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Hysteresis is a quality most often seen in magnetic and elastic materials where a response to stress or electromagnetic force upon the material lags behind the actual application of the force. The reaction is also dependent on the previous forces applied to the material, and not just the current stress conditions it is undergoing. Put more simply, it is the history dependence of a system, and the root term of the word actually means to come late or lag behind.
In ferromagnetic hysteresis, the principle is relied upon for the recording of information onto magnetic storage tape, credit cards strips, and more. As a magnetic hysteresis field is applied to the recording medium and released, the medium does not default back to a zero magnetization state. Instead, a new level of order is added to the magnetic particles in the material, which represents the structure of the data recorded there. This residual magnetic memory of sorts can only be erased by applying a magnetic charge in the opposite direction, known as a hysteresis loop. The implanted magnetic charge can otherwise be nearly permanent, which is a useful feature when storing information, and has been widely used for audio cassette tapes and computer hard drives.
The property of the hysteresis loop can also be used to erase magnetic data by applying a reverse magnetic field to the medium. One in the same direction can also be employed to overwrite the previous pattern. This repeatable feature or hysteresis cycle in ferromagnetics, however, is not present in the properties of other materials.
Memristors, or memory resistors, are components that demonstrate the principle of a hysteresis circuit. They have the ability to maintain a memory of hysteresis current that passes through them by changing their relative resistance in response to it. These devices mimic the way the synapse works in the human brain, and that has captured the attention of military researchers at the Defense Advanced Research Projects Agency (DARPA), in the United States. Research as of 2010 was aimed at developing supercomputer power which would be small enough to be packed into a two-liter volume, and have the equivalent of a cat's brain in intelligence.
Somewhat elastic materials, such as thin metals, can exhibit a thermal hysteresis effect. Changes in the alignment of metal atoms when bending the tines of a fork back and forth will demonstrate hysteresis, but, unlike magnetic materials, the metal becomes less responsive with repeated applications of force. This is referred to as work hardening, and eventually results in the metal becoming brittle and breaking. The metal builds up a lag in response to the force and eventually breaks, causing the loss of energy as heat, which is referred to as hysteresis loss.
The hysteresis model has applications in a range of science, engineering, and even economics disciplines. Russian mathematicians began to model nonlinear systems based on the principle in the 1970s. They subsequently developed theories such as the Preisach model, which could be used to describe hysteresis phenomenon in a wide range of sciences from economics to tectonics and superconductivity.