Magnetic Semiconductors

8.2.2.1. Magnetic Semiconductors

In the sections 5.4 and 5.5, the details of magnetization and Mössbauer experimental data are given for very small nano particles of magnetite within a glass matrix, which show several interesting magnetic phenomena even within this small nano range. These interesting aspects include most impor- tantly ‘anisotropy’, ‘magnons’ and ‘spin canting’, whose theoretical developments are neatly presented in the section 1.6.1. These properties may be very important also for the study of newer brand of nano- magnetic materials for information storage and processing. Thus, it opens up the door for the future possibilities in the field of nano-technology, which is presently directed towards the understanding of ‘hetero-structures’ of silicon, germanium and carbon. These are not new materials, but their ‘hetero-

NANO MATERIALS

structures’ are new to the world of nano-magnetics [24, 25], which are proving to be the key materials in today's field of information technology. One of the important category of such materials is III-V com- pounds.

The new class of materials such as ‘Magnetic Semiconductors’ are definitely fascinating, since they couple the complementary functionalities of the ‘spin properties’ of ferromagnets and the ‘elec- tronic properties’ of semiconducting materials. This brings us to a new category of materials, which are based on GaAs. The ferromagnetism in these materials is evident due to the presence of randomly substituted magnetic Mn ions.

The recent efforts have been on GaMnAs based materials such as Ga x Mn 1–x As alloys, which are grown by ‘Molecular Beam Epitaxy’ or MBE. In these materials, the Mn ions provides the charge carriers, i.e. the holes and carries spin (S = 5/2) making both the electrical and magnetic properties tunable. But, another important aspect is the ferromagnetic transition temperature, i.e. Curie Tempera-

ture or T c > 100 K in (Ga,Mn)As materials. It also shows the ‘long spin coherence times’ [26-28], which is a necessary ingredient in the rapid development of information storage technology based on magneto transport effects.

But in the above important investigations, the solubility of manganese ions in the GaAs matrix plays a crucial role. The equilibrium solubility of Mn in GaAs limits the T C . It should be mentioned that in many magnetic materials, T C is either below or near room temperature. This severely limits their technological applicability in devices at or above room temperature, and hence we require high T C materials. Therefore, both these limitations are to be tackled together.

One clever way is to choose a ‘ferromagnetic semiconductor system’ with a type of ‘digital superlattice structure’. By creating a thinner non-magnetic ‘spacer layer’ in the superlattice structure, the effective Mn concentration is increased and thus overcoming the solubility limit on Mn. At the same time, it is possible to have a higher Tc material [26-28]. In this type of system, the ferromagnetic order- ing of Mn layers and their magnetic coupling is unknown. Moreover, very little is known about the magnetic anisotropy of these systems, and the type of the interlayer coupling and its strength. Various newer type of experiments are to be performed on such materials, like it has been done on nano particles of magnetite, as described in the sections 5.4 and 5.5.