Diffusion and clustering of substitutional Mn in (Ga,Mn)As

The Ga vacancy mediated microstructure evolution of (Ga,Mn)As during growth and post-growth annealing is studied using a multi-scale approach. The migration barriers for the Ga vacancies and substitutional Mn together with their interactions are calculated from first principles, and temporal evolution at temperatures ranging from 200 to 350$^\circ$C is studied using Lattice Kinetic Monte Carlo simulations. We show that at the typical growth and annealing temperatures (i) gallium vacancies provide the diffusion mechanism for substitutional Mn and (ii) in 10--20 h the vacancy mediated diffusion of Mn promotes the formation of substitutional clusters. Clustering reduces the Curie temperature ($T_C$), and therefore the Mn clustering combined with the fast interstitial Mn diffusion explains the experimentally observed twofold annealing behavior of $T_C$

Ferromagnetism in (Ga,Mn)As and (Ga,Mn)N

(Ga,Mn)As and (Ga,Mn)N are so called diluted magnetic semiconductors, i.e. semiconductor based materials made ferromagnetic by inclusion of a magnetic element—in this case Mn. This type of materials bridge over the incompatibilities in metal–semiconductor interfaces in electronics components and have an enormous potential for future spintronics applications, where both charge and spin degrees of freedom can be employed simultaneously. In order to design new—or employ the existing—diluted magnetic semiconductor materials, the underlying mechanisms of magnetism must be understood. In this work a theoretical study of the structural and magnetic properties of the two most important prototype materials, (Ga,Mn)As and (Ga,Mn)N, is presented. Ferromagnetism arises from the quantum mechanical exchange interactions, but is by its very nature a macroscopic ordering effect. Therefore a multiscale approach is employed, beginning from quantum mechanical interactions. Both microscopic configurational energies and corresponding magnetic interactions are calculated from first principles. These energies are used in Monte Carlo simulations to study macroscopic and finite temperature properties. Curie temperatures are estimated using the Weiss molecular field theory, as well as a more sophisticated Monte Carlo approach. We show that both (Ga,Mn)As and (Ga,Mn)N consist largely of Mn clusters, and that the electronic and magnetic properties of these clusters differ significantly from those of single substitutional impurities. For (Ga,Mn)As we also show using lattice kinetic Monte Carlo methods that clustering occurs during growth and annealing via the Ga monovacancy mediated diffusion. For both materials clustering efficiently reduces the Curie temperatures even though the underlying band structure trends are different. The Curie temperatures are estimated for (Ga,Mn)As using the Weiss molecular field theory, while for (Ga,Mn)N we employ Monte Carlo methods in order to obtain the Curie temperatures.reviewe

Electronic and magnetic properties of substitutional Mn clusters in (Ga,Mn)As

The magnetization and hole distribution of Mn clusters in (Ga,Mn)As are investigated by all-electron total energy calculations using the projector augmented wave method within the density-functional formalism. It is found that the energetically most favorable clusters consist of Mn atoms surrounding one center As atom. As the Mn cluster grows the hole band at the Fermi level splits increasingly and the hole distribution gets increasingly localized at the center As atom. The hole distribution at large distances from the cluster does not depend significantly on the cluster size. As a consequence, the spin-flip energy differences of distant clusters are essentially independent of the cluster size. The Curie temperature $T_C$ is estimated directly from these spin-flip energies in the mean field approximation. When clusters are present estimated $T_C$ values are around 250 K independent of Mn concentration whereas for a uniform Mn distribution we estimate a $T_C$ of about 600 K.Comment: 7 pages, 5 figures, 2 tables; Revised manuscript 26. May 200

High Curie temperatures in (Ga,Mn)N from Mn clustering

The effect of microscopic Mn cluster distribution on the Curie temperature (Tc) is studied using density-functional calculations. We find that the calculated Tc depends crucially on the microscopic cluster distribution, which can explain the abnormally large variations in experimental Tc values from a few K to well above room temperature. The partially dimerized Mn_2-Mn_1 distribution is found to give the highest Tc > 500 K, and in general, the presence of the Mn_2 dimer has a tendency to enhance Tc. The lowest Tc values close to zero are obtained for the Mn_4-Mn_1 and Mn_4-Mn_3 distributions.Comment: To appear in Applied Phyiscs Letter

Ab initio tutkimus saostetuista magneettisista puolijohteista: Klusteroinnin vaikutus ferromagneettiseen kytkentään (Ga,Mn)As:ssa

Puolijohde-elektroniikka perustuu varauksen manipulointiin, kun taas magneettiset tallennuselementit perustuvat spinien järjestämiseen. Varausta ja spiniä voidaan hyödyntää samanaikaisesti magneettisissa puolijohteissa, joiden pohjalta suunnitellaan lukuisia tulevaisuuden spintroniikka -sovelluksia. Magneettisia puolijohteita voidaan valmistaa tavallisista puolijohteista saostamalla niitä magneettisilla epäpuhtauksilla, kuten Mn:lla. Tässä työssä tarkastellaan (Ga,Mn)As:a, jossa ferromagnetismin välittäjien tiedetään olevan aukkoja. Kokeellisesti on kuitenkin havaittu, että vain osa Mn -epäpuhtauksista järjestyy ferromagneettisesti, ja että aukkokonsenraatio on huomattavasti pienempi kuin mitä teoreettisesti ennustetaan. Uusien materiaalien ja niihin perustuvien sovellusten suunnittelun kannalta on ensiarvoisen tärkeää kattavasti tutkia ferromagnetismiin johtavia mekanismeja. Tässä työssä tutkitaan (Ga,Mn)As:a superkoppilaskuilla, joissa Mn -epäpuhtauksia käsitellään sekä tasaisesti jakautuneina että klustereina. Tiheysfunktionaalilaskut tehdään projektiotäydennetty-aalto -menetelmällä, joka on pseudopotentiaalimenetelmiä tarkempi. Ferro-, antiferro- ja paramagneettisten konfiguraatioiden kokonaisenergia laskettiin, ja ferromagnetismi havaittiin aina energeettisesti edullisimmaksi. Kahden Mn -atomin parikonfiguraatio, jossa lähinaapurikationit on korvattu Mn:lla, osoittautui perustilaksi, ja saman parikonfiguraation havaittiin kompensoivan yhden aukon. Tämä uusi aukkojen kompensaatiomekanismi selitettiin yksinkertaisella atomikoordinaatioon perustuvalla laskentasäännöllä. Aukkojen kompensaatiomekanismien kirjoa laajennettiin uudella mekanismilla, ja samalla selvennettiin miksi vain osa Mm:eista järjestyy ferromagneettisesti ja minne suuri osa aukoista katoaa