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The wider application of spintronic devices requires the development of several new material platforms that can address the following challenges: 1) efficient conversion between spin and charge currents, 2) storage of magnetic information that can be easily manipulated, and 3) transmission of spin information that can be easily detected. With respect to the first challenge, Bismuthate-based superconductors are systems that are generally thought to offer weak spin-orbit coupling, despite the heavy elements that make up such compounds. Here we use spin-torque ferromagnetic resonance to measure a large spin-orbit torque efficiency driven by spin polarization generated in heterostructures based on BaPb$_{1-x}$Bi$_{x}$O$_3$ in a non-superconducting state. We suggest that the unexpectedly large current-induced torques could stem from an orbital Rashba effect associated with local inversion symmetry breaking in BaPb$_{1-x}$Bi$_x$O$_3$.In response to the second challenge, Bismuth Ferrite has garnered considerable attention as a promising candidate for magnetoelectric spin-orbit coupled logic-in memory. While this model system offers a magnetic texture controllable by electric fields, epitaxial BiFeO$_3$ films have typically been deposited at temperatures higher than allowed for direction integration with silicon-CMOS platforms. Here, we solve this engineering problem by growing La-doped BiFeO$_3$ at temperatures reasonably compatible with silicon-CMOS integration on BaPb$_{1-x}$Bi$_x$O$_3$ electrodes. Despite the large lattice mismatch between the two materials, all layers are well-ordered with a [001] texture, and the La-doped BiFeO$_3$ exhibits desirable ferroelectric properties. These results provide a possible route for realizing epitaxial multiferroics on complex-oxide buffer layers at low temperatures and opens the door for potential silicon-CMOS integration. Furthermore, the incorporation of a material with efficient conversion between spin and charge with a multiferroic will drive innovation and application of new spintronic devices, such as all-oxide multiferroic magnonic memory architectures.In response to the second and third challenges, spin waves in magnetic materials, or magnons, are promising information carriers due to their ultra-low energy dissipation and long coherence length. Antiferromagnets are strong candidate materials for magnetic information storage due in part to their stability to external fields and larger group velocities. Multiferroic antiferromagnets such as BiFeO$_3$ have an additional degree of freedom stemming from magnetoelectric coupling, allowing for control of the magnetic structure, and thus magnons, with an electric field. Unfortunately, spin-wave propagation in BiFeO$_3$ is not well-understood due to the complexity of the magnetic structure. In this work, we discover an anisotropy in spin transport within the spin cycloid magnetic structure of BiFeO$_3$. We also show that through Lanthanum substitution, a single ferroelectric domain can be engineered with a stable, single-variant spin cycloid controllable by electric field. The strong anisotropy discussed is an important development in the understanding of magnon-spin currents in the spin cycloid magnetic texture.Finally, understanding the anisotropies and other characteristics of magnon-spin currents in a multiferroic requires an understanding of the symmetries inherent to the magnetic and polar orders of the multiferroic. In this work, we present a phenomenological model to elucidate the existence of magnon spin currents in generalized multiferroics. This model takes inspiration from the symmetries of multiferroics such as BiFeO$_3$, and is grounded in experimental data obtained from BiFeO$_3$ and its derivatives. By introducing this model, we address the issue of symmetry-allowed, switchable magnon spin transport in multiferroics, thereby establishing a critical framework for comprehending magnon transport within complex magnetic textures.Our responses to the three challenges posed above all point to a magnon-based multiferroic memory founded in epitaxial oxide heterostructures. In researching a path towards this application, we make new discoveries of the physics of spin-charge interconversion, heterostructure growth, and magnon dynamics in complicated magnetic textures. We hope that the following research will prove applicable not only towards the specific goal of multiferroic magnonic memory, but also to the wider field of epitaxial oxide spintronics.

The magnetic properties were investigated for C- and P-implanted MgO single crystals, which were irradiated by 80 keV C and P ions with the dose of 3 × 1017 ions/cm2. The magnetic properties of pristine MgO were apparently changed by C and P ion implantation. Room temperature ferromagnetism was presented in the C-implanted sample, while the P-implanted sample only displayed paramagnetism at 20 K. For the purpose of clarifying the correlation between the magnetic properties and microstructure, a comparative study was carried out using experimental and theoretical methods in both C and P ion-implanted samples. The defect types were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, photoluminescence, and absorption spectrum. The existence of intrinsic (Mg vacancies, O vacancies) and extrinsic (C-related and P-related) defects were verified by the experimental results. The magnetic properties induced by various single and composite defects were studied by first-principle calculations. The calculation results indicated that the configuration of VMg (Mg vacancy) + CO (C substitute O defect) was a key factor for the inducing ferromagnetic properties in C-implanted MgO. For the case of the P-implanted MgO, the configuration of P-related defects and intrinsic vacancies can only contribute to the total moment value but cannot induce ferromagnetism.

