This PDF file contains the front matter associated with SPIE Proceedings Volume 7398, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

We demonstrate spin noise spectroscopy as an efficient and surprisingly sensitive experimental tool to measure the spin dynamics of free and localized carriers in semiconductors. The technique suppresses perturbations and gives access to intrinsic spin relaxation times by omitting optical excitation. We show the power of spin noise spectroscopy for basic physics by measurements on n-type modulation doped (110) GaAs quantum wells. The measurements reveal that the spin relaxation times are limited by stochastic spin-orbit fields and that the spin can be used as marker for the observation of electron diffusion processes at thermal equilibrium. We show the power of spin noise spectroscopy for applied physics, by three dimensional measurements of the doping distribution in direct semiconductors.

We show that femtosecond time-resolved optical techniques are ideally suited to excite and probe collective spin organization and collective spin modes, in diluted magnetic semiconductors based quantum wells. The spin systems that we consider are low concentration CdMnTe quantum wells, which exhibit various spin phenomena under optical pulsed excitation, such as spontaneous magnetization patterning, soft mode of magnetization precession in p-doped quantum wells, and mixed electron-Mn precession in n-doped quantum wells. We examine also how the carrier-carrier Coulomb interactions affect these collective spin excitations.

We discuss the creation and evolution of the optically induced spin coherence in an ensemble of singly charged (In,Ga)As/GaAs quantum dots. The sample is periodically excited by the pulsed laser radiation which causes the creation of the discrete spin precession spectrum of electrons in an external magnetic field, known as mode-locking. The nuclei additionally help to focus all precessing spins into the discrete modes and even make it possible to realize the singlemode regime, where the whole inhomogeneous ensemble precesses without dephasing.

In this paper we describe the study of the magnetization dynamics after a short magnetic pulse, down to zero field and with a temporal resolution of a few ns, in (Cd,Mn)Te QWs with 0.2% to 1.5% Mn, and various densities of carriers. Short pulses of magnetic field were applied in the Faraday configuration by a small magnetic coil mounted at the surface of the sample. We analyzed the temporal evolution of the giant Zeeman shift of spectroscopic lines after the pulse with resolution down to a few nanoseconds. This evolution reproduces the dynamics of the magnetization of the Mn system. The dynamics in absence of magnetic field was found to be up to three orders of magnitude faster than that at 1 T. Hyperfine interaction and strain are mainly responsible for this fast decay. The influence of a hole gas is clearly visible: at zero field anisotropic holes stabilize the system of Mn ions, while in a magnetic field of 1 T they are known to speed up the decay by opening an additional relaxation channel.

The passage of spin-polarized currents through magnetic nanocontacts can lead to the excitation of self-sustained vortex oscillations in the free layer of a spin-valve stack. These oscillations involve the large amplitude translational motion of the vortex core about the contact region, with oscillation frequencies typically in the 200-500 MHz range. Here, we present a detailed experimental study of such current-driven vortex oscillations. In particular, we show that the oscillation mode is possible in zero applied magnetic field and is only stable within a range of in-plane applied fields.

We present the last developments of the approach of spin-transfer in terms of mesoscopic non-equilibrium thermodynamics. The modification of the ferromagnetic states due to the presence of a spin-accumulation process is investigated. The spin accumulation (also responsible for giant magnetoresistance) is generated by an electric current at the interfaces with a ferromagnetic nanostructure. The dynamical coupling that links the ferromagnetic degree of freedom to the spin of the conduction electrons is investigated with taking into account longitudinal and transverse spin accumulation. It seems that the coupling with the transverse spin-accumulation may be equivalent to the usual spin-transfer-torque mechanisms deduced from microscopic transmission- reflection coefficients at the interface. In contrast, the coupling with the longitudinal spin- accumulation generates fluctuations and seems to play important role in the activation process at larger time scales only.

Magnetoelectronic devices usually incorporate at least two magnetic layers, a "fixed" reference layer and a "free" active layer. We analyze the stabile configurations of such a bilayer in the presence of spin transfer torque acting on both layers. In the approximation of uniaxial anisotropy, we derive analytical stability conditions dependent on the parameters of the bilayer as well as the applied field and current. We illustrate our results with stability diagrams for several simple cases. We also determine the conditions for stable dynamical current-induced states. Our analysis provides criteria for the design of magnetic nanodevices with optimized response to electric current.

