Dynamic binding of driven interfaces in coupled ultrathin ferromagnetic layers

We demonstrate experimentally dynamic interface binding in a system consisting of two coupled ferromagnetic layers. While domain walls in each layer have different velocity-field responses, for two broad ranges of the driving field, H, walls in the two layers are bound and move at a common velocity. The bound states have their own velocity-field response and arise when the isolated wall velocities in each layer are close, a condition which always occurs as H->0. Several features of the bound states are reproduced using a one dimensional model, illustrating their general nature.Comment: 5 pages, 4 figures, to be published in Physical Review Letter

Magnetic domain structure and dynamics in interacting ferromagnetic stacks with perpendicular anisotropy

The time and field dependence of the magnetic domain structure at magnetization reversal were investigated by Kerr microscopy in interacting ferromagnetic Co/Pt multilayers with perpendicular anisotropy. Large local inhomogeneous magnetostatic fields favor mirroring domain structures and domain decoration by rings of opposite magnetization. The long range nature of these magnetostatic interactions gives rise to ultra-slow dynamics even in zero applied field, i.e. it affects the long time domain stability. Due to this additionnal interaction field, the magnetization reversal under short magnetic field pulses differs markedly from the well-known slow dynamic behavior. Namely, in high field, the magnetization of the coupled harder layer has been observed to reverse more rapidly by domain wall motion than the softer layer alone.Comment: 42 pages including 17 figures. submitted to JA

Model of bound interface dynamics for coupled magnetic domain walls

A domain wall in a ferromagnetic system will move under the action of an external magnetic field. Ultrathin Co layers sandwiched between Pt have been shown to be a suitable experimental realization of a weakly disordered 2D medium in which to study the dynamics of 1D interfaces (magnetic domain walls). The behavior of these systems is encapsulated in the velocity-field response v(H) of the domain walls. In a recent paper [P.J. Metaxas et al., Phys. Rev. Lett. 104, 237206 (2010)] we studied the effect of ferromagnetic coupling between two such ultrathin layers, each exhibiting different v(H) characteristics. The main result was the existence of bound states over finite-width field ranges, wherein walls in the two layers moved together at the same speed. Here, we discuss in detail the theory of domain wall dynamics in coupled systems. In particular, we show that a bound creep state is expected for vanishing H and we give the analytical, parameter free expression for its velocity which agrees well with experimental results.Comment: 9 page

Spin Pumping and Inverse Spin Hall Effect in Platinum: The Essential Role of Spin-Memory Loss at Metallic Interfaces

Through combined ferromagnetic resonance, spin-pumping and inverse spin Hall effect experiments in Co|Pt bilayers and Co|Cu|Pt trilayers, we demonstrate consistent values of spin diffusion length $\ell_{\rm sf}^{\rm Pt}=3.4\pm0.4$ nm and of spin Hall angle $\theta_{\rm SHE}^{\rm Pt}=0.051\pm0.004$ for Pt. Our data and model emphasize on the partial depolarization of the spin current at each interface due to spin-memory loss. Our model reconciles the previously published spin Hall angle values and explains the different scaling lengths for the ferromagnetic damping and the spin Hall effect induced voltage.Comment: 6 pages, 3 figures (main text) and 8 pages supplementary. Published with small modifications in Phys. Rev. Let

Spin injection in Silicon at zero magnetic field

In this letter, we show efficient electrical spin injection into a SiGe based \textit{p-i-n} light emitting diode from the remanent state of a perpendicularly magnetized ferromagnetic contact. Electron spin injection is carried out through an alumina tunnel barrier from a Co/Pt thin film exhibiting a strong out-of-plane anisotropy. The electrons spin polarization is then analysed through the circular polarization of emitted light. All the light polarization measurements are performed without an external applied magnetic field \textit{i.e.} in remanent magnetic states. The light polarization as a function of the magnetic field closely traces the out-of-plane magnetization of the Co/Pt injector. We could achieve a circular polarization degree of the emitted light of 3 % at 5 K. Moreover this light polarization remains almost constant at least up to 200 K.Comment: accepted in AP

