Gallium Manganese Arsenide - Properties

Properties

Regardless of the fact that room-temperature ferromagnetism has not yet been achieved, DMS materials such as (Ga,Mn)As, have shown considerable success. Thanks to the rich interplay of physics inherent to DMSs a variety of novel phenomena and device structures have been demonstrated. It is therefore instructive to make a critical review of these main developments.

A key result in DMS technology is gateable ferromagnetism, where an electric field is used to control the ferromagnetic properties. This was achieved by Ohno et al. using an insulating-gate field-effect transistor with (In,Mn)As as the magnetic channel. The magnetic properties were inferred from magnetization dependent Hall measurements of the channel. Using the gate action to either deplete or accumulate holes in the channel it was possible to change the characteristic of the Hall response to be either that of a paramagnet or of a ferromagnet. When the temperature of the sample was close to its TC it was possible to turn the ferromagnetism on or off by applying a gate voltage which could change the TC by ±1 K.

A similar (In,Mn)As transistor device was used to provide further examples of gateable ferromagnetism. In this experiment the electric field was used to modify the coercive field at which magnetization reversal occurs. As a result of the dependence of the magnetic hysteresis on the gate bias the electric field could be used to assist magnetization reversal or even demagnetize the ferromagnetic material. The combining of magnetic and electronic functionality demonstrated by this experiment is one of the goals of spintronics and may be expected to have a great technological impact.

Another important spintronic functionality that has been demonstrated in DMSs is that of spin injection. This is where the high spin polarization inherent to these magnetic materials is used to transfer spin polarized carriers into a non-magnetic material. In this example, a fully epitaxial heterostructure was used where spin polarized holes were injected from a (Ga,Mn)As layer to an (In,Ga)As quantum well where they combine with unpolarized electrons from an n-type substrate. A polarization of 8% was measured in the resulting electroluminescence. This is again of potential technological interest as it shows the possibility that the spin states in non-magnetic semiconductors can be manipulated without the application of a magnetic field.

(Ga,Mn)As offers an excellent material to study domain wall mechanics because the domains can have a size of the order of 100 µm. Several studies have been done in which lithographically defined lateral constrictions or other pinning points are used to manipulate domain walls. These experiments are crucial to understanding domain wall nucleation and propagation which would be necessary for the creation of complex logic circuits based on domain wall mechanics. Many properties of domain walls are still not fully understood and one particularly outstanding issue is of the magnitude and size of the resistance associated with current passing through domain walls. Both positive and negative values of domain wall resistance have been reported, leaving this an open area for future research.

An example of a simple device that utilizes pinned domain walls is provided by reference. This experiment consisted of a lithographically defined narrow island connected to the leads via a pair of nanoconstrictions. While the device operated in a diffusive regime the constrictions would pin domain walls, resulting in a giant magnetoresistance (GMR) signal. When the device operates in a tunnelling regime another magnetoresistance (MR) effect is observed, discussed below.

A further interesting property of domain walls is that of current induced domain wall motion. This reversal is believed to occur as a result of the spin-transfer torque exerted by a spin polarized current. It was demonstrated in reference using a lateral (Ga,Mn)As device containing three regions which had been patterned to have different coercive fields, allowing the easy formation of a domain wall. The central region was designed to have the lowest coercivity so that the application of current pulses could cause the orientation of the magnetization to be switched. Interestingly, this experiment showed that the current required to achieve this reversal in (Ga,Mn)As was two orders of magnitude lower than that of metal systems. It has also been demonstrated that current-induced magnetization reversal can occur across a (Ga,Mn)As/GaAs/(Ga,Mn)As vertical tunnel junction.

Another novel spintronic effect, which was first observed in (Ga,Mn)As based tunnel devices, is tunnelling anisotropic magnetoresistance (TAMR). This effect arises from the intricate dependence of the tunnelling density of states on the magnetization, and can result in MRs of several orders of magnitude. This was demonstrated first in vertical tunnelling structures and then later in lateral devices. This has established TAMR as a generic property of ferromagmetic tunnel structures. Similarly, the dependence of the single electron charging energy on the magnetization has resulted in the obersvation of another dramatic MR effect in a (Ga,Mn)As device, the so-called Coulomb blockade anisotropic magnetoresistance (CBAMR).

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