Magnetic anisotropy

Magnetic anisotropy describes the fact that magnetic materials can have a preferred direction or preferred plane for the magnetization. The measure of this is the magnetic anisotropy, which is defined as the work required to turn the magnetization out of a closed system (no exchange of particles ) from the " easy direction " ( the preferred orientation ).

The magnetic anisotropy causes the coupling of the magnetization of the crystal lattice and is, for example, responsible for rotating a magnetic needle (and thus the orientation of the spin- lattice later).

There are various forms of magnetic anisotropy. In addition to the magneto- crystalline anisotropy, the coupling of the magnetization and the crystal lattice, there are also effects on the modification of the shape of the body ( shape anisotropy ) and the elastic stress ( magnetoelastic anisotropy ) is based. 1956 new effect was discovered ( exchange bias, also unidirectional exchange anisotropy ), which causes a preferred direction of magnetization in a ferromagnet due to the interaction with an adjacent antiferromagnet and applications has in read heads of hard disks, which are based on thin ferromagnetic layers ( use eg the GMR effect ). There is also the interface anisotropy in thin magnetic layers.

Examples

• In a single-crystal iron cylinder whose length is substantially greater than the radius, preferably, the magnetization remains in the direction of the longitudinal axis. This is a so-called easy direction (English: easy axis ). Here the anisotropy is mainly determined by the shape of the sample, called shape anisotropy.

• Another example of which is thin ferromagnetic layers. The magnetization is not out of the surface, since the magnetic permeability of air is much worse. The magnetic domains in the balance and, in the ideal case (ie single-crystal layer with no defects ) are oriented such that magnetic field lines as little as possible out point from the layer. See also graphics.

• A single-crystal iron ball has, in spite of its isotropic form, also preferred directions of magnetization. This is due to the internal structure. It is crystal anisotropy (English: crystalline anisotropy ) called.

Explanation

The occurrence of the magnetic anisotropy is surprising at first sight. The exchange interaction, which is responsible for the collective order of the magnetic moments is isotropic. The Heisenberg spin Hamiltonian is ( as the scalar product ) is also isotropic. However, Magnetic anisotropy is a fact of experience. A thermodynamic consideration leads to the Gibbs free energy density ( a phenomenological approach in which considerations of symmetry play a leading role ) and thus to the terms that describe the anisotropy; which was first performed by the Russian physicist Akulov ( 1900-1976 ). The spontaneous magnetization is isotropic, ie, the same size for all directions. This follows from the observation that the magnetization of a ferromagnetic single crystal in a sufficiently high field is the same for all directions. All ferro- magnetic properties of a ferromagnetic material is lost in all directions at the same temperature, that is, the Curie point is isotropic.

Occurrence of magnetic anisotropy

However, depending on the direction of magnetization of a different behavior to be measured. A Eiseneinkristall reached its saturation magnetization very quickly if it is magnetized along its edges of the cube. Compared with the above case - - For magnetization along the face diagonals, the magnetization grows more slowly. A ferromagnetic crystal displays in different directions ia different magnetization curves. The magnetic anisotropy can be characterized by the exciting power. Iron in the magnetization along the working edges of the cube is the lowest, this direction is referred to as a light direction. Iron has three light and four heavy directions ( along the space diagonals ). In a light cobalt ( hexagonal axis), and infinitely many heavy directions are found.

The magnetic anisotropy describes the energy associated with the orientation of the magnetization. The size of the magnetic anisotropy energies are several orders of magnitude below those of the exchange energy, which is responsible for the spontaneous collective order of permanent magnetic moments. The fields lie in the exchange effect in 400-2000 Tesla, while the anisotropy are about 0.01 to 10 t.

Causes

Basically, the magnetic anisotropy has its roots in two physical interactions:

• Shape anisotropy,
• Crystal anisotropy ( in higher order of dipolar interactions determined )

• Crystal anisotropy,
• Surface anisotropy

The crystal anisotropy is influenced by mechanical stresses, so-called inverse magnetostriction.

In particular, the spin -orbit coupling plays a role of magnetocrystalline anisotropy, which entails difficulties for the theoretical derivation of the anisotropy of models because of their small size compared to about exchange interaction.

Applications and importance in practice

Outstanding significance of the study of the magnetic anisotropy in the development of new hard drives. More and faster access times and higher memory densities in particular always will lead in the near future to the so-called paramagnetic limit (see Moore's Law ). The magnetic anisotropy can for example be used specifically to enhance the stability of bits ( an overcoming power, as it exists in the anisotropy, always causes a certain stability of the system ), which can influence each other in smaller and smaller dimensions; The latter would have unwanted information losses.

Is particularly interesting in this context, the thin-film magnetic technology.

The positive magneto- elastic anisotropy of iron is used to locate near-surface residual stresses in ferrous materials and steel parts with the Barkhausen noise.