Controlling the formation of magnetic nanocrystals in DMS

Recent years have witnessed rapid progress in the demonstration of novel functionalities of ferromagnetic semiconductors and hybrid semiconductor/ferromagnetic metal structures. The expected applications of these systems require the development of materials supporting ferromagnetic order up to above room temperature. The concentrated effort in this direction has led to the observation of ferromagnetism in a number of transition metal- and rare earth-doped nitrides and oxides and even in materials nominally not doped with magnetic ions. These experimental findings have been supported and in many cases simulated by theoretical ab-initio works. However, despite this deceptive agreement between experiment and theory, there is growing evidence that the origin of the high temperature ferromagnetism in semiconductors is not yet elucidated. In particular, some most recent experimental studies, including our own on (Ga,Fe)N have shown that the presence of ferromagnetic behaviour at high temperature has to be associated with the occurrence of spinodal decomposition, i.e., with the formation of regions with respectively high- and low- concentration of magnetic ions. According to this paradigm, the ferromagnetic response comes from the regions with high concentration, which can be regarded as magnetic nanocrystals embedded in a non magnetic matrix. The key objective of the proposed project is to probe if and how the Fermi-level engineering in diluted magnetic semiconductors, in addition to controlling carrier-mediated spin-spin interaction and self-compensation, can affect also spinodal decomposition, as recently predicted and not yet demonstrated. In this case, the Fermi level tuning would be instrumental in determining the incorporation and aggregation of the magnetic ions in the semiconductor matrix. Uniformly- and -doped (Ga,Fe)N:Mg and (Ga,Fe)N:Si, as materials systems of choice, will be fabricated by means of metalorganic chemical vapour deposition and thoroughly characterized at the nano-scale level via state- of-the-art electron microscopy, scanning probe methods, and synchrotron radiation techniques. The magnetic, optical and electrical properties of the materials system will then be studied based on the acquired knowledge of the magnetic ions distribution and surface morphology. The determined characteristics of the system will be subsequently examined in view of possible applications in functional devices. It will also be explored to what extent the demonstrated properties of (Ga,Fe)N are general for the whole family of wide band-gap diluted magnetic semiconductors and diluted magnetic oxides.

Recent years have witnessed a rapid progress in the demonstration of novel functionalities of ferromagnetic semiconductors and hybrid semiconductor/ferromagnetic metal structures. The expected applications of these systems require the development of materials supporting ferromagnetic order up to above room temperature. While originally the expectations for reliable and flexible applications lied in homogeneous structures, it is becoming more and more evident that phase-separated systems reserve unexpected and remarkable properties. In the frame of this project, we could demonstrate that the formation of functional magnetic nanocrystals embedded in a nitride matrix can be controlled by co-doping with donors or acceptors, i.e. by engineering of the Fermi level. Furthermore, we could compile a phase diagram of the nanocrystals` formation as a function of the fabrication parameters, establishing a precise correlation between growth conditions, structural asset and magnetic properties, making a very significant step towards the nanoscale control of the macroscopic properties of technologically significant semiconductor/ferromagnetic metal nanocomposites.