A theoretical investigation of the structural, electronic, magnetic, and magneto-elastic properties is provided for the non-perovskite HoMnO3 in low symmetry. This study has been carried out using density functional theory (DFT + U) with the inclusion of the polarized spin and spin-orbit coupling for both ferromagnetic and antiferromagnetic configurations. The generalized gradient approximation (GGA), the GGA + U, and modified Becke-Johnson (mBJ) approach have been used. The stability of the ground state of this compound in ferromagnetic (FM) and antiferromagnetic (AFM) configurations has been highlighted, confirming the magnetic behavior of HoMnO3. A good agreement has been observed between the internal atomic relaxation calculations and the experimental data. The calculated total magnetic moment of HoMnO3, localized on Ho atom is about ~ 9.6 µB for the ferromagnetic configuration, in good agreement with the available experimental data. On other hand, the GGA and GGA + U (where U ~ 7 eV) approximations have predicted a metallic behavior behavior for the h-HoMnO3, whereas, the implementation of mBJ approach has highlighted a semiconductor feature of the HoMnO3 with spin-polarized band gap about 0.5 eV (FM) and 0.6 eV (AFM). The calculated Bulk modulus has been found to be about 162.23 GPa (FM), and 169.34 GPa (AFM).

Novel devices are being investigated as artificial synapse candidates for neuromorphic computing. These memory devices share the characteristics of an electronic element called memristor. The memristor can be regarded as a resistor with a history dependent resistance, which mimics the plasticity of a biological synapse. The present work presents various types of candidate devices that have been proposed in neuromorphic research, describes how they mimic a biological synapse and how they can be employed in artificial neuron network architectures.

We investigate the structural, electronic and magnetic properties of the full Heusler compounds Mn$_2$YSn (Y = Mo, Nb, Zr) by first-principles density functional theory using the generalized gradient approximation. It is found that the calculated lattice constants are in good agreement with the theoretical values. We observe that the Cu$_2$MnAl-type structure is more stable than the Hg$_2$CuTi type. The calculated total magnetic moments of Mn$_2$NbSn and Mn$_2$ZrSn are 1 $\mu_{\text{B}}$ and 2 $\mu_{\text{B}}$ at the equilibrium lattice constant of 6.18 \AA and 6.31 \AA, respectively, for the Cu$_2$MnAl-type structure. Mn$_2$MoSn have a metallic character in both Hg$_2$CuTi and Cu$_2$MnAl type structures. The total spin magnetic moment obeys the Slater-Pauling rule. Half-metal exhibits 100% spin polarization at the Fermi level. Thus, these alloys are promising magnetic candidates in spintronic devices., Comment: 9 pages, 3 figures, 2 tables

Here we review various themes of current research within mesoscopic magnetic systems., Comment: APL Special Topic Guest Editorial

With the intention to reveal the effect of the substitution, Ti-doped InSb alloy, we accomplished a first-principles prediction within the FPLAPW+lo method. We used GGA-PBEsol scheme attached with the improved TB-mBJ approach to predict structural, electronic, and magnetic properties of In$_{1-x}$Ti$_{x}$Sb with concentration $x = 0$, $0.125$, $0.25$, $0.50$, $0.75$, $0.875$, and $1$. Our lattice parameters are found in favorable agreement with the available theoretical and experimental data. The calculation shows that all structures are energetically stable. The substitutional doping transforms the ionic character of the InSb compound in half-metallic ferromagnetic comportment for concentration $x = 0$, $0.125$, $0.25$, and $0.50$, with a spin polarization of $100$% at the Fermi level, and metallic nature for In$_{0.25}$Ti$_{0.75}$Sb and In$_{0.125}$Ti$_{0.875}$Sb. The total magnetic moments are also estimated at about 1 $\mu_{\text{B}}$. In$_{0.875}$Ti$_{0.125}$Sb, In$_{0.75}$Ti$_{0.25}$Sb, and In$_{0.50}$Ti$_{0.50}$Sb have half-metallic ferromagnets comportment and they can be upcoming applicants for spintronics applications., Comment: 13 pages, 17 figures, 3 tables

Taking the full advantage of the conformal growth characterizing atomic layer deposition (ALD), the possibility to grow Co thin films, with thickness from several tens down to few nanometers on top of a granular topological insulator (TI) Sb2Te3 film, exhibiting a quite high surface roughness (2–5 nm), was demonstrated. To study the Co growth on the Sb2Te3 substrate, we performed simultaneous Co depositions also on sputtered Pt substrates for comparison. We conducted a thorough chemical-structural characterization of the Co/Sb2Te3 and Co/Pt heterostructures, confirming for both cases, not only an excellent conformality, but also the structural continuity of the Co layers. X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) analyses evidenced that Co on Sb2Te3 grows preferentially oriented along the [00ℓ] direction, following the underlying rhombohedric substrate. Differently, Co crystallizes in a cubic phase oriented along the [111] direction when deposited on Pt. This work shows that, in case of deposition of crystalline materials, the ALD surface selectivity and conformality can be extended to the definition of local epitaxy, where in-plane ordering of the crystal structure and mosaicity of the developed crystallized grains are dictated by the underlying substrate. Moreover, a highly sharp and chemically-pure Co/Sb2Te3 interface was evidenced, which is promising for the application of this growth process for spintronics. [Figure not available: see fulltext.].

