Determination of ferromagnetic phase diagram in nitrides

Over the very last years, a considerable progress has been achieved in the demonstration and control of novel functionalities of ferromagnetic semiconductors and hybrid semiconductor/ferromagnetic metal structures. The practical applications of these systems require the development of materials sustaining ferromagnetic order up to above room temperature. The focused 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 were supported, and in many cases suggested, by theoretical ab- initio works, indicating that the ground state is ferromagnetic in a number of diluted magnetic semiconductors. However, there is raising evidence that the agreement between experiment and theory was fortuitous, and that the origin of the high temperature ferromagnetism in semiconductors is actually far from being understood. This inadequate state of the art is at the origin of the lack of a report on a functioning device based on a ferromagnetic semiconductor and working up to room temperature. However, it has remarkably become obvious that the physics of these systems is exceedingly challenging, as it combines aspects associated to strongly correlated systems, disordered semiconductors, and mismatched alloys. The core objective of this project is to demonstrate - by taking advantage of the most recent breakthroughs in the field - the occurrence of hole-mediated ferromagnetism in diluted magnetic nitrides and to provide the corresponding phase diagram of the Curie temperature TC(x,p) as a function of the transition metal content and of the holes concentration in the material. In particular, beyond-state-of-the art device design and nanocharacterization tools will be employed to fabricate and to control the properties of Mn- and Mg-doped heterostructures and superlattices, in which modulation doping and interfacial electrostatic polarization will consent to prevail over e.g. magnetic ion aggregation and hole localization by strong p-d interaction, mechanisms that are considered to be at the origin of the failure of previous experiments. We expect that the here proposed comprehensive and beyond-state-of-the-art experimental and theoretical work will provide crucial inputs for the solution of one of the most controversial riddles in the field of magnetism in solid-state and spintronics, namely the origin of high TC ferromagnetism in transition metal-doped (diluted) semiconductors and oxides.

Especially due to his role as basic material for the functioning of commercial light emitting diodes, gallium nitride (GaN) has become one of the technologically most significant semiconductors. Furthermore, it is expected that in combination with magnetic elements it would open a plethora of perspectives for the next generation of spin-electronic devices for information storage and transfer. In the frame of this project, we have demonstrated that by controlling the combination of GaN with magnetic elements, it is possible to modulate the system on demand and obtain either a homogeneous magnetic semiconductors or an hybrid magnetic/semiconductor in which ordered arrays of functional magnetic nanostructures offer perspective applications in devices for transfer of information through magnetic spins. Moreover, we have shown that the formation of magnetic complexes in nitrides produces a material in which the magnetic state can be tuned and at the same time the photonic capabilities of GaN can be extended from the actual visible/ultraviolet to the infra-red in the telecommunication window.