Superparamagnetism

Superparamagnetism, also known as the superparamagnetic effect, referred to the magnetic property of very small particles of a ferromagnetic or ferrimagnetic material, to maintain, even at temperatures below the Curie temperature without permanent magnetization, when a previously applied magnetic field has been switched off.

Cause

Reason for this phenomenon are the Brownian relaxation and Néel relaxation ( simplified thermal excitations ), which change the direction of magnetization. In detail, the Néel relaxation occurs by thermal excitations, this magnetic moments of the particles are changed by thermal effects ( without the influence of a magnetic field ) again and again. The time within which rotates the magnetic moment is referred to as a Néel relaxation time. Since the time in which the magnetization is measured, takes longer than the Néel relaxation, magnetization appears average of 0 (since the changing magnetic moments are mutually offset in the measurement).

A collection of such particles therefore behaves macroscopically like a paramagnet, but still has the high magnetic saturation of a ferromagnet. In contrast to a paramagnet are not of individual atoms, but small magnetic particles which change their direction of magnetization regardless of each other.

Occurrence

Superparamagnetism occurs depending on the fabric to below a certain particle size.

The only prerequisite is that the direction of magnetization of the particles can rotate with very little energy. To change the magnetization direction of an energy barrier has to be overcome; This is primarily determined by the magnetic anisotropy of the material and the particle size. If this energy barrier is sufficiently low, occurs superparamagnetism. The particle size at which the particles no longer behave ferromagnetic but superparamagnetic is also referred to as superparamagnetic limit.

Superparamagnetism occurs above a specific, dependent on the material and the particle temperature, which is called the blocking temperature ( german).

Superparamagnetic particles are so small that they only form a magnetic domain.

Importance for magnetic storage media

In the magnetic data recording, eg on disks, the superparamagnetism is a physical upper limit of the possible recording density because for very small magnetic grains are needed. The reduction results in higher sensitivity to thermal stimulation and can lead to spontaneous loss of magnetization and thus the stored information.

Therefore, efforts to use materials with the highest possible magnetic anisotropy for hard drives, but that's just as far as possible as they can be re-magnetized even by the write head. By briefly heating the magnetic reversal (write) of such materials can be facilitated. This method is as HAMR technology ( engl. heat -assisted magnetic recording ) known and will allow an increase in the recording density of new magnetic storage media.

Another way to increase the storage density is to use only magnetic particles instead of many ( several hundred ) of a per bit. Then very high recording densities can be achieved despite the use of larger grains; However, these grains must be arranged so that the read and write head of the hard drive can follow the ranks of the grains. Such storage media can be by means of lithographic techniques such as electron beam or ion beam lithography to produce, but so far exist only in the laboratory and are as patterned media (English ) known.

Footnotes

• Magnetism
• Memory technology