Spintronics

Semiconductor nanostructures : magnetism inside !

The interest in semiconductor is due to the many opportunities offered by electrical doping and nano-structuration. Charge carriers (electrons and holes) can be introduced and manipulated by electric fields, using flexible configurations which combine bias voltage applied on gates and built-in interfaces. Here, the faster is the better. In semiconductors with a direct bandgap (such as III-Vs and II-VIs), optics further broadens the range of experimental tools and gives access to the spin states of the carriers.

By contrast, if magnetic systems have been widely used for information storage, this is because ferromagnetic domains are long lived. If the size is scaled down to the nanometer range, the anisotropy plays a crucial role in stabilizing the magnetization along well defined directions.

Can we introduce magnetic properties into semiconductor nanostructures ? One way is to dope the semiconductor with magnetic impurities : this is the concept of diluted magnetic semiconductors. In this case, a strong interaction exists between the spin of the impurity, and the carriers of the semiconductor. This gives rise to giant magneto-optical and magneto-electric effect. Two extreme cases can be contemplated :
- a single Mn impurity can be embedded in a quantum dot, and addressed optically through the carriers of the quantum dot.
- ferromagnetism can be induced by the interaction between an ensemble of magnetic impurities and a hole gas. (Ga,Mn)As is the most studied system, and spintronics devices are designed and studied in several laboratories worldwide.

Our approach is to use the specific properties of II-VI semiconductors (easy insertion of Mn which is an isoelectronic impurity, easy access to optical spectroscopy, all properties of semiconductors present) to design and grow nanostructures which behave as model systems, and obtain new functions in the frame of spintronics or quantum manipulation.

Present activities

Some of our present objectives are related to one question : can we use the confinement of carriers ? We illustrate this with two examples :

The single Mn atom in a quantum dot : speaking to a single spin with the language of nanoelectronics.

With a single Mn atom introduced in a II-VI quantum dot (Fig. 1a), the energy and polarization of the photon emitted or absorbed by the dot depends on the spin state of the S=5/2 magnetic atom : the exchange interaction between the confined electron-hole pair and the Mn spin splits the 2S+1=6 spin states of the Mn atom, leading to a 6-line optical spectrum for the quantum dot (Fig. 1b, top left). Laser excitation resonant with one of these optical transitions can be used to initialize the Mn spin and to probe its dynamics optically : the Mn atom behaves like an optically addressable long-lived spin-based memory. To go further, information processing using individual spins requires fast coherent control of a single spin and also tuning of the coupling between two spins. Preliminary steps are evidenced by the anticrossing appearing in the spectra of a quantum dot containing two Mn atoms (Fig. 1b, right and bottom) when the excitation intensity is increased.

Figure 1 : (a) AFM image of a layer of CdTe quantum dots deposited on a ZnTe substrate before their encapsulation by a ZnTe layer. Above : illustration of an encapsulated dot containing an individual Mn atom (green) and an optically-created electron hole pair (exciton). (b) At top : photoluminescence spectra resulting from the recombination of an exciton (X) in a CdTe quantum dot containing one Mn (left) or two Mn atoms (right). Below : Maps of photoluminescence intensity for the dot with two Mn atoms ; a single mode laser excites the dot above the range shown, around E1. In the upper panel we vary the laser power with the laser tuned to precise resonance with the highest energy state E1 (spin state |Sz1=+5/2 ; Sz2=+5/2 ; Jz=+1>, with Szi and Jz, the angular momentum of the Mn atom i and exciton respectively). In the lower panel, the laser is scanned through this energy. One observes a power-dependent and tuning-dependent splitting of the recombination of the lowest energy state |Sz1=+5/2 ; Sz2=+5/2 ;J=-1>. Since the ground state |Sz1=+5/2 ; Sz2=+5/2> is the same for both transitions, this shows that the Mn spins of the ground state are "dressed" by the resonant laser field.

An ensemble of magnetic impurities in a quantum dot : wavefunction and strain engineering of carrier-induced ferromagnetism.

At 3D (thick layers of (Ga,Mn)As and p-type (Zn,Mn)Te) and 2D ((Cd,Mn)Te quantum wells), the interaction between an ensemble of localized Mn spins and a gas of holes gives rise to ferromagnetism. The interest is twofold : the ferromagnetic character creates strong magneto-transport and magneto-optical properties, and ferromagnetic properties can be controlled by applying an electric field (Fig. 2a). One goal is now to move to 1D (nanowires) and 0D (quantum dots) in order to understand and optimize the role of confinement (can we increase the stability of the magnetic polaron by engineering the wavefunction of the carriers) and the anisotropy (can we enhanced the stability against reorientation by enhancing the anisotropy through strain engineering), as well as the coupling to the environment (surrounding spins and carriers, see Fig. 2b,c) and current-induced effects when inserting the quantum dot in a nanowire such as that in Fig. 2d.