When coupled with ferromagnetic (FM) layers, topological insulators (TI) are expected to boost the charge-to-spin conversion efficiency across the FM/TI interface. In this context, a thorough control and optimization of the FM/TI interface quality are requested. In this paper, the evolution of the chemical, structural, and magnetic properties of the Fe/Sb2Te3 heterostructure, as a function of a rapid mild (up to 200 °C) thermal annealing conducted on the Sb2Te3-TI only, is presented. The annealing of Sb2Te3 markedly improves its crystalline quality, leading to an increased fraction of ferromagnetic Fe atoms at the buried Fe/Sb2Te3 interface and a slight lowering of the Fe layer coercive field. This is quite surprising, since the bi-layer is not subjected to any thermal treatment upon Fe deposition. This method is an efficient tool to clean-up the Fe/Sb2Te3 interface, which may be extended to different FM/TI heterostructures. Simultaneously to the interface reconstruction, a constant 20% fraction of FeTe develops at the interface. Since ultrathin FeTe has been reported to be superconducting, these results can open perspective in exploiting phenomena at the edge of magnetism, superconductivity and topology.

Spin-charge interconversion phenomena at the interface between magnetic materials and topological insulators (TIs) are attracting enormous interest in the research effort towards the development of fast and ultra-low power devices for the future information and communication technology. We report a large spin-to-charge conversion efficiency in Au/Co/Au/Sb2Te3/Si(111) heterostructures based on Sb2Te3 TIs grown by metal organic chemical vapor deposition on 4 inches Si(111) substrates. By conducting room temperature spin pumping ferromagnetic resonance, we measure an inverse Edelstein Effect length {\lambda}IEE up to 0.75 nm, a record value for 3-dimensional chalcogenide-based TIs heterostructures. Our results open the path toward the use of chemical methods to produce TIs on large area Si substrates and characterized by highly performing spin-charge conversion, thus marking a milestone toward future technology-transfer., Comment: Main text: 28 pages, 5 figures and 2 tables. Supplementary information are also included in the file with additional 12 pages

We address the problem of electronic and nuclear spin resonance of an individual atom on a surface driven by a scanning tunnelling microscope. Several mechanisms have been proposed so far, some of them based on the modulation of exchange and crystal field associated to a piezoelectric displacement of the adatom driven by the RF tip electric field. Here we consider a new mechanism, where the piezoelectric displacement modulates the g factor anisotropy, leading both to electronic and nuclear spin flip transitions. We discuss thoroughly the cases of Ti-H (S = 1/2) and Fe (S = 2) on MgO, relevant for recent experiments. We model the system using two approaches. First, an analytical model that includes crystal field, spin orbit coupling and hyperfine interactions. Second, we carry out density functional based calculations. We find that the modulation of the anisotropy of the g tensor due to the piezoelectric displacement of the atom is an additional mechanism for STM based single spin resonance, that would be effective in S = 1/2 adatoms with large spin orbit coupling. In the case of Ti-H on MgO, we predict a modulation spin resonance frequency driven by the DC electric field of the tip., Comment: 15 pages, 7 figures

We use firstprinciples calculations to study the electronic and magneticproperties of undoped and Al-doped SnS and their dependence withspace-group symmetry. The formation of a magnetic moment due to animpurity element was analyzed by substitutional doping inPnma(62)-SnS and C2mb(39)-SnS symmetries. The holes introduced bycation Al-doping leads to the formation of a magnetic moment of 1.0 μB /impurity, and a half-metallic behavior. Orbital configurationswith local magnetic moments are established due to stronghybridization between {s;p} -Al orbitals and {s;p} -S orbitals. Thecollective magnetic order was analyzed by the effectiveexchange-coupling parameters between Al atoms at different atomiccationic sites, and different hole concentrations. These resultsindicate that the stabilization of the magnetic order presents astrong dependence with the spatial arrangement and the exchangepathways. For nearby dopants, and at the same holes concentration,both the Pnma and the C2mb symmetries present an AFM behavior. Thiscollective magnetic order changes to a ferromagnetic coupling for thecase in which the dopants are further away or when there is a higherholes concentration. The origin of such magnetic ordering can beunderstood based on the competition between hole-mediatedsuperexchange and double exchange mechanisms. Our predictive resultsoffer the possibility to distinguish between holes doping effect andthe role of symmetry in the coordination of the dopant, of greatimportance to the understanding of the origin of defect-inducedmagnetism.p { margin-bottom: 0.1in; line-height: 115%; } Fil: Zandalazini, Carlos Ivan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Física del Litoral. Universidad Nacional del Litoral. Instituto de Física del Litoral; Argentina Fil: Albanesi, Eduardo Aldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Física del Litoral. Universidad Nacional del Litoral. Instituto de Física del Litoral; Argentina

Funding: Framework of the nanoscience foundry and fine analysis (NFFA-MIUR Italy Progetti Internazionali) project; Leverhulme Trust (Grant No. RL-2016-006); Royal Society. Among Transition-Metal Dichalcogenides, mono and few-layers thick VSe2 has gained much recent attention following claims of intrinsic room-temperature ferromagnetism in this system, which have nonetheless proved controversial. Here, we address the magnetic and chemical properties of Fe/VSe2 heterostructure by combining element sensitive absorption spectroscopy and photoemission spectroscopy. Our x-ray magnetic circular dichroism results confirm recent findings that both native mono/few-layer and bulk VSe2 do not show any signature of an intrinsic ferromagnetic ordering. Nonetheless, we find that ferromagnetism can be induced, even at room temperature, after coupling with a Fe thin film layer, with antiparallel alignment of the moment on the V with respect to Fe. We further consider the chemical reactivity at the Fe/VSe2 interface and its relation with interfacial magnetic coupling. Postprint

