Alloys, Magnetic. Development and Application of Magnetic Alloys

The development and application of magnetic alloys in the twentieth century was driven largely by changes in the electronics industry. Magnetic alloys are used in three major applications. First, permanent magnets are required for devices like electric motors and generators, loudspeakers, and television tubes that use constant magnetic fields.

Second, electrical steels are used to make electromagnets, solenoids, transformers and other devices where changing magnetic fields are involved. Finally, they are used in magnetic storage media, which require magnetic materials that retain the impression of external magnetic fields.

The most important types of magnetic materials are the ferromagnets, of which the most commonly used are iron, cobalt, and nickel. Ferromagnets have high magnetic ability, which allows high magnetic inductions to be created using magnetic fields. They also retain magnetism so they can be used as a source of field in electric motors or for recording information. Ferromagnets are used in the manufacture of electrical steels and magnetic media.

In the early twentieth century, the most commonly used magnetic materials were steel alloys containing tungsten or chromium. Chromium steel came to dominate the market due to lower cost. In 1917, Honda and Takai found that the addition of cobalt doubled the coercivity of chromium steel. Cobalt-chromium steels were commercialized in the early 1920s. The first major permanent magnet alloy, Alnico, was discovered in 1930 by Mishima and commercially introduced in the late 1930s. An alloy of steel, aluminum, nickel, and cobalt, Alnico has magnetic properties roughly eight times better than chromium-cobalt steels.

Introduced in the 1950s, ferrites—ceramic ferromagnetic materials—are a class of magnets made from a mixture of iron oxide with other oxides such as nickel or zinc. Ferrites have greatly increased resistivity because they are oxides rather than alloys of metals. They are also very hard, which is useful in applications where wear is a factor, such as magnetic recorder heads. Unlike bulk metals, ceramic magnets can be molded directly. Although not as strong on a unit weight basis as Alnico, ferrite magnets are much cheaper, and account for the vast majority of magnets used in industry in the late twentieth century—roughly 90 percent by weight in the 1990s, for example.

The strongest magnetic materials are the ‘‘rare- earth’’ magnets, produced using alloys containing the rare earth elements samarium and neodymium. Samarium-cobalt magnets were introduced in the early 1970s, but increases in the price of cobalt due to unrest in Zaire limited their use. In 1983, magnets based on a neodymium-iron-boron alloy were introduced. Neodymium is cheaper and more widely available than samarium, and neodymium-iron-boron magnets were the strongest magnetic materials available at the end of the twentieth century.However, not all applications require the strongest magnetic field possible. Throughout the twentieth century, electromagnets and electromagnetic relays were constructed almost exclusively from soft iron. This material responds rapidly to magnetic fields and is easily saturated. It also has low remnance (a measure of how strong a remaining magnetic field is), so there is little residual field when the external magnetic field is removed.

For transformers, the material properties desired are similar but not identical to those for electromagnets. The primary additional property desired is low conductivity, which limits eddy current losses. First developed just after 1900, the primary material used for power transformers is thus a silicon-iron alloy, with silicon accounting for approximately 3 to 4 percent by weight. The alloy is heat-treated and worked to orient the grain structure to increase permeability in a preferred direction. Aluminum-iron alloy is also a suitable material for this application, although it is less used due to its higher cost.

Transformers for applications that involve audio and higher frequencies make use of nickel- iron alloys, with a nickel content of 30 percent or more. Common trade names for such alloys include Permalloy, Mumetal, and Supermalloy. These alloys were developed in the early twentieth century and were first manufactured in quantity for use as submarine cable shielding. The decline in cable production in the 1930s led to their use in transformers and related applications.

Magnetic recording was first developed by the Danish inventor Valdemar Poulsen at the beginning of the twentieth century and used for sound recording. The first material used for magnetic recording, solid steel wire or tape, was originally developed for other applications. For example, steel piano wire was used as the recording media for early wire recorders.

However, the property that makes particular steel alloys suitable for magnetic recording, strong remnant magnetism, is associated with increased stiffness. Thus, a recording tape or wire made from magnetic alloy steel is highly resistant to bending. This creates difficulties in the design of a mechanism for moving the recording media past the recording head.

As a result, by the late 1930s most recording media were divided into two parts. The first part was a suitable substrate, such as brass wire or plastic tape that could be fed easily through a reel or cassette mechanism. The second part was a coating that had suitable magnetic properties for recording. By the late 1940s, it was clear that the cheapest and most easily used recording media for sound recording was plastic tape coated with particles of a type of iron oxide (gamma ferric oxide).

This type of tape continued in use through the end of the twentieth century due to its low cost. During the 1970s, new tape particles of chromium dioxide and cobalt-doped ferric oxide were introduced because of their superior magnetic properties, but their higher cost meant that they were used only for more specialized audio recording applications.

Magnetic media based on ferric oxide particles were used for the recording of computer data beginning in the 1950s. Initial computer recording applications used coated plastic tape. Metal disks coated with iron oxide were introduced in the late 1950s for use in computer disk drives. In the 1990s, thin metal films of cobalt alloy largely replaced metal oxide as the recording media for hard disks.

In addition to ferromagnetic materials, two additional classes of magnetic alloys exist: paramagnets and diamagnets. Aside from use in the scientific study of magnetism, paramagnets have limited uses. One limited application is the production of very low temperatures. Paramagnetic salts can be cooled conventionally and then demagnetized, producing temperatures in the millikelvin range.

Superconducting materials are a subclass of diamagnets. When cooled to a sufficiently low temperature, superconducting materials experience a significant drop in resistance. Associated with this transition is the exclusion of magnetic flux from the conductor, with the flux moving to the surface of the conductor. These properties allow for the production of very high magnetic fields when using niobium-tin alloys as the conducting material.

These materials were also used in the development of magnetic resonance imaging (MRI) devices, although in the 1990s superconducting magnets were being replaced in this application by neodymium-iron-boron permanent magnet systems.