Remanence

Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed. Colloquially, when a magnet is "magnetized", it has remanence. The remanence of magnetic materials provides the magnetic memory in magnetic storage devices, and is used as a source of information on the past Earth's magnetic field in paleomagnetism. The word remanence is from remanent + -ence, meaning "that which remains". The equivalent term residual magnetization is generally used in engineering applications. In transformers, electric motors and generators a large residual magnetization is not desirable (see also electrical steel) as it is an unwanted contamination, for example a magnetization remaining in an electromagnet after the current in the coil is turned off. Where it is unwanted, it can be removed by degaussing. Sometimes the term retentivity is used for remanence measured in units of magnetic flux density. The default definition of magnetic remanence is the magnetization remaining in zero field after a large magnetic field is applied (enough to achieve saturation). The effect of a magnetic hysteresis loop is measured using instruments such as a vibrating sample magnetometer; and the zero-field intercept is a measure of the remanence. In physics this measure is converted to an average magnetization (the total magnetic moment divided by the volume of the sample) and denoted in equations as Mr. If it must be distinguished from other kinds of remanence, then it is called the saturation remanence or saturation isothermal remanence (SIRM) and denoted by Mrs. In engineering applications the residual magnetization is often measured using a B-H analyzer, which measures the response to an AC magnetic field (as in Fig. 1). This is represented by a flux density Br. This value of remanence is one of the most important parameters characterizing permanent magnets; it measures the strongest magnetic field they can produce.