Interface magnetic anisotropy in cobalt clusters embedded in a platinum or niobium matrix

A low concentration of cobalt clusters with a fcc structure and containing almost one thousand atoms are embedded in two different metallic matrices: platinum and niobium. Samples have been prepared using a co-deposition technique. Cobalt clusters preformed in the gas phase and matrix atoms are simultaneously deposited on a silicon substrate under Ultra High Vacuum conditions. This original technique allows to prepare nanostructured systems from miscible elements such as Co/Pt and Co/Nb in which clusters keep a pure cobalt core surrounded with an alloyed interface. Magnetic measurements performed using a Vibrating Sample Magnetometer (VSM) reveal large differences in the magnetic properties of cobalt clusters in Pt and Nb pointing out the key role of cluster/matrix interfaces.Comment: 7 pages (LaTeX), 12 PostScript figures, 1 PostScript tabl

Exchange bias in GeMn nanocolumns: the role of surface oxidation

We report on the exchange biasing of self-assembled ferromagnetic GeMn nanocolumns by GeMn-oxide caps. The x-ray absorption spectroscopy analysis of this surface oxide shows a multiplet fine structure that is typical of the Mn2+ valence state in MnO. A magnetization hysteresis shift |HE|~100 Oe and a coercivity enhancement of about 70 Oe have been obtained upon cooling (300-5 K) in a magnetic field as low as 0.25 T. This exchange bias is attributed to the interface coupling between the ferromagnetic nanocolumns and the antiferromagnetic MnO-like caps. The effect enhancement is achieved by depositing a MnO layer on the GeMn nanocolumns.Comment: 7 pages, 5 figure

Structure and magnetism of self-organized Ge(1-x)Mn(x) nano-columns

We report on the structural and magnetic properties of thin Ge(1-x)Mn(x)films grown by molecular beam epitaxy (MBE) on Ge(001) substrates at temperatures (Tg) ranging from 80deg C to 200deg C, with average Mn contents between 1 % and 11 %. Their crystalline structure, morphology and composition have been investigated by transmission electron microscopy (TEM), electron energy loss spectroscopy and x-ray diffraction. In the whole range of growth temperatures and Mn concentrations, we observed the formation of manganese rich nanostructures embedded in a nearly pure germanium matrix. Growth temperature mostly determines the structural properties of Mn-rich nanostructures. For low growth temperatures (below 120deg C), we evidenced a two-dimensional spinodal decomposition resulting in the formation of vertical one-dimensional nanostructures (nanocolumns). Moreover we show in this paper the influence of growth parameters (Tg and Mn content) on this decomposition i.e. on nanocolumns size and density. For temperatures higher than 180deg C, we observed the formation of Ge3Mn5 clusters. For intermediate growth temperatures nanocolumns and nanoclusters coexist. Combining high resolution TEM and superconducting quantum interference device magnetometry, we could evidence at least four different magnetic phases in Ge(1-x)Mn(x) films: (i) paramagnetic diluted Mn atoms in the germanium matrix, (ii) superparamagnetic and ferromagnetic low-Tc nanocolumns (120 K 400 K) and (iv) Ge3Mn5 clusters.Comment: 10 pages 2 colonnes revTex formatte

Electrical and thermal spin accumulation in germanium

In this letter, we first show electrical spin injection in the germanium conduction band at room temperature and modulate the spin signal by applying a gate voltage to the channel. The corresponding signal modulation agrees well with the predictions of spin diffusion models. Then by setting a temperature gradient between germanium and the ferromagnet, we create a thermal spin accumulation in germanium without any tunnel charge current. We show that temperature gradients yield larger spin accumulations than pure electrical spin injection but, due to competing microscopic effects, the thermal spin accumulation in germanium remains surprisingly almost unchanged under the application of a gate voltage to the channel.Comment: 7 pages, 3 figure