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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 stiffness. For 1 nm layers, the experimental data are consistent with an exchange stiffness A=20±2 pJ/m, which is slightly lower than its bulk counterpart. The thickness dependence of the exchange stiffness has strong implications for the numerous situations that involve...

The propagation of magnetic domain walls induced by spin-polarized currents has launched new concepts for memory and logic devices. A wave of studies focusing on permalloy (NiFe) nanowires has found evidence for high domain-wall velocities (100 m s(-1); refs,), but has also exposed the drawbacks of this phenomenon for applications. Often the domain-wall displacements are not reproducible, their depinning from a thermally stable position is difficult and the domain-wall structural instability (Walker breakdown) limits the maximum velocity. Here, we show that the combined action of spin-transfer and spin-orbit torques offers a comprehensive solution to these problems. In an ultrathin Co nanowire, integrated in a trilayer with structural inversion asymmetry (SIA), the high spin-torque efficiency facilitates the depinning and leads to high mobility, while the SIA-mediated Rashba field controlling the domain-wall chirality stabilizes the Bloch domain-wall structure. Thus, the high-mobility regime is extended to higher current densities, allowing domain-wall velocities up to 400 m s(-1).

The perpendicular magnetic anisotropy (PMA) of Pt/CoFe(B)/MgO bottom electrodes and the tunnel magnetoresistance (TMR) of CoFeB-based magnetic tunnel junctions (MTJ) have been analyzed as a function of Mg thickness for naturally oxidized barriers. Low PMA and TMR values are found for over-oxidized (small MgO thickness) and under-oxidized (large MgO thickness) barriers. When CoFe is used as bottom electrode, a strong correlation is observed between TMR and PMA variation as a function of MgO thickness with maxima in both quantities occurring for an MgO thickness around 1.2 nm. On the contrary, for CoFeB bottom electrodes, the sensitivity of PMA to MgO thickness is completely lost, as a probable consequence of boron diffusion towards the MgO interface.

Antiferromagnetic coupling between magnetic electrodes has been observed in non-epitaxial perpendicularly magnetized MgO tunnel junctions. This coupling becomes less negative with increasing annealing temperature up to 375degC. This can be possibly related to homogeneization of oxygen in the barrier and de-oxidation of the magnetic electrodes. However, the evolution of coupling field with both barrier and electrode thickness doesn't agree with existing coupling theories. Similar structures with in-plane magnetized electrodes exhibit classical ferromagnetic coupling.

We investigate thermally activated domain wall creep in a system consisting of two ultrathin Co layers with perpendicular anisotropy coupled antiferromagnetically through a 4 nm thick Pt spacer layer. The field driven dynamics of domain walls in the softer Co layer have been measured while keeping the harder Co layer negatively saturated. The effect of the interlayer interaction on the soft layer is interpreted in terms of an effective coupling field, H J , which results in an asymmetry between the domain wall speeds measured under positive and negative driving fields. We show that creep theory remains valid to describe the observed wall motion when the effective coupling field is included in the creep velocity law as a component of the total field acting on the wall. Using the resultant modified creep expression, we determine a value for the effective coupling field which is consistent with that measured from the shift of the soft layer's minor hysteresis loop. The net antiferromagnetic coupling is attributed to a combination of RKKY and orange-peel coupling.

X-ray spectroscopy measurements have been performed on a series of Pt/Co/AlOx trilayers to investigate the role of Co oxidation in the perpendicular magnetic anisotropy of the Co/AlOx interface. It is observed that high temperature annealing modifies the magnetic properties of the Co layer, inducing an enhancement of the perpendicular magnetic anisotropy. The microscopic structural properties are analyzed via X-ray Absorption Spectroscopy, X-ray Magnetic Circular Dichroism and X-ray Photoelectron Spectroscopy measurements. It is shown that annealing enhances the amount of interfacial oxide, which may be at the origin of a strong perpendicular magnetic anisotropy.