Recently, considerable attention has been paid to the Double Perovskite (DP) Sr2CrOsO6 due to its high magnetic polarization and its high Tc = 720 K. Several publications have appeared documenting some of its properties, but there is still a lack of enough technical and theoretical information about it. Therefore, to better understand how the 5d-Os atoms act on this Cr-based DP, it seems useful to take advantage from knowledge about the most studied Cr-based DP Sr2CrReO6, previously known to have the highest Tc = 610 K. This paper proposes a theoretical support by Monte Carlo Simulation (MCS) to study the magnetic properties of the diluted DP Sr2CrRexOs1−xO6 doped by Re-substitution. It has been found that the presence although minimal of Os-atoms in the system, is in favor of increasing Tc considerably. At 60% of Os-concentration, a certain competition has been noticed between ferromagnetic double-exchange (DE) and antiferromagnetic super-exchange (SE) interactions. It has been proposed that such a competition occurs at the half-metal-insulator transition due to 5d-band filling. Osmium crystal field effect on Tc, correlated with the Os-concentration in the system, has been also reported.

A first-principles approach is used to study the structural, electronic and magnetic properties of the Co 2 TaAl Heusler compound with CuHg 2 Ti-type structure. The investigation was done using the (FP-LAPW) method where the exchange-correlation potential was calculated with the frame of GGA by Perdew et al. (Phys. Rev. Lett. 77 (1996) 3865). At ambient conditions our calculations predict that Co 2 TaAl is half-metallic ferromagnet (HMF) with a magnetic moment of 2 μ B /fu and HM flip gap of 0.58 eV. In addition, the ferromagnetic phase is found to be energetically more favorable than paramagnetic phase. Therefore, the Co 2 TaAl Heusler compound is a candidate material for future spintronic applications.

The observation of perpendicular magnetic anisotropy (PMA) at MgO/Fe interfaces boosted the development of spintronic devices based on ultrathin ferromagnetic layers. Yet, magnetization reversal in the standard magnetic tunnel junctions (MTJs) with competing PMA and in-plane anisotropies remains unclear. Here we report on the field induced nonvolatile broken symmetry magnetization reorientation transition from the in-plane to the perpendicular (out of plane) state at temperatures below 50 K. The samples were 10 nm thick Fe in MgO/Fe(100)/MgO as stacking components of V/MgO/Fe/MgO/Fe/Co double barrier MTJs with an area of 20 × 20 μm2. Micromagnetic simulations with PMA and different second order anisotropies at the opposite Fe/MgO interfaces qualitatively reproduce the observed broken symmetry spin reorientation transition. Our findings open the possibilities to develop multistate epitaxial spintronics based on competing magnetic anisotropies., This work has been supported in part by Spanish MINECO (MAT2015-66000-P, EUIN2017-87474), SPINORBIT (MDM-2014-0377) and Comunidad de Madrid (NANOFRONTMAG-CM S2013/MIT-2850). C.T. acknowledges “EMERSPIN” grant ID PN-III-P4-ID-PCE-2016-0143, No. UEFISCDI:22/12.07.2017

We present a unified theoretical framework for the study of spin dynamics and relativistic transport phenomena in disordered two-dimensional Dirac systems with pseudospin-spin coupling. The formalism is applied to the paradigmatic case of graphene with uniform Bychkov-Rashba interaction and shown to capture spin relaxation processes and associated charge-to-spin interconversion phenomena in response to generic external perturbations, including spin density fluctuations and electric fields. A controlled diagrammatic evaluation of the generalized spin susceptibility in the diffusive regime of weak spin-orbit interaction allows us to show that the spin and momentum lifetimes satisfy the standard Dyakonov-Perel relation for both weak (Gaussian) and resonant (unitary) nonmagnetic disorder. Finally, we demonstrate that the spin relaxation rate can be derived in the zero-frequency limit by exploiting the SU(2) covariant conservation laws for the spin observables. Our results set the stage for a fully quantum-mechanical description of spin relaxation in both pristine graphene samples with weak spin-orbit fields and in graphene heterostructures with enhanced spin-orbital effects currently attracting much attention., 18 pages, 4 figures, 1 table

The spin galvanic effect (SGE) describes the conversion of a non-equilibrium spin polarization into a charge current and has recently attracted renewed interest due to the large conversion efficiency observed in oxide interfaces. An important factor in the SGE theory is disorder which ensures the stationarity of the conversion. Through this paper, we propose a procedure for the evaluation of the SGE on disordered lattices which can also be readily implemented for multiband systems. We demonstrate the performance of the method for a single-band Rashba model and compare our results with those obtained within the self-consistent Born approximation for a continuum model.

Fundamental research and technological applications of topological insulators are hindered by the rarity of materials exhibiting a robust topologically nontrivial phase, especially in two dimensions. Here, by means of extensive first-principles calculations, we propose a novel quantum spin Hall insulator with a sizable band gap of $\ensuremath{\sim}0.5\text{ }\text{ }\mathrm{eV}$ that is a monolayer of jacutingaite, a naturally occurring layered mineral first discovered in 2008 in Brazil and recently synthesized. This system realizes the paradigmatic Kane-Mele model for quantum spin Hall insulators in a potentially exfoliable two-dimensional monolayer, with helical edge states that are robust and that can be manipulated exploiting a unique strong interplay between spin-orbit coupling, crystal-symmetry breaking, and dielectric response.

