Spintronics is about control and manipulation of magnetic moments for new and improved functionality in electronic devices. The phenomenon of spin transfer emerged as a unique tool to control the magnetic state of a ferromagnet (F) with an electrical current. MacDonald and co-workers predicted that spin transfer could also occur in an antiferromagnet (AFM), where it might be even stronger under certain conditions. We recently showed that the exchange bias at an AFM/F interface, with AFM = FeMn and F = CoFe, could be either increased or decreased depending upon the polarity of the applied current. We attributed these changes to effects of the current on the AFM. Here we extend that study to a new AFM = IrMn and to a new F = Py = Ni_xFe_1-x with x ~ 0.2. Using exchange-biased spin-valves (EBSVs) of the form AFM/F(pinned)/Cu/F(free), where both Fs are the same alloy, we first compare data for F = CoFe with AFMs = FeMn or IrMn. The data for FeMn and IrMn are generally similar, with the current having clear effects upon the exchange bias, but little or none on the coercive field of the 'free' CoFe-layer. We then present data for F = Py with AFMs = FeMn or IrMn. With Py, the current generally affects both the exchange bias and the coercive field of the 'free' layer, in ways that we are not yet able to simply correlate with layer thicknesses or AFM.

We report on spin-polarized electron reflection experiments on Fe films on Ag(001). Upon reflecting from the Fe film the electron-spin polarization is found to vary in an oscillatory fashion as a function of the Fe thickness. Two different periods are identified. While the long period is related to the occurrence of quantum-size effects in the Fe layer, the short period of one monolayer is attributed to the periodic variations of the film morphology alternating between filled and incompletely filled atomic layers. This shows - because of total angular momentum conservation - that the transfer of spin-angular momentum from the incident electrons to the ferromagnetic film can be extremely sensitive to the morphology and structure of the film.

We investigate the impedance and direct-current (DC) electric response induced by ferromagnetic resonance (FMR) in a single layered ferromagnetic wire. The impedance difference in the FMR state rectifies part of the radio-frequency (RF) current and induces the DC voltage. The impedance measurement using a vector network analyzer reveals the characteristics of the rectifying effect. A phenomenological model interpreting the impedance changes derived from the magnetization precession is proposed. The rectifying effect lies in the ability to demodulate the RF signal for wireless mobile communications.

The aim of this paper is to demonstrate the relevance of the a non-equilibrium stochastic approach in the context of spin-transfer mechanism. Spin-transfer is the generic name for the effect of magnetization reversal (or magnetic excitation) produced by the injection of a spin-polarized current in a ferromagnetic layer. Deterministic vs. stochastic approaches are first defined in the context of the Landau-Lifshitz Gilbert equation of the magnetization. We then present a model based on non-equilibrium thermodynamics in which the spin-accumulation at the interface appears as a diffusion term in the Landau-Lifshitz equation. The expression of the critical current I_c is derived from this diffusive process and compared to the experimental results. Due to the definition of the critical current in terms of activation process, the phenomenological expression of I_c is identical to that derived in the deterministic case, after introducing an "efficiency parameter". Only the specific form of this efficiency parameter allows one to discriminate between the different models. The direct link to the integral of the magnetoresistance of the junction derived here allows some highly specific behaviour to be predicted.

We show that magnetic disequilibrium within a magnetic domain (e.g., by a magnetic field driving a domain wall) implies spin pumping of current within that domain. This has experimental implications for samples both with conducting leads and that are electrically isolated. For a two-band magnet these results are obtained first by simple arguments, and then by using irreversible thermodynamics to derive the full dynamical equations, with up and down spins each providing conduction and magnetism. It is known that in regions where the equilibrium magnetization is non-uniform, voltage gradients can drive both adiabatic and nonadiabatic bulk spin torques. Onsager relations then ensure that magnetic torques likewise drive related amounts of adiabatic and nonadiabatic currents what we call bulk spin pumping. As for recent spin-Berry phase work, we find that within a domain wall, the ratio of the effective electromotive force to the magnetic field is approximately given by P(2μ_Β/e), where Ρ is the spin polarization. The adiabatic spin torque and spin-pumping terms are shown to be dissipative. We also discuss the issue of Landau-Lifshitz damping vs Gilbert damping; both irreversible thermodynamics and Langevin theory with near-equilibrium thermodynamic fluctuations lead to Landau-Lifshitz damping.

A number of devices have been proposed and synthesized that exploit the spin torque effect. These systems are most straightforward to analyze in the absence of thermal effects. However, thermal effects are very important for the understanding of many experimental results, and for the design of devices. In spin torque MRAM, for example, although the very high-current behavior can be modeled without including thermal fluctuations, switching with currents low enough to be practical has a strong thermal component. Another example is the spin torque oscillator, whose usefulness in devices depends on its linewidth, which is strongly affected by thermal fluctuations. The statistical theory of spin torque systems has previously been worked out¹ using the Fokker- Planck equation, which describes the time evolution of the probability density ρ(M). In this paper we formulate the theory in terms of an effective energy, which has the advantage that the exact solution for the probability density has the familiar form exp(-VE_eff/k_ΒΤ ) in terms of the effective energy. We also generalize Eyring's 1935 transition state theory of rates to the spin torque case; it appears that in many practical cases this is a very good approximation.