International audience; The magnetic properties of double tunnel junctions with perpendicular anisotropy were investigated. Two synthetic antiferromagnetic references are used, while the middle storage magnetic layer can be either a single ferromagnetic or a synthetic antiferromagnetic FeCoB-based layer, with a critical thickness as large as 3.0 nm. Among the different achievable magnetic configurations in zero field, those with either antiparallel references, and single ferromagnetic storage layer, or parallel references, and synthetic antiferromagnetic storage layer, are of particular interest since they allow increasing the efficiency of spin transfer torque writing and the thermal stability of the stored information as compared to single tunnel junctions. The latter configuration can be preferred when stray fields would favour a parallel orientation of the reference layers. In this case, the synthetic antiferromagnetic storage layer is also less sensitive to residual stray fields. (C) 2015 AIP Publishing LLC.

Sputtered (Pt-Co)/sub n/-FeMn multilayers exhibit both perpendicular magnetic anisotropy and exchange bias at room temperature. Their magnetic properties after in-plane and perpendicular-to-plane magnetic annealing have been studied by means of extraordinary Hall effect and magnetic force microscopy or polar Kerr microscopy. A crossover from in-plane to perpendicular anisotropy was observed as a function of number of repeats n. The dynamics of magnetization switching has been investigated on both sides of the hysteresis loops. A clear asymmetry exists in the dynamics depending whether the field is parallel or antiparallel to the exchange bias field. Similarly, strong differences in the domain patterns at the coercive field were observed. Exchange-biased (Pt-Co)-Pt-(Pt-Co)-FeMn spin-valves were also investigated. Well-separated hysteresis loops were associated with the "soft" and pinned components of the stack. These structures were used to study the interlayer coupling through the Pt spacer.

We have investigated a very efficient way to produce patterned media for ultrahigh density magnetic recording by first preparing patterned wafers made of silicon. Two ways of patterning the wafers were investigated: direct e-beam lithography with reactive ion etching and nanoimprinting. Features as small as 30 nm with a pitch of 60 nm were realized by nanoimprint. Subsequently, a magnetic material was deposited on these patterned wafers, covering the top of the dots, the bottom of the trenches between dots, and to a lesser extent, the sidewalls of the dots. The magnetic information is stored in the magnetic deposit located on top of the dots. In most of our studies, the deposited magnetic materials were (Co-Pt) multilayers with perpendicular magnetic anisotropy. The influence of the magnetic deposit in the trenches as well as on the sidewalls of the dots was investigated. Two techniques were developed to manipulate the information written on the dots. One is a thermomagnetic writing process which consists of locally heating a single dot by flowing a pulse of current from a conductive atomic force microscopy tip to the dot. The second writing technique consists of using standard write heads for hard disk to locally apply a magnetic field on each individual dot. This paper presents a review of our work on these arrays of dots made from prepatterned wafers.

Spin-transfer torque switching has been studied on (Co/Pt)/Ta/CoFeB/MgO/FeCoB/Ta patterned perpendicular magnetic tunnel junctions with nanopillar diameters ranging between 100 and 300 nm. Plotting current-field state diagrams, the critical current density for writing was found to be around 3 MA· cm -2 . The field-current dependence of the state transition boundary is linear, with the slope depending on material parameters. Using different thicknesses of top FeCoB free layers, we were able to estimate the Gilbert damping parameter α .

A sputtered Ta 3 nm /[[Pt 2 nm /Co 0.4 nm )] 3 /IrMn 7 nm ] 7 /Pt 10 nm multilayer has been analyzed by laser-assisted tomographic atom probe, allowing to perform three-dimensional reconstructions of the layers and to determine their chemical composition at the atomic scale. From the concentration profiles, we show that the Co layer in contact with IrMn is strongly mixed. The Co/IrMn and IrMn/Pt interfaces are thus non-symmetric, the Co/IrMn interface being more diffuse than the IrMn/Pt one. This study demonstrates that the LATAP technique is extremely well suited for atomic scale structural and chemical characterizations of magnetic multilayers in relationship with their magnetic properties.