We evaluate the spin polarization (Edelstein or inverse spin galvanic effect) and the spin Hall current induced by an applied electric field by including the weak localization corrections for a two-dimensional electron gas. We show that the weak localization effects yield logarithmic corrections to both the spin polarization conductivity relating the spin polarization and the electric field and to the spin Hall angle relating the spin and charge currents. The renormalization of both the spin polarization conductivity and the spin Hall angle combine to produce a zero correction to the total spin Hall conductivity as required by an exact identity. Suggestions for the experimental observation of the effect are given.

Control of electron spin coherence via external fields is fundamental in spintronics. Its implementation demands a host material that accommodates the desirable but contrasting requirements of spin robustness against relaxation mechanisms and sizeable coupling between spin and orbital motion of the carriers. Here, we focus on Ge, which is a prominent candidate for shuttling spin quantum bits into the mainstream Si electronics. So far, however, the intrinsic spin-dependent phenomena of free electrons in conventional Ge/Si heterojunctions have proved to be elusive because of epitaxy constraints and an unfavourable band alignment. We overcome these fundamental limitations by investigating a two-dimensional electron gas in quantum wells of pure Ge grown on Si. These epitaxial systems demonstrate exceptionally long spin lifetimes. In particular, by fine-tuning quantum confinement we demonstrate that the electron Landé g factor can be engineered in our CMOS-compatible architecture over a range previously inaccessible for Si spintronics., Ge/Si heterojunctions are prominent candidates for spintronics, but the spin-dependent phenomena have been elusive. Here, Giorgioni et al. report long spin relaxation and coherence times in a two dimensional electron gas confined in quantum wells of pure Ge grown on SiGe-buffered Si substrates.

Theory has predicted that high temperature ferromagnetism (FM) should be found in cubic fake-diamonds, Mn-doped ZrO 2 . Experimentally, it is shown that Mn-doped ZrO 2 ceramics are not ferromagnetic, but the nanosized Mn-doped ZrO 2 thin films grown on LaAlO 3 substrates can be ferromagnets with T C above 400 K. The largest saturated magnetic moment ( M s ) is huge as of about 230 emu/cm 3 for the Mn 0.05 Zr 0.95 O 2 films, and it decreases as the Mn content increases. The intrinsic FM is strongly associated with the cubic structure of Mn-doped ZrO 2 , and the Mn–Mn interactions via oxygen intermediates are important. No electrical conductivity is observed. Mn-doped ZrO 2 thin films can be truly considered as excellent candidates for spintronic applications.

We have developed nanoscale double-gated field-effect-transistors for the study of electron states and transport properties of single deliberately implanted phosphorus donors. The devices provide a high-level of control of key parameters required for potential applications in nanoelectronics. For the donors, we resolve transitions corresponding to two charge states successively occupied by spin down and spin up electrons. The charging energies and the Lande g-factors are consistent with expectations for donors in gated nanostructures.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; Interplay between voltage induced magnetic anisotropy transition and voltage induced atomic diffusion is studied in epitaxial V/Fe (0.7 nm)/ MgO/ Fe(5 nm)/Co/Au magnetic tunnel junction where thin Fe soft electrode has in-plane or out-of-plane anisotropy depending on the sign of the bias voltage. We investigate the origin of the slow resistance variation occurring when switching bias voltage in opposite polarity. We demonstrate that the time to reach resistance stability after voltage switching is reduced when increasing the voltage amplitude or the temperature. A single energy barrier of about 0.2 eV height is deduced from temperature dependence. Finally, we demonstrate that the resistance change is not correlated to a change in soft electrode anisotropy. This conclusion contrasts with observations recently reported on analogous systems.

The shape instability of magnetic domain walls under current is investigated in a ferromagnetic (Ga,Mn)(As,P) film with perpendicular anisotropy. Domain wall motion is driven by the spin transfer torque mechanism. A current density gradient is found either to stabilize domains with walls perpendicular to current lines or to produce finger-like patterns, depending on the domain wall motion direction. The instability mechanism is shown to result from the non-adiabatic contribution of the spin transfer torque mechanism., 5 pages, 3 figures + supplementary materials

A theory for the electron (and hole) $g$ factor in multivalley lead-salt IV-VI semiconductor quantum wells (QWs) is presented. An effective Hamiltonian for the QW electronic states in the presence of an external magnetic field is introduced within the envelope-function approximation, based on the multiband $kp$ Dimmock model for the bulk. The mesoscopic spin-orbit (Rashba-type) and Zeeman interactions are taken into account on an equal footing and the effective $g$ factor in symmetric quantum wells (${g}_{\mathrm{QW}}^{*}$) is calculated analytically for each nonequivalent conduction-band (and valence-band) valley, and for QWs grown along different crystallographic directions.

The spin-orbit interaction strength for electrons in III-V semiconductor heterojunctions and the corresponding in-plane anisotropy are theoretically studied, considering Rashba and Dresselhaus contributions. Starting from a variational solution of Kane's effective Hamiltonian for the Rashba-split subbands, the total spin-orbit splitting at the Fermi level of the two-dimensional electron gas in III-V heterojunctions is calculated analytically, as a function of the electron density and wave-vector direction, by adding the Dresselhaus contribution within quasidegenerate first-order perturbation theory. Available GaAs and InGaAs experimental data are discussed. Effects of the barrier penetration are identified, and the spin-orbit anisotropy is shown to be determined by more than one parameter, even in the small-$k$ limit, contrary to the commonly used $\ensuremath{\alpha}/\ensuremath{\beta}$ (where $\ensuremath{\alpha}$ is the Rashba and $\ensuremath{\beta}$ the Dresselhaus interaction) single-parameter picture.