Spin-transfer torques (STT) provides a new mechanism to alter the magnetic configurations in magnetic heterostructures, a feat previously only achieved by an external magnetic field. A current flowing perpendicular through a magnetic noncollinear spin structure can induce torques on the magnetization, depending on the polarity of the current. This is because an electron carries angular momentum, or spin, part of which can be transferred to the magnetic layer as a torque. A spin-polarized current of a substantial current density (e.g., 10⁸ A/cm²) is required to observe the effect of the spin transfer torques. Consequently, switching by spin-polarized currents is often realized in small structures with sub-micron cross sections made by nanolithography. Here we demonstrate spin transfer torque effects using point-contact spin injection involving no lithography. In a continuous Co/Cu/Co trilayer, we have observed hysteretic reversal of sub-100 nm magnetic elements by spin injection through a metal tip both at low temperature and at room temperature. A small magnetic domain underneath the tip in the top Co layer can be manipulated to align parallel or anti-parallel to the bottom Co layer with a unique bias voltage. In an exchange-biased single ferromagnetic layer, we have observed a new form of STT effect which is the inverse effect of domain wall magnetoresistance effect rather than giant magnetoresistance effect. We further show that in granular solids, the STT effect that can be exploited to induce a large spin disorder when combined with a large magnetic field. As a result, we have obtained a spectacular MR effect in excess of 400%, the largest ever reported in any metallic systems.

Spin-transfer devices that incorporate a polarizer with its magnetization orthogonal to a switchable (free) layer offer the potential for ultra-fast switching, low power consumption and reliable operation. The non-collinear magnetizations lead to large initial spin-transfer torques, eliminating the incubation delay seen in devices with collinear magnetization. Here we present the basic electrical and magnetic characteristics of spin-valve nanopillars that incorporate a perpendicularly magnetized polarizer and demonstrate current-induced switching with short current pulses, down to 100 ps in duration. We have fabricated devices that have a CoNi polarizer with perpendicular magnetization and an in-plane magnetized 3 nm thick Co free layer and a 12 nm thick Co reference layer, each separated by thin (~ 10 nm) Cu layers. The magnetization of the reference layer is collinear with that of free layer to read out the device state. The reference layer also contributes to the spin-accumulation acting on the free layer and leads to a spin-torque that favors the parallel (P) or antiparallel (AP) state depending on the current pulse polarity, reducing the requirement of precise pulse timing in precessional reversal. The anisotropy field of the perpendicular polarizer is 1.3 T, i.e. it is high enough so that in-plane fields (< 0.3 T) applied to switch the magnetizations of the reference and free layers do not reorient the polarizer. Our typical nanopillar device lateral dimensions are between 60 nm and 300 nm and nanopillars are positioned on coplanar waveguides to allow for broadband electrical connections and studies with fast rise time pulses, generated by an arbitrary waveform generator. The switching probability has been determined for variable pulse amplitude and duration, from 0.1 to 10 ns at room temperature.

A recent experiment determined the magnetic moment /Mn, M, in the dilute Mn_xSi_1-x with x = 0.1% to be 5.0 µ_B/Mn. The existing calculated M values range from 2.37 to 3.1µ_B/Mn except the case with a fixed charge state, Mn²⁺, which gives 5.0µ_B/Mn. We address the issue: Can a single Mn at its neutral charge state in dilute MnxSi1-x alloys have M = 5.0 µ_B/Mn? After carrying out extensive calculations, the only model giving this M value involves a supercell having a total of 513 atoms with a Mn at a substitutional site and a Si at a tetrahedral interstitial site serving as a second neighbor to the Mn. Physically, the Mn contributes 4.0 µ_B due to the weakening of the d-p hybridization between the transition metal element and its nearest neighbor Si caused by the presence of the second neighbor Si. The additional 1.0 µ_B is the consequence of the exchange interaction through the remaining weak overlap of the wave functions between the d-state of the Mn and the sp³ state of the nearest neighbor Si atom. Evidences for the weakening of the d-p hybridization are presented.

Single ion spins in semiconductors with sizable spin-orbit interaction can be optically or electrically manipulated. The highly extended anisotropic wave function of the hole bound to each magnetic ion is susceptible to external non-magnetic control fields. The spin-orbit coupling between the orbital and spin character of the hole permits indirect manipulation and readout of the ionic spin state. The spin-spin interaction between the magnetic ions is electrically or structurally controllable.

Spin-polarized currents are able to change the magnetic configuration of nanostructures through the spintransfer effect proposed more than a decade ago. Intensive research is currently directed at understanding the basic physics of this non-equilibrium interaction and designing magnetic nanodevices controlled by electric current. In those devices spin transfer torques play a key role creating dynamic regimes that are not present in conventional magnetic systems. Unfortunately full dynamic study of the phenomenon is not straightforward even for simple spin transfer devices due to the nonlinearity of the Landau-Lifshitz-Gilbert equation governing the magnetization motion. Devices with complicated magnetic anisotropy feature many dynamic regimes: "canted states", multiple precession states, "magnetic fan" regimes, etc. which are often studied by numeric methods. One special case of magnetic anisotropy, a dominating easy plane, turns out to be ubiquitous in experimental designs. This case is characterized by a simplification of the dynamic equations which permits analytical treatment by means of an effective planar equation. The planar equation is presented and discussed for a number of regimes. It is shown how planar description gives an intuitively clear picture of magnetic dynamics and allows to predict new phenomena.

