Magnetic nitrides probed via microwave resonances

One of the principal aims of nowadays science is to develop tools for the bottom-up assembling of nanostructures with predefined properties and functionalities. One example of such system are semiconductor quantum dots that are expected to revolutionize the performance of various commercial devices, such as flash memories, low current semiconductor lasers, and single photon emitters. Gallium nitride (GaN) and its alloys have already demonstrated their potential in photonics and also as high power electronic systems, reaching the status of the technologically most significant semiconductor materials next to silicon (Si). The addition of magnetism opens now wide research and application prospects, as proven by the rich range of magnetic effects reported for the most extensively studied alloy (Ga,Mn)N. Our recent works of the applicant on magnetically doped nitrides combining a comprehensive growth program with a wide-ranging (nano)characterization, have allowed disentangling the magnetic signatures of diluted spins from those brought about by self-assembled embedded magnetic nanocrystals. Moreover, it has been demonstrated that by appropriate growth conditions and co-doping with shallow impurities one can engineer either ferromagnetic (FM) or antiferromagnetic (AF) interactions between diluted spins in (Ga,Mn)N as well as control the formation of Fe-rich nanocrystals in (Ga,Fe)N with either a FM or an AF ground state. The key objective of this project is to exploit this breakthrough progress in materials science to address, by low and high frequency microwave techniques, supported by magnetotransport and synchrotron spectroscopy: [i] the nature of the exchange coupling in dilute magnetic insulators, such as (Ga,Mn)N, depending on the charge and spin state of the transition metal ions; [ii] the signatures of percolation and quantum criticality at the ferromagnet/spin-glass verge; [iii] the evolution of the magnetic and electric characteristics when the self-organized aggregation of the magnetic ions, drives the magnetic nanocrystals across the insulator-to-metal transition; [iv] the possibility of magnetic refrigeration and/or current generation with ferromagnetic/semiconductor nanocomposite systems. We expect that the beyond-state-of-the-art experimental and theoretical work proposed here will on one side shed light on new mechanisms of ferromagnetism in magnetic semiconductors and on the other, will address methods of self organized growth and unforeseen functionalities of nanocomposites consisting of magnetic nanocrystals embedded in semiconductors.

One of the principal aims of nowadays science is to develop tools for the bottom-up assembling of low-dimensional structures with predefined properties and functionalities. Gallium nitride (GaN) and its alloys representing the family of materials employed for most of the commercially available light emitting diodes (LEDs) have already demonstrated their potential in photonics and also as high power electronic systems, reaching the status of the technologically most significant semiconductor materials next to silicon (Si). The addition of magnetism opens now wide research and application prospects, as proven by the rich range of magnetic effects reported for the most extensively studied alloy (Ga,Mn)N. In the frame of this project and through a comprehensive protocol of epitaxial growth, nanocharacterization and micro-wave-based methods supported by magnetotransport and synchrotron spectroscopy, we have: [i] unraveled the nature of the exchange coupling in dilute magnetic insulators in particular (Ga,Mn)N depending on the charge and spin state of the transition metal ions; [ii] gained deep insight into the magnetic anisotropy of self-assembled embedded magnetic nanocrystals with magnetic response tuned on-demand by varying the stoichiometry through the adjustment of the growth parameters; [iii] shown the possibility to exploit the piezoelectric nature of the material to change the magnetization direction in the insulator (Ga,Mn)N, and [iv] demonstrated the possibility of generating through spin pumping pure spin currents in non-magnetic GaN doped with Si, with a spin-to-charge conversion efficiency at least one order of magnitude greater than in other technologically relevant semiconductors.We are confident, that our results open wide avenues for nitride-based heterostructures as building-blocks for the next generation of non-volatile, energy-sparing and green electronic spintronic and spin-orbitronic devices exploiting respectively the spin of electrons and the spin-orbit coupling in non-magnetic materials to generate, manipulate and detect pure spin currents.