The renormalization of the electron $g$ factor by the confining potential in semiconductor nanostructures is considered. A new effective $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ Hamiltonian for the electronic states in III--V semiconductor nanostructures in the presence of an external magnetic field is introduced. The mesoscopic spin-orbit (Rashba type) and Zeeman interactions are taken into account on an equal footing. It is then solved analytically for the electron effective $g$ factor in symmetric quantum wells (${g}_{\mathrm{QW}}^{*}$). Comparison with different spin quantum beat measurements in GaAs and InGaAs structures demonstrates the accuracy and utility of the theory. The quantum size effects in ${g}_{\mathrm{QW}}^{*}$ are easily understood and its anisotropy $\ensuremath{\Delta}{g}_{\mathrm{QW}}^{*}$ (i.e., the difference between the in-plane and perpendicular configurations) is shown to be given by a mesoscopic spin-orbit effect having the same origin as the Rashba one.

We demonstrate optical orientation in Ge/SiGe quantum wells and study their spin properties. The ultrafast electron transfer from the center of the Brillouin zone to its edge allows us to achieve high spin-polarization efficiencies and to resolve the spin dynamics of holes and electrons. The circular polarization degree of the direct-gap photoluminescence exceeds the theoretical bulk limit, yielding ~37% and ~85% for transitions with heavy and light holes states, respectively. The spin lifetime of holes at the top of the valence band is found to be ~0.5 ps and it is governed by transitions between heavy and light hole states. Electrons at the bottom of the conduction band, on the other hand, have a spin lifetime that exceeds 5 ns below 150 K. Theoretical analysis of the electrons spin relaxation indicates that phonon-induced intervalley scattering dictates the spin lifetime., 5 pages, 3 figures

A spin-dependent variational theory is used to analyze the Rashba spin-orbit splitting in two-dimensional electron gases formed in III--V semiconductor inversion layers. The spin split conduction subbands in CdTe/InSb, insulator/InAs, InP/InGaAs, InAlAs/InGaAs, and AlGaAs/GaAs heterojunctions are calculated. The theory, presented here in detail, is based on the 8 $\ifmmode\times\else\texttimes\fi{}$ 8 $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ Kane model and on the introduction of simple and convenient spin-dependent Fang-Howard trial functions, and leads to analytical expressions for the split subbands, as well as allows for a detailed knowledge of the Rashba spin-orbit coupling, including its explicit dependence on structure parameters and its decomposition into separate contributions. The Rashba coupling parameter and the population difference in the spin-split subbands, as experimentally determined from the beating pattern of the Shubnikov-de Haas (SdH) oscillations, are obtained as a function of the electron density (${n}_{s}$). The separate contributions to the particularly large Rashba splitting in CdTe/InSb heterojunctions are also computed and discussed. It is shown, for example, that due to the spin-dependent boundary conditions, the direct Rashba spin-orbit coupling term in the effective Hamiltonian dominates the splitting only for ${n}_{s}g{10}^{10}$ cm${}^{\ensuremath{-}2}$ while it is the barrier penetration kinetic energy term that gives the largest contribution to the Rashba effect at lower densities.

In this work, we investigated the magnetotransport properties of a two dimensional electron gas hosted in an AlGaN/AlN/GaN heterostructure and one-dimensional devices fabricated on it. At cryogenic temperature, high mobility and long mean free path is achieved, allowing ballistic transport experiments. Longitudinal resistivity measured in Hall bar geometry shows well-developed Shubnikov–de Haas oscillations with amplitude modulation. Amongst possible mechanisms, the zero-field spin splitting may be the origin of the observed effects. Split gate quantum point contacts were fabricated by electron beam lithography. Linear conductance measurements at zero magnetic field show clear quantized conductance plateaus at 2e 2/h and 4e 2/h. Non-perfectly quantized conductance values are found for higher plateaus, suggesting the presence of impurity scattering.

Magnetic tunnel junctions sandwiching a superlattice thin film of iron oxide nanocrystals (NCs) have been investigated. The transport was found to be controlled by Coulomb blockade and single-electron tunneling, already at room temperature. A good correlation was identified to hold between the tunnel magnetoresistance (TMR), the expected magnetic properties of the NC arrays, the charging energies evaluated from current-voltage curves, and the temperature dependence of the junction resistance. Notably, for the first time, a switching from negative to positive TMR was observed across the Verwey transition, with a strong enhancement of TMR at low temperatures.