Heterogeneous Co x Ag 1− x and Ni x Ag 1− x alloys have giant magnetoresistance properties. Such alloys, with atomic concentrations x = 0.20 and 0.37, were studied by transmission X-ray Absorption Spectroscopy at the Co or Ni K -edge after in situ thermal annealing. For alloys as-deposited and annealed at 200 °C, Extended X-ray Absorption Fine Structure analysis displays both Co-Co (Ni-Ni) bonds related to Co (Ni) atoms agglomerated in magnetic particles and Co-Ag (Ni-Ag) bonds related to Co (Ni) atoms in substitutional sites in the Ag matrix. At the same alloy concentration, the miscibility in the Ag matrix is found larger for Ni than for Co. After annealing around 250 °C, the marked decrease of the Ag neighbour peak corresponds to a diffusion of magnetic atoms outside the Ag matrix. The Co-Co coordination number increases regularly with annealing temperatures up to 450 °C reflecting a progressive expansion of Co particles. On the contrary, for Ni alloys, no further particle expansion has been observed in the same annealing range. This different behaviour may be linked both to the difference between the as-deposited structural states and to the Co/Ag and Ni/Ag interfaces energies. Using a simple model, evolution of the mean particle size has been estimated as a function of annealing.

The influence of annealing on the properties of an ultrathin permalloy/silver multilayer is analysed using Mossbauer effect, x-ray diffraction, magnetization and magnetoresistance experiments. A thick permalloy film is studied for comparison. From Mossbauer experiments, the existence of a `magnetically dead' layer is ruled out. An interfacial zone corresponding to about 1.2 monolayers is observed in the low hyperfine field range in the as-deposited film and disappears after annealing at , when the layer structure is destroyed. The evolution of the hyperfine field is associated with both densification of the NiFe layers and interface improvement. From Mossbauer and magnetization measurements, the individual Ni and Fe magnetic moments have been evaluated. They exhibit different thermal dependences, in agreement with results on multilayers based on pure Fe or Ni layers.

International audience; We investigated the magnetic switching of MgO/Co/Pt pillars with perpendicular magnetic anisotropy, for lateral pillar sizes from 30 nm to 2 lm and for Co layer thicknesses between 1.8 and 2.6 nm. For large pillars, both the coercivity and the remanent magnetization decrease for increasing Co thickness. For all Co thicknesses, the coercivity strongly increases upon decreasing the pillar size. A comparison with micromagnetic simulations shows that the change in coercivity is determined by size-dependent demagnetizing effects. Our results show that small pillars with perpendicular magnetization and a tunable coercivity can be fabricated from continuous layers with in-plane magnetization.

From the observation of magnetic and structural properties of Ag/Ni 81 Fe 19 multilayers, we show that two diffusion mechanisms are involved in the thermal evolution of such structures: the diffusion of Ag at interfaces and at grain boundaries. Upon annealing, they first lead to a decrease in interface roughness and then cause the layers to become discontinuous. We have also observed that thermal treatments under an applied magnetic field have an influence on the magnetoresistive properties of the samples. This could be due to some modification of the domain structure, resulting in a different contribution of electron scattering by domains walls.

International audience; Ultrathin Ta layers were inserted in the bottom hard (Co/Pt)/Ta/CoFeB/MgO magnetic electrode of perpendicular magnetic tunnel junctions. The magnetization of the top part of this electrode abruptly falls in-plane when the Ta thickness exceeds 0.45 nm. This results from the balance between the various energy terms acting on this layer (exchange-like coupling through Ta, demagnetizing energy, and perpendicular anisotropy at the CoFeB/MgO interface). For small Ta thicknesses, this insertion leads to a strong improvement of the tunnel magnetoresistance, as long as the magnetization of all layers remains perpendicular-to-plane.

Domain wall propagation has been measured in continuous, weakly disordered, quasi-two-dimensional, Ising-like magnetic layers that are subject to spatially periodic domain wall pinning potentials. The potentials are generated non-destructively using the stray magnetic field of ordered arrays of magnetically hard [Co/Pt]$_m$ nanoplatelets which are patterned above and are physically separated from the continuous magnetic layer. The effect of the periodic pinning potentials on thermally activated domain wall creep dynamics is shown to be equivalent, at first approximation, to that of a uniform, effective retardation field, $H_{ret}$, which acts against the applied field, $H$. We show that $H_{ret}$ depends not only on the array geometry but also on the relative orientation of $H$ and the magnetization of the nanoplatelets. A result of the latter dependence is that wall-mediated hysteresis loops obtained for a set nanoplatelet magnetization exhibit many properties that are normally associated with ferromagnet/antiferromagnet exchange bias systems. These include a switchable bias, coercivity enhancement and domain wall roughness that is dependent on the applied field polarity., Comment: 12 pages, 9 figures

