Hysteresis
Hysteresis is a physics term that means, literally, to be late. It describes systems that do not directly follow the forces applied to them, but react slowly, or don't return completely to their original state: that is, systems whose states depend on their immediate history. For instance if you push on a piece of putty it will assume a new shape, and when you remove your hand it will not return to its original shape, or at least not entirely.Hysteresis phenomena do not only show up in magnetic and ferromagnetic materials, but are present in the elastic and electromagnetic behavior of materials, in which a lag occurs between the application and the removal of a force or field and its subsequent effect. Electric hysteresis occurs when applying a varying electric field, and elastic hysteresis occurs in response to a varying force. Although the hysteresis loop depends on the material property behaviours, there is no complete theoretical description that explains the physical phenomenon.
The family of hysteresis loops, from the results of different applied varying voltages or forces, form a closed space in three dimensions, called the hysteroid.
Hysteresis was initially considered to be a dirty, unwanted, phenomena of materials. But its behaviour is now considered to be of very great importance in technology, and the property is for example used when constructing permanent memory for computers.
The phenomena of hysteresis can conceptually be explained as follow. Given a system, it can be divided into subsystems or domains, much larger than an atomic volume but even microscopically small. Such domains normally occur in ferroelectric and ferromagnetic systems, since individual dipoles tend to group with each other, forming a small isotropic region. Each of the system's domainss can be shown to have a metastable state. The metastable domains can in turn have two or more substates. Such a metastable state fluctates very much from domain to domain, but the average represents the configuration of lowest energy. The hysteresis is simply the sum of all domains, or the sum of all metastable states.
The term is actually used almost entirely to describe an effect seen in magnetism, specifically in ferromagnetic materials. When an external magnetic field is applied to a ferromagnet, the ferromagnet "takes up" some of the external field. Even when the external field is removed, the magnet will retain some field, it has become magnetized.
A hysteresis loop occurs when an alternating magnetic field is applied to a ferromagnetic material. As the magnetic field increases, the magnetization also increases, but it has a lower value than the equilibrium magnetization. The magnetic field then reverses direction and starts to decrease - again the magnetization lags, this time staying above the equilibrium value. If the field is plotted against magnetization during this cycle, it will appear as a loop.
This is a very important effect in many situations, notably magnetic tape and other related storage media like hard disks. In these materials it would seem obvious to have one polarity represent a bit, say north for 1 and south for 0. However, if you want to change the storage from one to the other, the hysteresis effect requires you to know what was already there, because the needed field will be different in every case. In order to avoid this problem, recording systems first overdrive the entire system into a known state, a process known as bias. Different materials require different biasing, which is why there is a selector for this on the front of most cassette recorders.
The term "hysteresis" is sometimes used in other fields, for example economics. In such cases it describes some sort of memory or lagging effect.
Electrical hysteresis typically occurs in ferroelectric material, where domainss of polarisation contribute to the total polarisation. Polarisation is the electrical dipole moment (either C·m-2 or C·m).
Hysteresis is also a known phenomenon in thermodynamics. Here the molecules change properties during heat transfer (via a potential field or a higher temperature increase) and it is this change that causes hysteresis.
When hysteresis occurs with extensive and intensive variables, the work done on the system is the area under the hysteresis graph.
Hysteresis represents states, and the characteristic curve shape sometimes remiscent of a two-value state, or a so-called bistable state. However, the hysteresis curve does really contain infinitely many states. But a simple application is to let the threshold regions (usually to the left and to the right) represent respective on/off-state. In this way, it will work as a bistable system. Note that even if no external field is applied, the position of the hysteresis curve might change with time; it is not necessarily stationary, i.e. it does not say that the system will stay in the exact same state as it had previously. The system might need new energy transfer to be stationary.
The hysteresis effect can be used when connecting complex circuits with the so-called passive matrix addressing. This scheme is praised as a technique that can be used in modern nanoelectronics, electrochrome cells, memory effect, etc. In this scheme, shortcuts are made between adjacent components (see crosstalk) and the hysteresis helps to keep the components in a particular state while the other components change states, i.e. one can address all rows at the same time instead of doing each individually.Definition
Magnetic hysteresis
Electrical hysteresis
Liquid-solid phase transitions
Energy
Applications