Tailoring magnetic properties of Mn-Cr chalcogenide alloys and heterostructures

We shall develop new, thin-layer materials composed of transition metal elements (Cr, Mn) combined with group VI elements (S, Se,Te); these little-known materials offer the prospect of satisfying the requirements for a wide range of spin-dependent electronic ("spintronic") devices. This proposal aims to take the first steps in making and investigating single- and multi-layer materials from this family with the necessary compositions and crystal quality, but we shall also manufacture and study selected demonstrator devices within the timescale of the project.

The term "spintronics" encompasses many proposed devices (for example, but not exclusively, sensors, memory elements, diodes and transistors) in which it is the spin of the electrons that is manipulated in order to sense, store, carry or process information. In other words, these devices exploit the intrinsic magnetic properties of electrons as well as their charge and, as a result, may offer improvements in energy efficiency, speed or size. The prizes for the realization of such devices are enormous; for example, the phenomenal commercial success of the device at the heart of hard disk read heads (which can be classed as a spintronic device) has stimulated great excitement and intense research efforts aiming at wider applications of spin. Despite this, it has not proved possible to produce industrially-useful magnetic semiconductors from the first few candidate materials identified by early theoretical predictions. As a result, many groups in the worldwide spintronics community are now engaged in widening both the scope of the materials considered, and the types of magnetic behaviour that can be exploited; this search has revitalised the whole field of spintronics. This proposal addresses a candidate material family that has been proposed in the theoretical literature but (apart from our trials) has not yet been produced in the laboratory.

The (Cr, Mn)(S,Se,Te) material family satisfies several crucial requirements. Firstly, thin layers of these compounds will be grown on industry standard GaAs substrates and will adopt the same crystal structure as the substrate. This makes the resulting structures highly compatible with existing semiconductor technologies. Secondly, preliminary studies of ours and theoretical studies of several groups imply that we will be able to produce all the potentially useful types of magnetic behaviour by tuning the composition. These include ferromagnets and half-metals (where the transition metal magnetic moments align parallel to each other and add), antiferromagnets (where they align oppositely and cancel) and ferrimagnets (where dissimilar transition metals align oppositely but are not equivalent and so cannot exactly cancel). Layers of any of the above magnetic types can form the active layer in different types of spintronic devices. Thirdly, these materials are chemically and structurally compatible with non-magnetic semiconductors (e.g, ZnSe, MgS) that we can grow as parts of multi-layer structures; these allow the necessary electrical contacts and electrical barriers to be formed.

Our programme will involve a substantial effort in growing these new materials to obtain layers of high crystal quality; preliminary work indicates that there will be no fundamental obstacles to success. The structural, magnetic and electrical properties of the materials will be investigated to identify the most promising compositions and most appropriate target device designs based on them. Demonstrator devices will then be produced to test how the materials perform in realistic device contexts and to promote interest in the work. This work requires a broad base of experience and so we have formed a team having expertise in MBE growth, electrical device fabrication and measurement, magneto-optical spectroscopy and magnetometry.

At the earliest stages in the fabrication of these materials and heterostructures, which do not exist in nature and which are known only through theoretical predictions, we expect the immediate impact of this project (during its 42 month period) to be amongst academic groups, as discussed in the Academic Beneficiaries section above. This impact is likely to be significant independent of whether the most ambitious objectives of this project are achieved, since the new insights into material growth and into the modelling of magnetic behaviour of these systems that will be generated will be of fundamental interest to those respective communities.

In the medium term (most likely, towards the conclusion of the project and onwards), a successful demonstration of high temperature ferromagnetic behaviour, or half-metallicity, and successful spin injection would impact also on industrial groups interested in exploitation of spin effects for novel semiconductor devices. Significant research and development work would then be needed to optimise device design and fabrication with a much more restricted set of the best candidate materials and that would be likely to require a programme of academic/industrial collaboration. We can anticipate significant continuing involvement of the present project partners, especially since expertise in the growth of these materials will be unique to HWU, but a much wider network with suitable device engineering skills would become necessary. How we will reach out to such industrial groups is discussed in the Pathways to Impact section of the main proposal. Part of the appeal of our proposed materials is their relatively easy integration with existing III-V technologies, and this is why we take the ambitious step of including demonstrator devices already within this project.

The ultimate economic and societal impact of novel semiconductor technologies is potentially enormous; past breakthroughs in, for example, blue light emitting diodes and lasers, or GMR hard disk read heads, were commercialised on the scale of 10 to 15 years following the initial materials science discoveries and these are now the bases of large-scale industrial activity and huge international markets. Furthermore, the resulting transformations in information technology have (in both cases) had a wide societal impact. Some of the possible device applications of magnetic semiconductors (which are not discussed in detail in the proposal, note, since our project is mainly focused on a much earlier stage in the overall programme, of fundamental materials science) are based on providing similar functionality to existing types of semiconductor device but at much reduced power consumption and/or smaller physical size. As is widely recognised, such devices, whilst not breaking new grounds in terms of function, could have significant positive implications for sustainability and for wider access to information technology. Beyond this, more disruptive concepts such as quantum information technology and quantum computing require the manipulation of quantum bits. Many possible strategies for this exist, one of which is based on the use of electron spins within nanoscale ferro- and paramagnetic materials. Here again, the materials we propose to study could have a role to play and the fabrication of quantum dot structures from them will be feasible by well-established means.