We have prepared spin valves exhibiting perpendicular magnetic anisotropy [perpendicular spin valves (PSVs)] by sputtering. These PSVs associate a “free” (Co/Pt) multilayer with a “pinned” (Co/Pt)/FeMn multilayer separated by various spacer materials (Pt, Cu, Al2O3). We carried out a comprehensive study of the magnetic and magnetotransport properties of the biased multilayers and of the complete spin valves. When the number of repeats in the (Co/Pt) exchange-biased multilayer is larger than 3, the samples present 100% remnant magnetization in the perpendicular configuration. The major hysteresis cycles exhibit two well-separated loops associated with the free and the exchange-biased (Pt/Co) multilayers. When optimized, the exchange-bias field can be larger than the coercivity of the pinned layer. Metallic PSVs with Cu spacers exhibit giant magnetoresistance but the amplitude is only of the order of 1% due to significant current shunting. In contrast, perpendicularly magnetized tunnel junctions are very promising.

Domain wall motion induced by nanosecond current pulses in nanostripes with perpendicular magnetic anisotropy ($\mathrm{Pt}/\mathrm{Co}/{\mathrm{AlO}}_{x}$) is shown to exhibit negligible inertia. Time-resolved magnetic microscopy during current pulses reveals that the domain walls start moving, with a constant speed, as soon as the current reaches a constant amplitude, and no or little motion takes place after the end of the pulse. The very low ``mass'' of these domain walls is attributed to the combination of their narrow width and high damping parameter $\ensuremath{\alpha}$. Such a small inertia should allow accurate control of domain wall motion by tuning the duration and amplitude of the current pulses.

We detail measurements of field-driven expansion and zero-field relaxation of magnetic mirror domains in antiferromagnetically coupled perpendicularly magnetized ultrathin Co layers. The zero-field stability of aligned ('mirror') domains in such systems results from non-homogeneous dipolar stray fields which exist in the vicinity of the domain walls. During field-driven domain expansion, we evidence a separation of the domain walls which form the mirror domain boundary. However, the walls realign, thereby reforming a mirror domain, if their final separation is below a critical distance at the end of the field pulse. This critical distance marks the point at which the effective net interaction between the walls changes from attractive to repulsive.

MRAM are already used in low density applications such as microcontrollers or for spatial/avionics applications where they benefit from their radiation hardness. Thermally assisted writing allows extending the downsize scalability of both FIMS and STT-MRAM below the 20nm node. Within a few years, MRAM can play an important role in the memory hierarchy of electronic circuits. They may be first used as DRAM replacement below the 20nm node and in a second step even closer the logic units (Fig.10).

We study the anisotropy of the spin and orbital magnetization of ultrathin Co layers characterized by structure inversion asymmetry, namely Pt/Co/AlO${}_{x}$ trilayers with Co thicknesses between 0.6 and 2 nm. We use x-ray magnetic circular dichroism (XMCD) to probe the Co layer along two orthogonal measurement geometries. By combining transverse and collinear XMCD, we achieve a vector measurement of the Co spin and orbital magnetic moments. We find an enhanced and anisotropic orbital magnetic moment localized at the Co interface and verify the connection between the Co orbital moment anisotropy and the macroscopic magnetic anisotropy of the trilayer. These results provide a link between diverse interfacial phenomena originating from spin-orbit coupling, such as the Rashba effect and perpendicular magnetic anisotropy. Using simple considerations derived from tight-binding models, we show that the Rashba constant is proportional to the ratio between out-of-plane and in-plane orbital magnetic moments.