Figure 2 : (a) control of the magnetic properties of a (Cd,Mn)Te quantum well by applying an electric field in a Field Effect Transistor structure. When the applied bias drives the carriers into the Mn-containing quantum well, a spontaneous magnetization appears at low temperature (blue symbols) ; this ferromagnetic character is suppressed when the electric field drives the carriers out of the quantum well (red symbols). (b) a single carrier polarizes the Mn spins around it : this is the so-called “magnetic polaron” ; (c) the formation of the magnetic polaron is favoured in a quantum dot, but it interacts with hot Mn spins present around the dot ; (d) a ZnTe nanowire.
Cohérence quantique - CQ

Cohérence quantique - CQ

Révéler des phénomènes quantiques dans des circuits électroniques nanométriques
Hélium : du fondamental aux applications - HELFA

Hélium : du fondamental aux applications - HELFA

Hélium comme système modèle, hydrodynamique et turbulence, spatial et astrophysique, instrumentation et développement cryogénique.
Magnétisme et Supraconductivité - MagSup

Magnétisme et Supraconductivité - MagSup

Equipe Magnétisme et supraconductivité à l’Institut NEEL - Systèmes impliquant différents degrés de liberté comme la charge, le spin ou le réseau.
Optique et Matériaux - OPTIMA

Optique et Matériaux - OPTIMA

Rassembler une chaine de compétence complète qui va de la synthèse et l’élaboration de matériaux nouveaux à l’étude des propriétés optiques non linéaires et plasmoniques
Matériaux, Rayonnements, Structure - MRS

Matériaux, Rayonnements, Structure - MRS

Compréhension des propriétés physico-chimiques de matériaux complexes sur la base d’une connaissance fine de leur structure
Micro et NanoMagnétisme - MNM

Micro et NanoMagnétisme - MNM

Complementary expertise in fabrication, characterisation, and simulations for studies in nanomagnetism with applications in spin electronics and micro-systems
Nano-Electronique Quantique et Spectroscopie - QNES

Nano-Electronique Quantique et Spectroscopie - QNES

Transport électronique et spectroscopie locale de structures quantiques
Nano-Optique et Forces - NOF

Nano-Optique et Forces - NOF

Nano - optique et forces
Nanophysique et Semiconducteurs - NPSC

Nanophysique et Semiconducteurs - NPSC

Élaboration de nanostructures de semi-conducteurs III-V et II-VI et étude de leurs propriétés physiques en vue de nouvelles fonctionnalités
Nanospintronique et Transport Moléculaire - NanoSpin

Nanospintronique et Transport Moléculaire - NanoSpin

Studying magnetism at the nanoscale, where classical and quantum properties can be combined and used for molecular quantum spintronics
Semi-conducteurs à large bande interdite - SC2G

Semi-conducteurs à large bande interdite - SC2G

De la physique du diamant et autres semi-conducteurs à grand gap vers les applications en électronique et biotechnologies
Surfaces, Interfaces et Nanostructures - SIN

Surfaces, Interfaces et Nanostructures - SIN

Etudes expérimentales et théoriques de systèmes de basse dimensionnalité
Systèmes Hybrides de basse dimensionnalité - HYBRID

Systèmes Hybrides de basse dimensionnalité - HYBRID

Propriétés électroniques, optiques, vibrationnelles, mécaniques, et leur couplage à l’échelle quantique, de nouveaux systèmes hybrides (nanotubes, graphène, matériaux bi-dimensionnels, fonctionnalisés) que l’équipe développe.
Théorie de la Matière Condensée -TMC

Théorie de la Matière Condensée -TMC

Phénomènes physiques nouveaux dans les matériaux et systèmes modèles.
Thermodynamique et biophysique des petits systèmes - TPS

Thermodynamique et biophysique des petits systèmes - TPS

Instrumentation ultrasensible pour sonder les propriétés électronique et thermique : de la matière condensée à basse température aux systèmes biologiques à l’ambiante.
Théorie Quantique des Circuits - ThQC

Théorie Quantique des Circuits - ThQC

Étude théorique du transport électronique dans des dispositifs nanométriques aux propriétés quantiques remarquables.
Ultra-basses températures - UBT

Ultra-basses températures - UBT

La physique quantique à la limite des ultra-basses températures.

Semiconductor nanowires for ultimate magnetic objects

Our research aims to fabricate, study, and manipulate different forms of magnetic polarons embedded in semiconductors nanowires. One challenge is to make a link between the quantum limit (single magnetic impurity, single carrier), and ferromagnetic-like system involving an ensemble of magnetic impurities and several carriers confined in a quantum dot, or a one dimensional hole gas. It may open also new routes for semiconductor structures embedding ferromagnetic elements, in the search for higher ordering temperatures by wavefunction engineering, for controlled anisotropy by strain engineering, and for strong magneto-electric effects.

Optically dressed magnetic atoms

The continuing decrease of the size of the structures used in semiconductor electronics and in magnetic information-storage devices has dramatically reduced the number of atoms necessary to process and store one bit of information : An individual magnetic atom would represent the ultimate size limit for storing and processing information. Towards this goal, we have demonstrated that an individual manganese atom embedded in a semiconductor quantum dot may act as a spin-based memory. Further, a pair of Mn atoms can act as a prototype of a pair of coupled memory units. We can exploit the optical absorption and emission of the quantum dot in order to initialize and to read out the spin state of the magnetic atoms. Under resonant optical excitation, we can enter the "strong coupling" regime where hybrid states of matter and the electromagnetic field are created, and this could be used for a coherent, optical “manipulation" of the Mn spin.

Optical control of an individual spin

People : L. Besombes, H. Boukari, T. Clement, D. Ferrand, H. Mariette Former members : Y. Leger, L. Maingault, J. Bernos Contact : lucien.besombes grenoble.cnrs.fr Overview Our group has active research activities in optical and magnetic interactions in semiconductor quantum structures... > suite

Spectroscopy and magnetic properties of wide band gap diluted magnetic semiconductors

People D. Ferrand, R. Giraud, D. Halley, H. Mariette, S. Marcet, W. Pacuski, E. Sarigiannidou, A. Titov Overview and results In II-VI and III-V diluted magnetic semiconductors (DMS), the spin carrier coupling between delocalized holes and localized spins is particularly efficient to induce... > suite

© Institut Néel 2012 l Webdesign chrisgaillard.com l Propulsé par spip l Dernière mise à jour : Monday 11 December 2017 l