analog or digital form. Examples are magnetic tapes, drums, and disks.

Huge electromagnets are used to move automobiles or other metal objects in automobile recycling yards and junkyards. Gigantic electromagnets are essential to nuclear fusion experiments. Magnetic-levitation (maglev) trains are held above the ground by superconducting electromagnets. Superconducting electromagnets are also used in magnetic resonance imaging (MRI) body scanners, devices that produce detailed images of the inside of the body and provide diagnostic data to doctors.