International audience; Domain formation in a [Pt/Co] multilayer with out-of-plane anisotropy leads to a shift of the hysteresis loop of an adjacent NiFe thin film with in-plane anisotropy in an analogous manner to the exchange-bias phenomenon typically observed in ferromagnetic/antiferromagnetic systems. This paper shows that the loop shift can be gradually tuned through modification of the magnitude of the perpendicular anisotropy of the [Pt/Co]/NiFe system by simply varying the thickness of an underlying Pt buffer layer. Magnetometry, magnetotransport, and magnetic force microscopy experimental measurements were made. It appears that the key parameter which controls the exchange-bias effect in this system is the presence of a vortex core in between magnetic domains in the [Pt/Co] multilayer. The magnitude of the exchange-bias is related to the ability to unpin this vortex core, which is related to the magnitude of the perpendicular anisotropy. Micromagnetic simulations illustrate the magnetic configuration responsible for the exchange-bias phenomenon in this nonstandard exchange-biased type system.

Publisher Summary Spinelectronics is a very rapidly expanding area of research and development that merges magnetism and electronics. It aims at taking advantage of the quantum characteristic of the electrons that is its spin to create new functionalities and new devices. Spintronic devices comprise magnetic layers that serve as spin polarizers or analyzers separated by non-magnetic layers through which the spin-polarized electrons are transmitted. It provides a new way to manipulate the magnetization of magnetic nanostructures by a spin-polarized current. Spinelectronics has found applications in hard disk drives and more recently in nonvolatile standalone memories, such as magnetic random access memory (MRAM). Furthermore, in addition to MRAMs, hybrid complementary metaloxide-semiconductor (CMOS)/magnetic technology can yield a totally new approach in the way electronic devices are designed. The hybrid CMOS/magnetic tunnel junction (MTJ) technology is progressing fast and gradually becomes more and more reliable. This hybrid CMOS/ magnetic technology has very important application in the field of lowpower, reprogrammable, and non-volatile logic.

Arrays of silicon lines covered by a [Co/sub 5/ /spl Aring//Pt/sub 18/ /spl Aring/] multilayer have been studied by soft X-ray Resonant Magnetic Scattering (XRMS). The magnetic peaks, which appear at the Co L/sub 3/ resonance, reveal the magnetic periodicity in the array, resulting from the magnetic coupling between the top of the lines, with a varying degree of influence from the trenches. In agreement with MFM images, the evolution of the magnetic peak intensities shows that small interline spacings and deep trenches favor antiferromagnetic order, which can be further reinforced by a specific demagnetization process.

The spin-orbit interaction constitutes a weak but essential perturbation to the Hamiltonian of magnetic systems. Linking spins with atomic structure, spin-orbit coupling assumes a prominent role in structures of reduced dimensionality, where it defines the internal anisotropy fields. In this paper, we discuss interface-enhanced spinorbit effects that arise in metallic multilayers in the presence of an electric current. We demonstrate that a novel type of spin torque can be induced in ferromagnetic metal films lacking structure inversion symmetry through the Rashba effect. Owing to the combination of spin-orbit and exchange interactions, we show that electrons flowing in the plane of a Co layer with asymmetric Pt and AlOx interfaces produce an effective transverse magnetic field of 1 T per 108 A/cm2 of applied current. This torque does not require a current flowing through noncollinear magnetic structures, opening new perspectives for room temperature applications in spintronics.

The influence of magnetic layer thickness on the interlayer coupling through a tunnel barrier is investigated in Co/MgO/Co structures with perpendicular anisotropy. Despite the rather large MgO thickness, a clear antiferromagnetic coupling is observed. It oscillates with increasing magnetic layer thickness, in agreement with theories on indirect coupling through insulating spacers. Although the average interlayer coupling strength decreases with increasing annealing temperature, constant period and amplitude of these oscillations are observed for all annealing temperatures.

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., 5 pages, 4 figures, to be published in Physical Review Letters

The blocking temperature $({T}_{\text{B}})$ distribution of various polycrystalline ferromagnetic (F)/antiferromagnetic (AF) bilayers exhibits two distinct peaks. This is ascribed to the existence of two families of magnetic entities determining the temperature dependence of exchange bias. One is formed by the AF grains which undergo thermally activated magnetic reversal. The other is formed by F/AF disordered interfacial spins which exhibit spin-glass-like behavior. The bulk versus interfacial nature of these two families is indicated by the dependence of the ${T}_{\text{B}}$ distribution on the AF thickness.

