Exchange stiffness in ultrathin perpendicularly-magnetized CoFeB layers determined using spin wave spectroscopy

We measure the frequencies of spin waves in nm-thick perpendicularly magnetized FeCoB systems, and model the frequencies to deduce the exchange stiffness of this material in the ultrathin limit. For this, we embody the layers in magnetic tunnel junctions patterned into circular nanopillars of diameters ranging from 100 to 300 nm and we use magneto-resistance to determine which rf-current frequencies are efficient in populating the spin wave modes. Micromagnetic calculations indicate that the ultrathin nature of the layer and the large wave vectors used ensure that the spin wave frequencies are predominantly determined by the exchange stiffness, such that the number of modes in a given frequency window can be used to estimate the exchange. For 1 nm layers the experimental data are consistent with an exchange stiffness A= 20 pJ/m, which is slightly lower that its bulk counterpart. The thickness dependence of the exchange stiffness has strong implications for the numerous situations that involve ultrathin films hosting strong magnetization gradients, and the micromagnetic description thereof.Comment: 5 pages, 4 figures, submitted to PR

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

Pinned synthetic ferrimagnets with perpendicular anisotropy and tuneable exchange bias

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.Pinned synthetic ferrimagnets (syFerri) with perpendicular-to-plane magnetic anisotropy, of the form AP1/Ru/AP2/FeMn [where AP1 and AP2 are (Co/Pt) multilayers], have been prepared and characterized. The magnitudes of the exchange bias fields of both AP1 and AP2 can be tuned at room temperature by simply varying the relative number of (Co/Pt) repeats in each multilayer. This effect can be quantitatively interpreted by considering the different energy contributions involved during magnetization reversal. Moreover, from the values of these fields, the characteristic parameters of the system (i.e., coupling strength through the Ru and AP2/FeMn pinning energy), can be evaluated. Interestingly, an extended plateau with a virtually constant magnetization is observed around zero field when the number of Co/Pt repeats in AP1 is equal or larger than in AP2. This is very appealing for field sensor or memories applications using spin valves or tunnel junctions with perpendicular anisotropy, since it offers a large dynamic range over which the magnetic configuration of the syFerri remains stable

High domain wall velocities induced by current in ultrathin Pt/Co/AlOx wires with perpendicular magnetic anisotropy

Current-induced domain wall (DW) displacements in an array of ultrathin Pt/Co/AlOx wires with perpendicular magnetic anisotropy have been directly observed by wide field Kerr microscopy. DWs in all wires in the array were driven simultaneously and their displacement on the micrometer-scale was controlled by the current pulse amplitude and duration. At the lower current densities where DW displacements were observed (j less than or equal to 1.5 x 10^12 A/m^2), the DW motion obeys a creep law. At higher current density (j = 1.8 x 10^12 A/m^2), zero-field average DW velocities up to 130 +/- 10 m/s were recorded.Comment: Minor changes to Fig. 1(b) and text, correcting for the fact that domain walls were subsequently found to move counter to the electron flow. References update

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

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

Modulating spin transfer torque switching dynamics with two orthogonal spin-polarizers by varying the cell aspect ratio

We study in-plane magnetic tunnel junctions with additional perpendicular polarizer for subnanosecond-current-induced switching memories. The spin-transfer-torque switching dynamics was studied as a function of the cell aspect ratio both experimentally and by numerical simulations using the macrospin model. We show that the anisotropy field plays a significant role in the dynamics, along with the relative amplitude of the two spin-torque contributions. This was confirmed by micromagnetic simulations. Real-time measurements of the reversal were performed with samples of low and high aspect ratio. For low aspect ratios, a precessional motion of the magnetization was observed and the effect of temperature on the precession coherence was studied. For high aspect ratios, we observed magnetization reversals in less than 1 ns for high enough current densities, the final state being controlled by the current direction in the magnetic tunnel junction cell.Comment: 6 pages, 7 figure

Size effects on exchange bias in sub-100 nm ferromagnetic-antiferromagnetic dots deposited on prepatterned substrates

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.Exchange bias effects have been investigated in ferromagnetic (FM)-antiferromagnetic (AFM) square dots, with lateral sizes of 90 nm, sputtered on a prepatterned Si substrate. The magnetic behavior of the dots has been compared with that of a continuous FM-AFM bilayer with the same composition. Along the unidirectional direction, the dots exhibit square hysteresis loops and preserve an exchange bias field, HE, of 70 Oe at room temperature, which is about 40% smaller than HE in the continuous film. In addition, the distribution of blocking temperatures in the nanostructures is found to be shifted toward lower values with respect to that in the continuous film. These results can be interpreted assuming that the reduced lateral dimensions of the nanostructures impose some constraints on the formation and pinning of domain walls in the AFM layer

Analysis of anisotropy crossover due to oxygen in Pt/Co/MOx trilayer

Extraordinary Hall effect and X-ray spectroscopy measurements have been performed on a series of Pt/Co/MOx trilayers (M=Al, Mg, Ta...) in order to investigate the role of oxidation in the onset of perpendicular magnetic anisotropy at the Co/MOx interface. It is observed that varying the oxidation time modifies the magnetic properties of the Co layer, inducing a magnetic anisotropy crossover from in-plane to out-of-plane. We focused on the influence of plasma oxidation on Pt/Co/AlOx perpendicular magnetic anisotropy. The interfacial electronic structure is analyzed via X-ray photoelectron spectroscopy measurements. It is shown that the maximum of out-of-plane magnetic anisotropy corresponds to the appearance of a significant density of Co-O bondings at the Co/AlOx interface

Influence of Co layer thickness on the structural and magnetic properties of multilayers

International audienceThe correlated effects of the insertion of a Pt spacer between ferromagnetic and antiferromagnetic layers and of the variation of the Co layers thickness on the structural and magnetic properties of [ (Pt/Co tCo) 3 /Pt tPt /IrMn ] n multilayers have been studied. Samples with n = 1 and 7, t Co = 0.4 and 0.6 nm, t Pt = 0 and 0.4 nm have been investigated by tomographic atom probe and superconducting quantum interference device magnetometry. For spacer free samples (t Pt = 0), the structural investigation shows that when t Co = 0.4 nm, Mn and Ir atoms diffuse deeply in the (Pt/Co) multilayers. In contrast for t Co = 0.6 nm, the Mn and Ir diffusion is much reduced. Because Pt acts as a barrier against the Mn and Ir diffusion, this difference is less pronounced in samples with Pt insertion. The hysteresis loops shapes, the exchange bias fields and the saturation magnetization values were correlated with the structural properties of these samples and discussed, taking into account the susceptibility, exchange stiffness, and perpendicular magnetic anisotropy