International audience; Time-resolved scanning transmission x-ray microscopy measurements were performed to study the current-induced magnetization switching mechanism in nanopillars exhibiting strong perpendicular magnetic anisotropy. This technique provides both short-time (70 ps) and high-spatial (25 nm) resolutions. Direct imaging of the magnetization demonstrates that, after an incubation time of ∼1.3 ns, a 100 × 300 nm 2 ellipsoidal device switches in ∼1 ns via a central domain nucleation and opposite propagation of two domain walls toward the edges. High domain-wall velocities on the order of 100 m/s are measured. Micromagnetic simulations are shown to be in good agreement with experimental results and provide insight into magnetization dynamics during the incubation and reversal periods. Spin-polarized current-induced magnetization switching (CIMS) has now been reported in many experimental works involving a wide variety of geometries including point contacts, nanopillars (spin valves or tunnel junctions), and nanowires with or without notches. 1 These systems are extensively studied, in part, because they hold the potential for applications in spin-transfer magnetic random access memory. 2 Interest in materials with perpendicular magnetic anisotropy (PMA) has grown considerably as a pathway for lowering the critical current required to switch the magnetization while maintaining thermal stability as compared with in-plane systems. 3,4 This interest has, to a large degree, stemmed from calculations of the switching behavior using a macrospin approximation. 5

Recently, we have proposed a spin quantum cross structure (SQCS) device toward the realization of novel spintronics devices. In this paper, we have investigated thermoelectric effects in point contacts (PCs) of Ni ferromagnetic metals using SQCS devices, theoretically and experimentally. The calculated results show that the thermoelectric voltage Vq changes from 0.48 mV to 2.12 mV with the temperature difference of PCs increasing from 10 K to 50 K. Also, the magnitude of the theoretical thermoelectric voltage agrees very well with that of the experimental result. PCs of SQCS devices with Ni electrodes can serve as spin dependent thermobatteries.

By means of ab initio calculations and spin-polarized scanning tunneling microscopy experiments the creation of a complex energy dependent magnetic structure with a tailored spin-polarized interface is demonstrated. We show this novel effect by adsorbing organic molecules containing $\ensuremath{\pi}({p}_{z})$ electrons onto a magnetic surface. The hybridization of the out-of-plane ${p}_{z}$ atomic-type orbitals with the $d$ states of the metal leads to the inversion of the spin polarization at the organic site due to a ${p}_{z}\mathrm{\text{\ensuremath{-}}}d$ Zener exchange-type mechanism. As a key result, we demonstrate the possibility to selectively and efficiently inject spin-up and spin-down electrons from a ferromagnetic-organic interface, an effect which can be exploited in future spintronic devices.

International audience; An experimental technique has been developed to characterize spin-transfer driven magnetization dynamics. It was tested on a nanopillar spin valve with perpendicular anisotropy by measuring the nanopillar voltage under ac injected current ͑dV / dI͒, and ac magnetic field ͑dV / dH͒. Both the amplitude and the sign of the signals are different which reveals the different influences of the current and the field on the magnetization dynamics. Comparison between experiments and macrospin simulation shows that dV / dH measurements reveal the presence of a " canted state " demonstrating that dV / dH and dV / dI measurements are complementary techniques to probe magnetic states and their dynamics. It is now well established that magnetization switching or precession can be induced by spin polarized injected current due to the spin transfer torque ͑STT͒ effect. 1,2 This topic has been extensively studied because of the interesting physics and the potential applications for spin transfer magnetic random access memories and high-frequency oscillators.

The zero bias anomaly in the conductance due to Kondo correlations in a quantum dot at Coulomb blockade with an even number of electrons has been only clearly experimentally seen at the singlet–triplet crossing. However, S = 1 Kondo correlations should also be present in a dot in the absence of magnetic field if Hund's rule enforces a triplet ground state. We propose that the elusive appearance of the S = 1 Kondo anomaly in semiconductor quantum dots is due to spin–orbit interactions, which we find to be a relevant perturbation of the Kondo fixed point in the NRG flow.

Symposium on Novel Materials and Devices for Spintronics, San Francisco, CA, APR 14-17, 2009; International audience; The spin dynamics of resident holes in singly p doped InAs/GaAs quantum dots is studied by pump probe photo-induced circular dichroism experiments We show that the hole spin dephasing is controlled by the hyperfine interaction between the hole spin and nuclear spins We find a characteristic hole spin dephasing time of 12 ns, in close agreement with our calculations based on a dipole-dipole coupling between the hole and the quantum dot nuclei Finally we demonstrate that a small external magnetic field typically 10 mT quenches the hyperfine hole spin dephasing

The magnetic configuration of a nanostructure can be altered by an external magnetic field, by spin-transfer torque or by its magnetoelastic response. Here, we explore an alternative route, namely the possibility of switching the sign of the exchange coupling between two magnetic centres by means of an electric potential. This general effect, which we name electrostatic spin crossover, occurs in insulating molecules with super-exchange magnetic interaction and inversion symmetry breaking. As an example we present the case of a family of di-cobaltocene-based molecules. The critical fields for switching, calculated from first principles, are of the order of 1 V nm−1 and can be achieved in two-terminal devices. More crucially, such critical fields can be engineered with an appropriate choice of substituents to add to the basic di-cobaltocene unit. This suggests that an easy chemical strategy for achieving the synthesis of suitable molecules is possible. Controlling the magnetic properties by using an electric field is a promising route towards spintronics or magnetic data storage applications. Using dicobaltocene as a test case, it is now demonstrated theoretically that an electrostatic potential can be used to control the spin states in molecules.