5 páginas, 4 figuras.-- et al., Methods to manipulate the magnetization of ferromagnets by means of local electric fields or current-induced spin transfer torque allow the design of integrated spintronic devices with reduced dimensions and energy consumption compared with conventional magnetic field actuation. An alternative way to induce a spin torque using an electric current has been proposed based on intrinsic spin–orbit magnetic fields and recently realized in a strained low-temperature ferromagnetic semiconductor. Here we demonstrate that strong magnetic fields can be induced in ferromagnetic metal films lacking structure inversion symmetry through the Rashba effect. Owing to the combination of spin–orbit and exchange interactions, we show that an electric current flowing in the plane of a Co layer with asymmetric Pt and AlOx interfaces produces an effective transverse magnetic field of 1 T per 108 A cm−2. Besides its fundamental significance, the high efficiency of this process makes it a realistic candidate for room-temperature spintronic applications., This work was supported by the European Research Council (Starting Grant 203239).

International audience; The effect of electrical current pulses on magnetic domain walls is studied in a nanometer-scale magnetic track defined in a He +-irradiated Pt/Co0.5 nm/Pt film with out-of-plane anisotropy. Current pulses in a wide range of intensities and durations are shown to result either in track demagnetization, or in polarity-independent domain-wall propagation, due to Joule heating during the current pulse. None of the expected spin transfer effects is shown to occur. To explain this, the spin-dependent current-density distribution in the stack is evaluated, in the frame of an adapted Fuchs-Sondheimer model. The spin-polarized current density in the cobalt layer is shown to be unexpectedly low as compared to the charge current in the whole stack, which causes Joule heating. This is explained by a low electron transmission at Co/Pt interfaces, as is deduced from resistance measurements on a series of samples with different cobalt thicknesses. This leads us to emphasize that the balance between spin and total charge current densities should be carefully considered when addressing spin transfer torque effects.

Structural and magnetic investigations of ${\text{Ta}}_{3\text{ }\text{nm}}/{[{({\text{Pt}}_{2\text{ }\text{nm}}/{\text{Co}}_{0.4\text{ }\text{nm}})}_{3}/{\text{Pt}}_{t\text{Pt}}/{\text{IrMn}}_{7\text{ }\text{nm}}]}_{7}/{\text{Pt}}_{10\text{ }\text{nm}}$ multilayers with ${t}_{\text{Pt}}=0$ and ${t}_{\text{Pt}}=0.4\text{ }\text{nm}$ have been carried out using x-ray reflectometry, tomographic atom probe, and superconducting quantum interference device magnetometry. The structural investigations show that the Co/IrMn interface is more diffuse in the absence of the Pt spacer. The consequences on the magnetic properties are discussed. The exchange-bias field and the anisotropy direction of these two specimens are analyzed in comparison to ${\text{Ta}}_{3\text{ }\text{nm}}/[{({\text{Pt}}_{2\text{ }\text{nm}}/{\text{Co}}_{0.4\text{ }\text{nm}})}_{3}/{\text{Pt}}_{t\text{Pt}}/{\text{IrMn}}_{7\text{ }\text{nm}}]/{\text{Pt}}_{10\text{ }\text{nm}}$ multilayers containing only one ferromagnetic/antiferromagnetic repeat and correlated with the structural investigations.

In addition to a storage function through the magnetization of nanowires, domain wall propagation can be used to trigger magnetic logic functions. Here, we present a new way to realize a pure magnetic logic operation by using magnetic nanowires with perpendicular anisotropy. Emphasis is given on the generation of the logic function 'NOT' that is based on the dipolar interaction between two neighbouring magnetic wires, which favours the creation of a domain wall. This concept has been validated on several prototypes and the results fit well with the expectations.

We report the direct measurement of the nonadiabatic component of the spin torque in domain walls. Our method is independent of both the pinning of the domain wall in the wire as well as of the Gilbert damping parameter. We demonstrate that the ratio between the nonadiabatic and the adiabatic components can be as high as 1, and explain this high value by the importance of the spin-flip rate to the nonadiabatic torque. In addition to their fundamental significance these results open the way for applications by demonstrating a significant increase of the spin torque efficiency.

