Magnetic hysteresis occurs when an external magnetic field from outside is applied to a ferromagnetic such as iron and the atomic dipoles align themselves with it. Even when external field is removed, part of the alignment will be retained: the material has become magnetized forever or certain period of time. Once magnetized, the magnet will stay magnetized indefinitely. To demagnetize the ferromagnetic, it requires heat or a magnetic field in the opposite direction to remove the field. This is the effect that provides the element of memory in a hard disk drive..
The association between field strength which is denoted as H and magnetization which is denoted as M is not linear in such materials. If a magnet is demagnetized (H=M=0) by the outside sources and the relationship between H which is field strength and M which is magnetization is plotted for increasing levels of field strength follows the initial magnetization curve. At first, the curve increases rapidly and after a certain time an asymptote called magnetic saturation. If the H-M association is plotted for all strengths of applied magnetic field, the result of this plot is called a hysteresis loop called the main loop. The width of the central section along the H axis is twice the coactivity of the material.
The occurrence of hysteresis in ferromagnetic materials is resulted from two major effects such as revolution of magnetization for the materials and variations in size or number of magnetic domains. The researchers suggested that Hysteresis loop is a closed curve screening the variation of the magnetic flux density of a ferromagnetic material with the external magnetic field producing it, when this field is changed through a complete cycle. The low magnetic coercivity generally reduces energy loss associated with hysteresis. In general, the magnetization varies in direction such as same direction or opposite direction but not magnitude) across a magnet, but in sufficiently small magnets, it doesn't. In these single-domain magnets, the magnetization responds to a magnetic field by rotating. Single-domain magnets are used wherever a strong, stable magnetization is needed (for example, magnetic recording).
Larger magnets are divided into areas called domains. Within each area, the magnetization does not differ; but between areas are comparatively thin domain walls in which the direction of magnetization rotates from the way of one area to another. If the magnetic field changes, the walls move, changing the relative dimensions of the domains. Because the areas are not magnetized in the same direction, the magnetic moment per unit volume is smaller than it would be in a single-domain magnet; but domain walls involve rotation of only a small part of the magnetization, so it is much stress-free to change the magnetic moment. The magnetization can also change by addition or subtraction of domains (called nucleation and denucleation).
There are an immeasurable variety in application of the theory of hysteresis in magnetic materials. Majority of this make use of their ability to recall a memory, for example magnetic tape, hard disks, and credit cards. In these applications, hard magnets (high coercivity) like iron are necessary so the memory is not easily erased.
Soft magnets (low coercivity) are used as centers in modifiers and electromagnets. The response of the magnetic moment to a magnetic field enhance the response of the coil wrapped around it. Low coercivity reduces that energy loss associated with hysteresis.
Magnetic hysteresis material (soft nickel-iron rods) has been used in hampering the angular motion of satellites in low earth orbit since the dawn of the space age.
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