An electron spin confined to a semiconductor quantum dot is not subject to the classical spin relaxation mechanisms known for free carriers but it strongly interacts with the nuclear spin system via the hyperfine interaction. We show in time resolved photoluminescence spectroscopy experiments on ensembles of self assembled InAs quantum dots in GaAs that this interaction leads to strong electron spin dephasing. By analysing the polarization state of photons absorbed or emitted by individual dots we show how optical pumping of electron spins leads in turn to a strong nuclear polarisation that can be measured via a drastic change in the Zeeman splitting in magneto-photoluminescence experiments.

The talk summarizes results of our recent optical studies related to spin states in II-VI and III-V semiconductor quantum dot (QD) systems. First the influence of in-plane anisotropy on the QD excitonic spin states is recalled. Then various ways of circumventing, compensating, or exploiting this influence are discussed. Short lifetime of neutral excitons (governed by inter-dot tunneling) allowed us to transfer their spin polarization to another QD before its destruction by the anisotropic exchange interaction. This spin polarization, as well as single carrier spin memory effects in quantum dots are demonstrated using trion states, negligibly perturbed by the anisotropy. Modification of the anisotropy by external perturbations (electric and magnetic field) is shown. In particular, full compensation of the anisotropy by in-plane electric field is demonstrated using optical orientation of neutral excitons. Finally, the influence of the anisotropy will be exploited to achieve circular-to-linear and linear-to-circular polarization conversion in single QDs and in coupled QD pairs.

We consider spin-dependent tunneling through a gallium arsenide barrier, a material which has no inversion symmetry. We are dealing with free electrons, with one effective mass and a spin-splitting in the barrier material. When we take into account both the spin-orbit interaction and the absence of the inversion symmetry, the evanescent states in the barrier are spin split and the tunneling process can become rather involved: Depending on the crystallographic direction, the incident wave experiences spin filtering during the tunneling or a spin precession around an effective magnetic field. These results open stimulating perspectives for spin manipulation in tunnel devices.

The Spin Hall Effect and related transport phenomena originating from the coupling of the charge and spin currents due to spin-orbit interaction were predicted in 1971 by Dyakonov and Perel [1, 2]. Following the suggestion in [3], the first experiments in this domain were done by Fleisher's group at Ioffe Institute in Saint Petersburg [4, 5], providing the first observation of what is now called the Inverse Spin Hall Effect. As to the Spin Hall Effect itself, it had to wait for 33 years before it was experimentally discovered by two groups in Santa Barbara (US) [6] and in Cambridge (UK) [7]. These observations aroused considerable interest and triggered intense research, both experimental and theoretical, with hundreds of publications. The Spin Hall Effect consists in spin accumulation at the boundaries of a current-carrying conductor, the directions of the spins being opposite at the opposing boundaries. For a cylindrical wire the spins wind around the surface. The boundary spin polarization is proportional to the current and changes sign when the direction of the current is reversed. The term "Spin Hall Effect" was introduced by Hirsch [8] in 1999. It is indeed somewhat similar to the normal Hall effect, where charges of opposite signs accumulate at the sample boundaries due to the action of the Lorentz force in magnetic field. However, there are significant differences. First, no magnetic field is needed for spin accumulation. On the contrary, if a magnetic field perpendicular to the spin direction is applied, it will destroy the spin polarization. Second, the value of the spin polarization at the boundaries is limited by spin relaxation, and the polarization exists in relatively wide spin layers determined by the spin diffusion length, typically on the order of 1 μm (as opposed to the much smaller Debye screening length where charges accumulate in the normal Hall effect).

Surfaces and interfaces of complex oxides materials provide a rich playground for the exploration of novel magnetic properties not found in the bulk but also the development of functional interfaces to be incorporated into applications. We have recently been able to demonstrate a new type of hybrid spin filter/ magnetic tunnel junction. Our hybrid spin-filter/magnetic-tunnel junction devices are epitaxial oxide junctions of La_0:7Sr_0:3MnO₃ and Fe₃O₄ electrodes with magnetic NiMn₂O₄ barrier layers. Depending on whether the barrier is in a paramagnetic or ferromagnetic state, the junction exhibits magnetic tunnel junction behavior where the spin polarized conduction is dominated by the electrode-barrier interface or spin filter behavior where conduction is dominated by barrier layer magnetism.

Manipulation of magnetically ordered states by electrical means is a promising approach towards novel spintronics devices. We report on the electric control of surface magnetism in Cr₂O₃ thin films and uniaxial anisotropy in ferroelectric/ferromagnetic heterostructures, respectively. Artificial magnetoelectricity is realized in a BaTiO₃/Fe heterostructure. Here, thermally induced coercivity changes of the Fe hysteresis loop are used to derive the stress imposed by the ferroelectric BaTiO₃ substrate on the adjacent Fe film. Electrically induced coercivity changes give rise to a giant magnetoelectric susceptibility in the vicinity of the magnetic coercive field.

