Dilute magnetic semiconductor nanostructures

The prospect of a new generation of electronic devices based on the fundamental quantum property of angular momentum, known as spin, has led to the rapidly developing field of spintronics. It is envisioned that these advanced devices will have significant advantages over traditional charge based electronics in properties such as speed, power consumption and long coherence times. By combining the properties of magnetics with that of semiconductors, the novel class of materials known as dilute magnetic semiconductors (DMSs) are considered a promising system for exhibiting spintronic functionality. These materials are created by using molecular beam epitaxy (MBE) to incorporate into traditional semiconductors a quantity of transition metal atoms sufficient that ferromagnetism is exhibited. The most widely studied DMS is (Ga,Mn)As which has well characterised behaviour and can be processed using standard III-V fabrication techniques, thus providing an excellent basis for further study. In this research the properties of (Ga,Mn)As based systems are studied as the material dimensions are reduced to nanometre length scales. Three complementary approaches are used for this purpose. The first is to use ultra-high-resolution electron-beam lithography to construct devices. By being able to selectively remove material, laterally patterned structures can have sizes as small as 10 nm. The second approach is to exploit the atomic layer growth of MBE to allow the construction of epilayers and heterostructures with well defined vertical compositions. Thirdly, a theoretical k.p kinetic-exchange model allows the simulation of multilayer structures and an exploration of the parameter spaces available in such materials. Two systems are considered: lateral nanoconstricted magnetic tunnel junctions and vertically defined magnetic superlattices. The nanoconstrictions are analysed using low temperature magnetotransport techniques and novel anisotropic magnetoresistance (MR) effects are measured. Primarily, tunnelling anisotropic magnetoresistance (TAMR) is observed, demonstrating that it is a generic property of ferromagnetic tunnel devices and is therefore of wide interest for other spintronic systems. Secondarily, anisotropic switching behaviour is observed and is interpreted as Coulomb blockade anisotropic magnetoresistance (CBAMR). Additionally, the significance of the processing stages and material properties are highlighted. The magnetic superlattices are firstly considered on a theoretical basis in order to determine structural parameters in which a new MR effect might be observed. This effect derives from the interlayer exchange coupling (IEC) between the magnetic layers which can either be in parallel or opposed orientations. Based on the calculations, samples are measured using low temperature magnetotransport and magnetometry techniques in order to explore the possibility of some of the dramatic properties predicted in magnetic superlattice structures