Superparamagnetism
Super paramagnetism , also known as the superparamagnetic effect , describes the magnetic property of very small particles of a ferromagnetic or ferrimagnetic material not to hold permanent magnetization even at temperatures below the Curie temperature if a previously applied magnetic field has been switched off.
root cause
The reason for this phenomenon are the Brown relaxation and the Néel relaxation (simplified: thermal excitations), through which the direction of the magnetization changes.
In detail, the Néel relaxation takes place through thermal excitations. Here, the magnetic moments of the particles are repeatedly changed by thermal influences ( without the influence of a magnetic field). The time within which the magnetic moment rotates is also known as the Néel relaxation time. Since the time in which the magnetization is measured takes longer than the Néel relaxation and the changing magnetic moments in the measurement compensate for each other, the magnetization seems to have the value 0 on average. A collection of such particles behaves macroscopically like a paramagnet , but still has the high magnetic saturation of a ferromagnet. In contrast to a paramagnet, it is not individual atoms that change their direction of magnetization independently of one another, but rather small magnetic particles.
Occur
Depending on the substance, superparamagnetism occurs below a certain particle size.
The prerequisite for this is that the direction of magnetization of the particles can rotate without great expenditure of energy. To change the direction of magnetization, an energy barrier must be overcome; this is essentially determined by the magnetic anisotropy of the material and the particle size. When this energy barrier is sufficiently low, superparamagnetism occurs. The particle size below which the particles no longer behave ferromagnetically but rather superparamagnetically is also referred to as the superparamagnetic limit .
M s has different values depending on the substance. Noticeable: after switching off the magnetic field there is no remanence (curve runs through the origin).
Superparamagnetism occurs below the Curie temperature, but above a certain temperature that depends on the material and the particle size, which is known as the blocking temperature .
Superparamagnetic particles are so small that they only form a magnetic domain .
Importance for magnetic storage media
In magnetic data recording , e.g. B. on hard drives , the superparamagnetism represents a physical upper limit of the possible recording density, because very small magnetic grains are required. The reduction in size leads to higher sensitivity to thermal excitation and can lead to spontaneous loss of magnetization and thus of the stored information.
Therefore, attempts are made to use materials with the highest possible magnetic anisotropy for hard drives . However, this is only possible to the extent that they can still be magnetized by the write head .
Brief heating can make it easier to remagnetize (write) such materials. This method is known as HAMR technology ( heat-assisted magnetic recording ) and is intended to enable an increase in the recording density of future magnetic storage media.
Another way to increase storage densities is to use only one per bit instead of many magnetic grains (a few hundred) . Then, despite the use of larger grains, very high recording densities can be achieved; however, these grains must be arranged in such a way that the read and write head of the hard disk can follow the rows of grains. Such storage media can be used with the help of lithography techniques such. B. produce electron beam or ion beam lithography , but so far only exist in the laboratory and are known as patterned media .