We discuss spin transfer and its reverse process, the generation of electric current by dynamic magnetization textures. Both these processes acquire corrections due to spin relaxation. In the case of layered structures the latter contribute to the effective-field-like spin torque, which in the continuum case results in the dissipative spin transfer torque, or "β-term". The corresponding corrections to spin pumping and spin motive forces are discussed with emphasis on, and applications to, field-driven domain walls.

Zero-bias spin separation generated by homogeneous optical excitation with terahertz radiation in quantum wells is reviewed. In gyrotropic semiconductor structures spin-dependent asymmetry of electron scattering induces a pure spin current which results in a spin separation. We consider the relaxation mechanism yielding the spin current due to the energy relaxation of a heated electron gas and the excitation mechanism caused by the scattering assisted free carrier absorption. An experimental access to these phenomena provides the application of an external magnetic field converting the spin current into a measurable net electric current. We discuss microscopic and phenomenological theory of these effects, give an overview of experimental data, and address several applications.

In non-centrosymmetric semiconductors with zinc-blende structure grown along the [110] crystallographic direction, electrons with up and down spins undergo different quantum phase shifts upon tunneling, which can be wieved as resulting from spin precession around a complex magnetic field. There is no spin filtering but a pure spin dephasing. The phase shift of the transmitted wave is proportional to the overall barrier-material thickness. We show that a device incorporating a number of resonant tunnel barriers constitutes an efficient quantum-phase shifter.

The spin states of single Mn atoms embedded in InAs quantum dots (QD) were investigated by optical means. In (In,Ga)As, the Mn impurity acts both as acceptor and a magnetic moment. In the low density limit, which is relevant here, the acceptor is in the neutral state A⁰ with an effective spin J = 1. Using magneto-photoluminescence experiments, we probed the exchange interaction between the dot confined carriers and the Mn spin. Peculiar properties are found such as a large influence of the QD strain field on the acceptor spin states, a ferromagnetic hole-Mn spin coupling and a very small interaction with electrons. The Mn atoms were deposited during the QD growth by molecular beam epitaxy. Although the substrate temperature favored large segregation of the Mn atoms, we could measure a few dots containing single magnetic impurities. The zero-magnetic field exciton of a Mn-doped single dot shows a complex spectrum consisting of two doublets and a singlet. They correspond to the excitonic recombination of electron-hole pairs, affected by the exchange coupling with the Mn magnetic moment in the J_z = ±1, 0 states. The fine structure of each doublet is due to the QD in-plane anisotropy which partially mixes the J_z = ±1 states. The magnetic field dependence exhibits even more striking modifications, with several crossing and anti-crossing again linked to the QD strain field. We developed a model taking into account the spin interactions between the Mn impurity and the carriers in the dot. A very good agreement is found with the experimental data.

Compact proximity focused vacuum tubes with GaAs(Cs,O) photocathodes are used for experimental studying spindependent phenomena. Firstly, spin-dependent emission of optically oriented electrons from p-GaAs(Cs,O) into vacuum in a magnetic field normal to the surface was observed in a nonmagnetic vacuum diode. This phenomenon is explained by the jump in the electron g-factor at the semiconductor-vacuum interface. Due to this jump, the effective electron affinity on the semiconductor surface depends on the mutual direction of optically oriented electron spins and the magnetic field, resulting in the spin-dependent photoemission. It is demonstrated that the observed effect can be used for the determination of spin diffusion length in semiconductors. Secondly, we developed a prototype of a new spin filter, which consists of a vacuum tube with GaAs(Cs,O) photocathode and a nickel-covered venetian blind dynode. Preliminary results on spin-dependent reflection of electrons from the oxidized polycrystal nickel layer are presented.

Spin transport in graphene devices has been investigated in both local and non-local spin valve geometries. In the nonlocal measurement, spin transport and spin precession in single layer and bilayer graphene have both been achieved with transparent Co contacts. Gate controllable non-local spin signal was also demonstrated in this system. For the local graphite spin valve device, we report MR up to 12% for devices with tunneling contacts. We observe a correlation between the nonlinearity of the I-V curve and the presence of local MR and conclude that tunnel barriers can be employed to surmount the conductance mismatch problem in this system. These studies indicate that the improvement of tunnel barriers on graphene, especially to inhibit the formation of pinholes, is an important step to achieve more efficient spin injection into graphene.