Multilayered magnetic nanowires provide ideal platforms for nanomagnetism and spin-transport studies. They exhibit complex magnetization reversal behaviors as dimensions of the magnetic components are varied, which are difficult to probe since the magnetic entities are buried inside the nanowires. We have captured magnetic and magnetoresistance "fingerprints" of Co nanodiscs in Co/Cu multilayered nanowires as they undergo a single domain to vortex state transition, using a first-order reversal curve (FORC) method. The Co/Cu multilayered nanowires have been synthesized by pulsed electrodeposition into nanoporous polycarbonate membranes. In 50 nm diameter nanowires of [Co(5nm)/Cu(8nm)]₄₀₀, a 10% magnetoresistance effect is observed at 300 K. In 200 nm diameter nanowires, the magnetic configurations can be tuned by adjusting the Co nanodisc aspect ratio. The thinnest nanodiscs exhibit single domain behavior. The thicker ones exhibit vortex states, where the nucleation and annihilation of the vortices are manifested as butterfly-like features in the FORC distributions. The magnetoresistance effect shows different characteristics, which correspond to the different magnetic configurations of the Co nanodiscs.

(Ga,Mn)As is the prototypical ferromagnetic semiconductor for spintronics devices, largely because of its rich magnetic and transport anisotropy. Until recently, the lack of local anisotropy control limited device design as all elements inherited the anisotropy of the parent layer. We report here on anisotropy control through lithographically engineered strain relaxation. By patterning the layer, we allow local and strain relaxation and controlled deformation of the crystal. Because of the strong spin orbit coupling, this leads to a new anisotropy term that can be tuned independently for each element of a compound device. We use this method to demonstrate a novel non-volatile memory element.

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 nano-columns 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, the magneto-transport characteristics of the whole structure strongly depend on the magnetic configuration of the nano-columns.

We investigate the magnetization dynamics induced by a current pulse in Permalloy nanowires by means of Lorentz microscopy and electron holography, together with simultaneous transport measurements. A variety of magnetization dynamics is observed below the Curie temperature. Local transformation, displacement of magnetic domain wall and nucleation and annihilation of magnetic domain, i.e. magnetization reversal are presented as a function of current density flowing into the wire and wire resistance. Shift of threshold current densities for domain wall displacement and magnetization reversal when changing current pulse duration and thermal conductance of the sample supports that observed behavior of magnetic domains and domain walls is associated with the spin transfer torque and thermal excitation. For the well-controlled magnetization reversal, we microscopically demonstrate that applying small in-plane magnetic field is very effective to controllably nucleate and erase the magnetic domain using a current pulse. Stochastic nature of the magnetization reversal due to spin-wave and thermal excitation in the absence of magnetic field completely disappears and turns into deterministic in the presence of small magnetic field, which enables the magnetization reversal control using current.

Manipulating individual spins in a solid, such as for quantum information processing or a spintronic device, requires the ability to quickly and coherently reorient a spin while leaving its neighbors unaffected. Using traditional electron spin resonance methods is problematic because of the difficulty of confining oscillating magnetic fields to small volumes. In contrast, g-tensor modulation resonance, which has been demonstrated in quantum wells, uses the electric field to exploit differences in the spin-orbit interaction in and around the confining structure and should be scalable. I will present theoretical calculations of g-tensor modulation resonance spin manipulation in quantum dots and donors and show that such schemes are feasible for manipulation of single spins. For InAs/GaAs quantum dots it is possible to rapidly reorient the spin in an arbitrary direction with only the application of a static magnetic field and the application of pulsed electric fields from a gate. Donors behave much like quantum dots, with the advantage that they do not suffer from variations in composition and size.

A spin current carries spin angular momentum in a spintronics device. Its interaction with a magnetic nanostructure not only gives rise to spin-dependent transport but also excites dynamics in the magnetic state. Unlike the spin-polarized electrical current, a pure spin current is useful for both fundamental and applied research because neither Oersted fields nor electrical current-related spurious effects are produced. Nonlocal electrical spin injection is a feasible way to produce the pure spin current. Here we demonstrate that the nonlocal spin valve signal is increased by an order of magnitude by improving the interface quality in a new device structure using a clean, in situ fabrication process. The generated pure spin current enables the magnetization reversal of a nanomagnet as efficiently as electrical current-induced magnetization switching. These results will open the door towards the realization of a pure-spin-current-driven device.

Using spin-flip model (SFM) of vertical-cavity surface-emitting lasers (VCSELs) subject to polarized injection, it is found that reducing the spin relaxation, or increasing the birefringence and pumping terms, can increase elliptically polarized injection locking (EPIL) stability for the slave VCSEL. The nonlinear dynamic is investigated in optically injected VCSELs with numerical simulation techniques for six fundamental VCSEL rate equations. A novel phenomenon as quasi stability which affects the EPIL is analyzed.