Strain in crystalline core-shell nanowires

The strain configuration induced by the lattice mismatch in a core-shell nanowire is calculated analytically, taking into account the crystal anisotropy and the difference in stiffness constants of the two materials. The method is applied to nanowires with the wurtzite structure or the zinc-blende structure with the hexagonal / trigonal axis along the nanowire, and the results are compared to available numerical calculations and experimental data. It is also applied to multishell nanowires, and to core-shell nanowires grown along the $$ axis of cubic semiconductors

Electric-field control of the magnetic anisotropy in an ultrathin (Ga,Mn)As/(Ga,Mn)(As,P) bilayer

We report on the electric control of the magnetic anisotropy in an ultrathin ferromagnetic (Ga,Mn)As/(Ga,Mn)(As,P) bilayer with competing in-plane and out-of-plane anisotropies. The carrier distribution and therefore the strength of the effective anisotropy is controlled by the gate voltage of a field effect device. Anomalous Hall Effect measurements confirm that a depletion of carriers in the upper (Ga,Mn)As layer results in the decrease of the in-plane anisotropy. The uniaxial anisotropy field is found to decrease by a factor ~ 4 over the explored gate-voltage range, so that the transition to an out-of-plane easy-axis configuration is almost reached

Magnetization dynamics down to zero field in dilute (Cd,Mn)Te quantum wells

The evolution of the magnetization in (Cd,Mn)Te quantum wells after a short pulse of magnetic field was determined from the giant Zeeman shift of spectroscopic lines. The dynamics in absence of magnetic field was found to be up to three orders of magnitude faster than that at 1 T. Hyperfine interaction and strain are mainly responsible for the fast decay. The influence of a hole gas is clearly visible: at zero field anisotropic holes stabilize the system of Mn ions, while in a magnetic field of 1 T they are known to speed up the decay by opening an additional relaxation channel

X-ray magnetic circular dichroism in (Ge,Mn) compounds: experiments and modeling

X-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectra at the L$_{2,3}$ edges of Mn in (Ge,Mn) compounds have been measured and are compared to the results of first principles calculation. Early \textit{ab initio} studies show that the Density Functional Theory (DFT) can very well describe the valence band electronic properties but fails to reproduce a characteristic change of sign in the L$_{3}$ XMCD spectrum of Mn in Ge$_3$Mn$_5$, which is observed in experiments. In this work we demonstrate that this disagreement is partially related to an underestimation of the exchange splitting of Mn 2$p$ core states within the local density approximation. It is shown that the change in sign experimentally observed is reproduced if the exchange splitting is accurately calculated within the Hartree-Fock approximation, while the final states can be still described by the DFT. This approach is further used to calculate the XMCD in different (Ge,Mn) compounds. It demonstrates that the agreement between experimental and theoretical spectra can be improved by combining state of the art calculations for the core and valence states respectively.Comment: 8 page

Optical properties of single ZnTe nanowires grown at low temperature

Optically active gold-catalyzed ZnTe nanowires have been grown by molecular beam epitaxy, on a ZnTe(111) buffer layer, at low temperature 350\degree under Te rich conditions, and at ultra-low density (from 1 to 5 nanowires per micrometer^{2}. The crystalline structure is zinc blende as identified by transmission electron microscopy. All nanowires are tapered and the majority of them are oriented. Low temperature micro-photoluminescence and cathodoluminescence experiments have been performed on single nanowires. We observe a narrow emission line with a blue-shift of 2 or 3 meV with respect to the exciton energy in bulk ZnTe. This shift is attributed to the strain induced by a 5 nm-thick oxide layer covering the nanowires, and this assumption is supported by a quantitative estimation of the strain in the nanowires

From diluted magnetic semiconductors to self-organized nanocolumns of GeMn in Germanium

While achieving high Curie temperatures (above room temperature) in diluted magnetic semiconductors remains a challenge in the case of well controlled homogeneous alloys, several systems characterized by a strongly inhomogeneous incorporation of the magnetic component appear as promising. Incorporation of manganese into germanium drastically alters the growth conditions, and in certain conditions of low temperature Molecular Beam Epitaxy it leads to the formation of well organized nanocolumns of a Mn-rich material, with a crystalline structure in epitaxial relationship with the Mn-poor germanium matrix. A strong interaction between the Mn atoms in these nanocolums is demonstrated by x-ray absorption spectroscopy, giving rise to a ferromagnetic character as observed through magnetometry and x-ray magnetic circular dichroism. Most interesting, intense magneto-transport features are observed on the whole structure, which strongly depend on the magnetic configuration of the nanocolumns.Comment: SPIE Optics & Photonics Symposium, San Diego : \'Etats-Unis d'Am\'erique (2008