The influence of thermal annealing on Pt/Co/AlOx trilayers has been investigated up to $450\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ as a function of the plasma oxidation time of the AlOx layer. Both magnetic properties of the Co layer and transport properties are strongly modified upon annealing. The perpendicular magnetic anisotropy reaches very large values, while the Hall angle increases with annealing temperature. This study reveals that this trilayer system possesses tunable magnetic anisotropy properties, which can be controlled by varying either oxidation time or annealing temperature. These results are compared with those obtained on standard Pt/Co/Pt trilayers which show, on the contrary, a continuous degradation of their magnetic properties upon annealing.

International audience; X-ray reflectivity (XRR) is used to determine the oxidation front at the nanometer scale in sputtered perpendicular semi tunnel junctions, as the form Pt/Co/AlOx, by varying the oxidation time tOx of the capping layer. From XRR simulations, we show that the nature of the stack is gradually defined according to the value of tOx. For low tOx values (

Silver–nickel multilayers, with elementary layers comprising a few to a few ten atomic planes, have been prepared by cathodic sputtering on glass substrates. X‐ray diffraction experiments show that highly textured, polycrystalline superlattices are formed. For a deposition temperature of 100 K, there exists a critical number of atomic planes nc = 3, below which noncrystalline samples are obtained. This critical number can even be lowered in samples with nonequal numbers of Ag and Ni atomic planes. Transmission x‐ray and electron diffraction experiments show that equivalent crystallographic directions are aligned in the Ag and Ni (111) planes, with a noticeable relaxation of the in‐plane distances at the interfaces. The simulation of the periodic structure assuming a rectangular composition profile with interplanar distances slightly larger than the bulk values leads to a very good agreement with the experimental diffraction spectra.

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 updated

From results at one given temperature $(300\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, Malinowski et al. [Phys. Rev. B 75, 012413 (2007)] draw the conclusion that lateral confinement of ferromagnetic-antiferromagnetic exchange-biased structures does not enhance thermally activated unpinning of the antiferromagnetic spins, which would thus contrast with a recent report [Phys. Rev. Lett. 94, 117201 (2005)], as explicitly mentioned in their manuscript. In this Comment, we discuss why such a conclusion might need revision above a ``crossover temperature,'' as evidenced in the literature. The value of such a crossover temperature certainly depends on the magnetic parameters of each system studied, e.g., anisotropy and exchange stiffness. From the above reasons, and contrary to the statement of Malinowski et al., we rather think that their results might well agree with the report to which they refer to. In our Comment we notably aim at complementing the conclusion of Malinowski et al. by explaining why some differences between the two studies are observed at one given temperature, and why it might be expected to observe similar trends over a whole range of temperatures.

We report on magnetic domain-wall velocity measurements in ultrathin $\mathrm{Pt}/\mathrm{Co}(0.5--0.8\text{ }\text{ }\mathrm{nm})/\mathrm{Pt}$ films with perpendicular anisotropy over a large range of applied magnetic fields. The complete velocity-field characteristics are obtained, enabling an examination of the transition between thermally activated creep and viscous flow: motion regimes predicted from general theories for driven elastic interfaces in weakly disordered media. The dissipation limited flow regime is found to be consistent with precessional domain-wall motion, analysis of which yields values for the damping parameter, $\ensuremath{\alpha}$.

International audience; X-ray spectroscopy measurements have been performed on a series of Pt/Co/AlOx trilayers to investigate the role of Co oxidation in the perpendicular magnetic anisotropy of the Co/AlOx interface. It is observed that high temperature annealing modifies the magnetic properties of the Co layer, inducing an enhancement of the perpendicular magnetic anisotropy. The microscopic structural properties are analyzed via X-ray Absorption Spectroscopy, X-ray Magnetic Circular Dichroism and X-ray Photoelectron Spectroscopy measurements. It is shown that annealing enhances the amount of interfacial oxide, which may be at the origin of a strong perpendicular magnetic anisotropy.

