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.
Quantum coherence - CQ

Quantum coherence - CQ

Revealing quantum phenomena in electronic nano-circuits
Helium : from fundamental to applications - HELFA

Helium : from fundamental to applications - HELFA

Helium as model system, hydrodynamic and turbulence, space and astrophysics, instrumentation and cryogenic development, kinetic inductance detectors.
Magnetism and Superconductivity - MagSup

Magnetism and Superconductivity - MagSup

Team Magnetism and Superconductivity at Institut NEEL - Systems involving charge, spin or lattice degrees of freedom.
Optics and materials - OPTIMA

Optics and materials - OPTIMA

a complete chain of competences that goes from the design and elaboration of new materials to the study of nonlinear optical properties and plasmonics
Materials, Radiations, Structure - MRS

Materials, Radiations, Structure - MRS

Understanding of the physico-chemical properties of complex materials based on the precise description of their structure
Micro and NanoMagnetism - MNM

Micro and NanoMagnetism - MNM

Complementary expertise in fabrication, characterisation, and simulations for studies in nanomagnetism with applications in spin electronics and micro-systems
Quantum Nano-Electronics and Spectroscopy - QNES

Quantum Nano-Electronics and Spectroscopy - QNES

Electron transport and local spectroscopy of quantum structures
Nano-Optics and Forces - NOF

Nano-Optics and Forces - NOF

Nano - optics and forces
Nanophysics and Semiconductors - NPSC

Nanophysics and Semiconductors - NPSC

Growth of III-V and II-VI semiconductor nanostructures and their physics in search of new functions for potential applications.
Nanospintronics and Molecular Transport - NanoSpin

Nanospintronics and Molecular Transport - NanoSpin

Studying magnetism at the nanoscale, where classical and quantum properties can be combined and used for molecular quantum spintronics
Wide bandgap semiconductors - SC2G

Wide bandgap semiconductors - SC2G

Physics of diamond and other wide bandgap semiconductors towards applications in electronics and biotechnologies
Surfaces, Interfaces and Nanostructures - SIN

Surfaces, Interfaces and Nanostructures - SIN

Experimental and theoretical studies of low dimensional systems
Hybrid systems at low dimensions - HYBRID

Hybrid systems at low dimensions - HYBRID

Electronic, optical, vibrational, mechanical properties, as well as their interplay at the nanoscale, of novel hybrid systems (nanotubes, graphene, two-dimensional and functionalized materials) which are developed by the group .
Condensed Matter Theory -TMC

Condensed Matter Theory -TMC

Novel physical phenomena in materials and model systems.
Thermodynamics and Biophysics of small systems - TPS

Thermodynamics and Biophysics of small systems - TPS

Ultra-sensitive instrumentation for electrical and thermal measurements: from biophysics to low temperature condensed matter physics.
Theory of Quantum Circuits - ThQC

Theory of Quantum Circuits - ThQC

Theoretical studies of electronic transport in nanometer-scale devices showing remarkable quantum effects.
Ultra-low temperatures - UBT

Ultra-low temperatures - UBT

Quantum physics at the ultra-low temperature frontier.

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... > 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

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