Diluted Magnetic Semiconductors: Basic Physics and Optical Properties. Second edition

International audienceDiluted Magnetic Semiconductors (DMS) form a new class of magnetic materials, which fill the gap between ferromagnets and semiconductors. In the early literature these DMS were often named semimagnetic semiconductors, because they are midway between non magnetic and magnetic materials.DMS are semiconductor compounds (A1−xMxB) in which a fraction x of thecations is substituted by magnetic impurities , thereby introducing magnetic properties into the host semiconductor AB. This makes a great difference with semiconducting ferromagnets, i.e., ferromagnetic materials exhibitingsemiconductor-like transport properties, which have been known for some time (see a review in [2]). A DMS is expected to retain most of its classical semiconducting properties, and to offer the opportunity of a full integration into heterostructures, including heterostructures with the host material. The greatchallenge and ultimate goal of the research in this field is to obtain DMS ferromagnetic at room temperature, which can be integrated in semiconductor heterostructures for electronic or optoelectron c applications. This is one of the key issue for the advent of spintronics devices. Among the principal DMS families, II-VI and, to a less extent, III-V based DMS, with Mn as the magnetic impurity, are best understood. For this reason the present chapter will be mainly based on these compounds to introduce the well established basic physics of DMS. More details can be found in reviewpapers such as [3–5]. Some issues related to work in progress, generally on novel materials, will be also discussed but only briefly

VANNES DE SPIN ET JONCTIONS TUNNEL A BASE D'OXYDE DE NICKEL (LES BRIQUES ELEMENTAIRES D'UN TRANSISTOR MAGNETIQUE)

LES VANNES DE SPIN A BASE D'OXYDE DE NICKEL DU TYPE NIO / CO / CU / NI 8 0FE 2 0 SONT DES MULTICOUCHES MAGNETIQUES, DANS LESQUELLES L'AIMANTATION DE LA COUCHE FERROMAGNETIQUE DE COBALT EST PIEGEE PAR INTERACTION D'ECHANGE AVEC LA COUCHE ANTIFERROMAGNETIQUE D'OXYDE DE NICKEL. CES VANNES DE SPIN SONT PREPAREES PAR PULVERISATION CATHODIQUE MAGNETRON A PARTIR D'UNE CIBLE D'OXYDE DE NICKEL. SELON L'INCIDENCE DU DEPOT D'OXYDE DE NICKEL, IL EST POSSIBLE D'INDUIRE DANS CES SYSTEMES UN AXE D'ANISOTROPIE MARQUE, D'OU UNE REPONSE MAGNETORESISTIVE EN CRENEAUX. L'INTERACTION D'ECHANGE ENTRE L'OXYDE DE NICKEL ET LE COBALT SE TRADUIT PAR UN RENFORCEMENT MARQUE DU CHAMP COERCITIF DU COBALT A TEMPERATURE AMBIANTE ET PAR UN DECALAGE DE CYCLE A BASSE TEMPERATURE. L'AMPLITUDE DE MAGNETORESISTANCE PEUT ATTEINDRE 12% A TEMPERATURE AMBIANTE AVEC UNE SENSIBILITE DE L'ORDRE D'1%/OE. LES PROPRIETES STRUCTURALES, MAGNETIQUES ET DE TRANSPORT EN GEOMETRIE DE COURANT PLANAIRE ONT ETE ETUDIEES EN DETAIL. CES VANNES DE SPIN SONT DES CANDIDATS INTERESSANTS POUR DES APPLICATIONS EN TANT QUE CAPTEURS DE CHAMPS MAGNETIQUES OU MEMOIRES MAGNETIQUES NON VOLATILES, MAIS PEUVENT EGALEMENT ETRE INTEGREES DANS UN DISPOSITIF PLUS COMPLEXE APPELE TRANSISTOR A EFFET VANNE DE SPIN. CE TRANSISTOR DONT LA BASE EST CONSTITUEE D'UNE VANNE DE SPIN, COMPREND RESPECTIVEMENT AUX INTERFACES EMETTEUR / BASE ET BASE / COLLECTEUR UNE BARRIERE TUNNEL D'OXYDE DE NICKEL ET UNE JONCTION SCHOTTKY SI / PT. L'INTERET D'UN TEL DISPOSITIF EST DE PERMETTRE L'ETUDE DU TRANSPORT D'ELECTRONS CHAUDS DANS LES MULTICOUCHES MAGNETIQUES, C'EST-A-DIRE D'ELECTRONS EXCITES A DES ENERGIES SUPERIEURES A L'ENERGIE DE FERMI. UNE PREMIERE TENTATIVE D'INTEGRATION DES VANNES DE SPIN DANS LE TRANSISTOR A ETE REALISEE.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF