Tunnel magnetoresistance

The magnetic tunnel resistance (English tunnel magnetoresistance TMR ) or TMR effect is a magnetoresistive effect in magnetic tunnel junctions ( engl. magnetic tunnel junction MTJ ) occurs. It is a device consisting of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough ( typically a few nanometers), so electrons can tunnel between the two ferromagnets. This process can not be explained by classical physics and is therefore a purely quantum mechanical phenomenon.

Magnetic tunnel junctions are fabricated in thin film technology. For film formation on an industrial scale magnetron sputtering is used for this purpose, on a laboratory scale as well as molecular beam epitaxy, Laserstrahlverdampfen, electron and ion beam sputtering. The actual contacts are produced by photolithography.

Descriptive Description

By using an external magnetic field, the direction of magnetization of the two magnetic layers may be controlled independently. When the magnetizations are oriented the same, the probability that electrons tunnel through the insulator layer is greater than at opposite ( anti-parallel ) orientation. Thus, the electrical resistance of the contact between two different resistance states - that is binary 0 and 1 - can be switched back and forth.

History

The effect in 1975 by M. Jullière (University of Rennes, France) in Fe / Ge -O / Co contacts at 4.2 K. As was discovered in the relative change in resistance at room temperature was below 1%, the discovery initially received little attention. 1991 was Terunobu Miyazaki ( Tohoku University, Japan), an effect of 2.7 % at room temperature and 1994 a "giant TMR effect " of 18% at room temperature (iron layers separated by an amorphous alumina insulator ). The highest previously observed effects in the alumina-based contacts amounted to 70% at room temperature.

Since 2000, the tunnel barrier of magnesium oxide ( MgO) are developed. Today ( 2009), contacts up to 600 % achieved by CoFeB / MgO / CoFeB effects at room temperature at 4.2 K even more than 1100 %.

Application

The read heads of modern hard disk drives are now working on the basis of magnetic tunnel junctions. A novel non-volatile data memory, referred to as MRAM, is developed based on the TMR. Even for sensor applications (eg ABS sensors in the motor vehicle ) are magnetic tunnel junctions for use.

Physical explanation

The relative change in resistance or the effect of the amplitude is defined as

Wherein the electrical resistance of the anti-parallel state, and the electrical resistance in the parallel condition described.

The TMR effect has been attributed to the spin polarization of Jullière of each ferromagnetic electrode of a magnetic tunnel junction. The spin polarization arises from the spin-dependent density of states ( engl. density of states, Abk: DOS ) of electrons at the Fermi level:

The spin-up electrons are those whose spin orientation is parallel to the magnetization of the spin - down electrons, those with an antiparallel spin orientation. The relative resistance change is now obtained from the spin polarizations of the two ferromagnets, and:

No voltage is applied to the electrodes, electrons tunnel in both directions with equal rate. If you place an voltage, electrons tunnel präferenziert toward the positive electrode. Assuming that the spin is maintained at tunneling, the flow can be described with a two- flow model; is decomposed here the total current in a spin-up and spin -down portion. These vary in size, depending on the magnetic state of the contact.

In order to obtain a defined anti-parallel state, there are two possibilities. On the one hand you can use with different coercive ferromagnetic electrodes ( by different materials or different layer thicknesses ). On the other hand, one of the two layers are coupled to an antiferromagnet (English: exchange bias ). In this case, the magnetization of the non-coupled electrode remains "free".

The TMR decreases with both increasing temperature and with increasing voltage. Both can in principle be understood by Magnonanregung or interaction with magnons.

Obviously, is that the TMR becomes infinite if and are equal to 1, or both electrodes are spin polarized 100 %. In this case, the magnetic tunnel junction is a switch that can switch between a finite ( small ) resistance and infinite resistance on a magnetic basis. Materials which are eligible, referred to as ferromagnetic semimetals. Their conduction electrons are fully spin polarized. Theoretically predicted this property is for a number of materials (eg CrO2, various Heusler alloys ), but has not yet been confirmed experimentally.

Tunnel barriers of MgO have a special role. If the interfaces between the ferromagnet and the MgO epitaxially, ie the crystal lattice dislocations fit each other, additional filtering effects can occur. In this case, electrons can be suppressed with a specific orbital symmetry, while others can almost freely tunnel. The electrons, which can then pass almost unhindered, stem bands, which have a particularly high polarization.