Optical spin control in nanocrystalline magnetic nanoswitches

We investigate the optical properties of (Cd,Mn)Te quantum dots (QDs) by looking at the excitons as a function of the Mn impurities positions and their magnetic alignments. When doped with two Mn impurities, the Mn spins, aligned initially antiparallel in the ground state, have lower energy in the parallel configuration for the optically active spin-up exciton. Hence, the photoexcitation of the QD ground state with antiparallel Mn spins induces one of them to flip and they align parallel. This suggests that (Cd,Mn)Te QDs are suitable for spin-based operations handled by light

Antiferromagnetic order in (Ga,Mn)N nanocrystals: A density functional theory study

We investigate the electronic and magnetic properties of (Ga,Mn)N nanocrystals using the density functional theory. We study both wurtzite and zinc-blende structures doped with one or two substitutional Mn impurities. For a single Mn dopant placed close to surface, the behavior of the empty Mn-induced state, hereafter referred to as "Mn hole", is different from bulk (Ga,Mn)N. The energy level corresponding to this off-center Mn hole lies within the nanocrystal gap near the conduction edge. For two Mn dopants, the most stable magnetic configuration is antiferromagnetic, and this was unexpected since (Ga,Mn)N bulk shows ferromagnetism in the ground state. The surprising antiferromagnetic alignment of two Mn spins is ascribed also to the holes linked to the Mn impurities located close to surface. Unlike Mn holes in (Ga,Mn)N bulk, these Mn holes in confined (Ga,Mn)N nanostructures do not contribute to the ferromagnetic alignment of the two Mn spins

First-principles calculations of the magnetic properties of (Cd,Mn)Te nanocrystals

We investigate the electronic and magnetic properties of Mn-doped CdTe nanocrystals (NCs) with 2 nm in diameter which can be experimentally synthesized with Mn atoms inside. Using the density-functional theory, we consider two doping cases: NCs containing one or two Mn impurities. Although the Mn d peaks carry five up electrons in the dot, the local magnetic moment on the Mn site is 4.65 mu_B. It is smaller than 5 mu_B because of the sp-d hybridization between the localized 3d electrons of the Mn atoms and the s- and p-type valence states of the host compound. The sp-d hybridization induces small magnetic moments on the Mnnearest- neighbor Te sites, antiparallel to the Mn moment affecting the p-type valence states of the undoped dot, as usual for a kinetic-mediated exchange magnetic coupling. Furthermore, we calculate the parameters standing for the sp-d exchange interactions. Conduction N0\alpha and valence N0\beta are close to the experimental bulk values when the Mn impurities occupy bulklike NCs' central positions, and they tend to zero close to the surface. This behavior is further explained by an analysis of valence-band-edge states showing that symmetry breaking splits the states and in consequence reduces the exchange. For two Mn atoms in several positions, the valence edge states show a further departure from an interpretation based in a perturbative treatment. We also calculate the d-d exchange interactions |Jdd| between Mn spins. The largest |Jdd| value is also for Mn atoms on bulklike central sites; in comparison with the experimental d-d exchange constant in bulk Cd0.95Mn0.05Te, it is four times smaller

Polymorphic MnAs nanowires of a magnetic shape memory alloy

We describe a magnetic shape memory alloy, in which it is the nanostructural confinement that influences both the crystal geometry and the electronic and magnetic properties. We use calculations from first-principles on shape memory MnAs nanowires to study the influence of strain on the resulting crystallographic phases, which arise at their surfaces. We show that MnAs nanowires as thin as two nanometers can be stable in a new crystal geometry which is induced by one-dimensionality and hence is unknown in the bulk, typically hexagonal. The changes between phases caused by differences in strain require the existence of twin domains. Our analysis suggests that the strain-induced structural transition-which is here described for MnAs compounds-could be applied to other (magnetic) shape memory nanowire systems for applications in a range of devices from mechanical to magneto-electronic. © 2014 the Owner Societies.This study was supported by funding from the Basque Government through the Nanomaterials project (Grant No. IE05-151) under the ETORTEK Program (iNanogune), the Spanish Ministerio de Ciencia y Tecnologia of Spain (Grant No. FIS 2010-19609-C02-02), and the University of the Basque Country (Grant No. IT-366-07).Peer Reviewe

Control of hole localization in magnetic semiconductors by axial strain

Mn and Fe-doped GaN are widely studied prototype systems for hole-mediated magnetic semiconductors. The nature of the hole states around the Mn and Fe impurities, however, remains under debate. Our self-interaction corrected density-functional calculations show that the charge neutral Mn0 and positively charged Fe+ impurities have symmetry-broken d5+h ground states, in which the hole is trapped by one of the surrounding N atoms in a small polaron state. We further show that both systems also have a variety of other d5+h configurations, including symmetric, delocalized states, which may be stabilized by axial strain. This finding opens a pathway to promote long-range hole-mediated magnetic interactions by strain engineering and clarifies why highly strained thin-films samples often exhibit anomalous magnetic properties.This work was funded by JSPS Grant-in-Aid for Scientific Research (C) No. 17K05494. H.R. is grateful to the São Paulo Research Foundation FAPESP and S.B. thanks the Rotary Yoneyama Memorial Foundation for support. A.A. acknowledges support of the Project FIS2016-76617-P of the Spanish Ministry of Economy and Competitiveness MINECO.Peer Reviewe

Antiferromagnetism in nanofilms of Mn-doped GaN

We theoretically study the role of surfaces in the electronic and magnetic properties of nanofilms made of wurtzite and zinc-blende (Ga,Mn)N. The studied doping reactions suggest that Mn impurities replacing Ga cations preferably stay just below the unsaturated surface rather than near the substrate. The hole-mediated ferromagnetism, typical of (Ga,Mn)N bulk, is absent from these films, and Mn moments for the most stable cationic positions become antiferromagnetically coupled. The holes ascribed to dopants in (Ga,Mn)N semiconductors are here occupied by electrons from dangling bonds. In other less-stable sites, Mn atoms are ferromagnetic; the surface moment can be then parallel to them and small, as in the wurtzite and zinc-blende (111) geometries, or antiparallel and large, as in zinc-blende (001). Hence, the magnetic interplay between surfaces and Mn impurities depends on the surface orientation, which could be useful for the design of magnetic nanodevices.This work has been supported by the Basque Government through the NANOMATERIALS project (Grant No. IE05-151) under the ETORTEK Program (iNanogune), the Spanish MCyT (FIS2010-19609-C02-02), and the UPV (Grant No. IT-366-07).Peer reviewe