The effect of a magnetic domain wall on the electronic transport in disordered materials is studied in an exchange-coupled amorphous ${\text{Gd}}_{40}{\text{Fe}}_{60}/{\text{Gd}}_{10}{\text{Fe}}_{90}$ bilayer. In this amorphous system, the size and the shape of an interfacial domain wall is controlled by an external magnetic field. Current-in-plane transport measurements are performed on single GdFe layers, ${\text{Gd}}_{40}{\text{Fe}}_{60}/{\text{Gd}}_{10}{\text{Fe}}_{90}$ bilayer, and on a ${\text{Gd}}_{40}{\text{Fe}}_{60}/\text{Si}/{\text{Gd}}_{10}{\text{Fe}}_{90}$ trilayer where the Si layer prevents the formation of the interfacial magnetic domain wall. Different contributions to the resistance are evidenced. In all types of samples, a linear positive magnetoresistance contribution is observed at high field which can be linked to the amorphous structure of the GdFe alloys. The comparison between the bilayer and the trilayer allows to eliminate this contribution and evidences that anisotropic magnetoresistance is the main effect induced by the interfacial domain wall. Beyond the anisotropic magnetoresistance signal, a supplementary negative magnetoresistance is evidenced. The origin of this effect is discussed qualitatively using previous theoretical predictions on magnetotransport through a magnetic domain wall in disordered metals.

International audience; In this work we present a magneto-structural study of MnAs/Fe bilayers, epitaxially grown on GaAs(1 0 0). The coexistence of ferromagnetic and non-magnetic phases is observed by scanning tunneling microscopy (STM) at room temperature. The magnetic force microscopy (MFM) images of the samples show a stripe-patterned domain structure. The magnetization reversal at the coercive field is observed at MFM micrographs measured at different fields. The temperature dependence of the ferromagnetic resonance (FMR) spectra allows us to follow the evolution of the phase coexistence up to the Curie temperature. Two main lines can be identified in the FMR spectra in the phase coexistence region: one of them is associated to the FMR of the Fe layer. The other one, near 3000 Oe, could be associated to the resonance of Mn ions. The general appearance of the FMR spectra is strongly temperature dependent. The resonance field, line width, and shape of the Fe line is strongly altered in the α and β coexistence region, where a severe drop of the resonance field is observed. The temperature dependence of the iron resonance nicely follows the transition of the underlying MnAs layer.

International audience; Soon afterwards the discovery of the giant magnetoresistance in metallic multilayers, researchers have attempted to integrate spintronic properties with semiconductor materials. They came up against several difficulties related to the structural and electronic properties of the ferromagnetic metal-semiconductor interface. We will report on the recent progress made in this field of spintronic with semiconductors. First of all we will explain the interfacial resistance conditions required to inject and detect efficient spin current in a semiconductor and in a second part we will show that efficient spin injection experiments have been now achieved thanks to the addition of a tunnel resistance at the interface. We will then report on the magnetoresistance experiment performed with diluted magnetic semiconductors as ferromagnetic material. This type of material can constitute an alternative road to achieving electrical control spintronic devices. Finally, we will finish by reporting on research for a highly spin-polarized source to inject spin-polarized current in a semiconductor. It will be mainly focused on tunnel magnetoresistance junctions with semiconductor barriers and hot electron transistor. To cite this article: J.-M. George et al., C R. Physique 6 (2005). (c) 2005 Academie des sciences. Published by Elsevier SAS. All rights reserved.

For spintronics, both, the properties of small ferromagnetic particles as well as the properties of ferromagnet-semiconductor hybrid structures are of importance. Below we describe how micro-Hall magnetometry provides information about the magnetisation switching of small ferromagnetic particles. In the second part we demonstrate spin dependent transport through thin GaAs membranes. The latter experiments point to the important role of spin-flip scattering in ferromagnet-semiconductor hybrid structures.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; The strong impact of molecular beam epitaxy growth and Synchrotron Radiation characterization tools in the understanding of fundamental issues in nanomagnetism and spintronics is illustrated through the example of fully epitaxial MgO-based Magnetic Tunnel Junctions (MTJs). If ab initio calculations predict very high tunnel magnetoresistance (TMR) in such devices, some discrepancy between theory and experiments still exists. The influence of imperfections in real systems has thus to be considered like surface contaminations, structural defects, unexpected electronic states, etc. The influence of possible oxygen contamination at the Fe/MgO(001) interface is thus studied, and is shown to be not so detrimental to TMR as predicted by ab initio calculations. On the contrary, the decrease of dislocations density in the MgO barrier of MTJs using Fe1-xVx electrodes is shown to significantly increase TMR. Finally, unexpected transport properties in Fe1-XCox/MgO/Fe1-XCox (001) are presented. With the help of spin and symmetry resolved photoemission and ab initio calculation, the TMR decrease for Co content higher than 25% is shown to come from the existence of an interface state and the shift of the empty Delta 1 minority spin state towards the Fermi level.

The purpose of this study is to further understanding of the structural, electronic, magnetic properties of CdO doped with transition metal (Mn) and non metal element (N). The calculations are performed by the developed full-potential augmented plane wave plus local orbitals method within the spin density functional theory. As exchange–correlation potential we used the generalized gradient approximation (GGA) form. Moreover, the electronic structure study for our compounds was performed with and without oxygen deficiency. We treated the ferromagnetic and antiferromagnetic states and we found that all compounds are stable in the ferromagnetic structure, and all doped materials CdO:Mn and CdO:N adopt the half metallic character. In addition, we notice that the oxygen vacancy destroyed the ferromagnetism in N doped CdO, while Mn doped CdO becomes semiconductor.