International audience; Intra-dot inter-layer magnetostatic coupling have been investigated in trilayers which consist of two [Co/Pt] multilayers with out-of-plane anisotropy and different coercivities separated by a thick Pt layer. For single domain nanodots, the strong intra-dot inter-layer magnetostatic interactions manifest as a shift of the soft layer hysteresis loop. The amplitude of this shift decreases when the size of the dots is increased and/or when the temperature is increased. Satisfactory agreement is found between calculations and experimental results. PACS. 75 Magnetic properties and materials-75.70.-i Magnetic properties of thin films, surfaces, and interfaces-75.60.-d Domain effects, magnetization curves, and hysteresis-75.60.Ch Domain walls and domain structure-75.75.+a Magnetic properties of nanostructures

The effects of domain replication resulting from interlayer magnetostatic coupling have been investigated in trilayers which consist of two $\mathrm{Co}∕\mathrm{Pt}$ multilayers with out-of-plane anisotropy separated by a thick Pt layer. For demagnetized films, the strong correlation between the domain configurations of the two multilayers gives rise to an evolution of the equilibrium domain size with the spacer layer thickness. A good correspondence is found between our numerical calculations and the experimental measurements. These results are an extension of Kaplan's theory of domain size in single ferromagnetic films with out-of-plane magnetization, to the case of interacting ferromagnetic films.

International audience; 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.

Spintronics materials have recently been considered for radio-frequency devices such as oscillators by exploiting the transfer of spin angular momentum between a spin-polarized electrical current and the magnetic nanostructure it passes through. While previous spin-transfer oscillators (STOs) were based on in-plane magnetized structures, here we present the realization of an STO that contains a perpendicular spin current polarizer combined with an in-plane magnetized free layer. This device is characterized by high-frequency oscillations of the free-layer magnetization, consistent with out-of-plane steady-state precessions induced at the threshold current by a spin-transfer torque from perpendicularly polarized electrons. The results are summarized in static and dynamic current-field state diagrams and will be of importance for the design of STOs with enhanced output signals.

The main role played by the Pt buffer layer on the magnetic configuration of the [Pt/Co] multilayer, i.e. on the pattern consisting of closure domains, and the possibility of doubling HE by annealing the[Pt/Co]/NiFe system is shown.

Exchange bias (EB) refers to the shift of the hysteresis loop, typically observed in exchange interacting ferromagnetic (FM)-antiferromagnetic (AFM) materials. During the last decades, EB properties have been extensively investigated, mainly in thin films, due to their technological applications in magnetic random access memories and magnetoresistive read heads based on spin valves or tunnel junctions. Recently, the drastic increase in the areal density of magnetic recording media has motivated the study of EB properties in systems of reduced lateral dimensions. The reduction of the lateral dimensions of an EB system down to length scales comparable to the FM or to the AFM magnetic domain sizes is also interesting from a fundamental point of view since this results in a confinement and subsequent alteration of the FM and AFM domain structures, hence allowing to probe the role of domains on EB.

A shift in the hysteresis loop of a $\mathrm{NiFe}$ thin film (with in-plane anisotropy) exchange coupled to a $[\mathrm{Pt}∕\mathrm{Co}]$ multilayer (with out-of-plane anisotropy) is observed after in-plane saturation of the system. The origin of this effect and the related magnetic properties are investigated by means of in-plane and out-of-plane magnetometry techniques, magnetic force microscopy imaging, and micromagnetic simulations. Both the number of $\mathrm{Pt}∕\mathrm{Co}$ repetitions in the multilayer and the $\mathrm{NiFe}$ thickness are found to have an influence on the magnitude of the loop shift and the in-plane and out-of-plane coercivity values of the system. This is correlated with variations in the number and average size of the magnetic domains formed in the $[\mathrm{Pt}∕\mathrm{Co}]$ multilayer which, as revealed by micromagnetic simulations, pin the $\mathrm{NiFe}$ magnetization via formation of closure domains with a preferential orientation at the interface between the $[\mathrm{Pt}∕\mathrm{Co}]$ multilayer and the $\mathrm{NiFe}$.

International audience; Magnetostatic interactions in perpendicularly magnetized Co/ Pt bilayers lead to the formation of mirror domains in both soft and hard layers. We show that it is possible to take advantage of domain replication in order to probe the microscopic domain configuration of the hard layer through macroscopic hysteresis minor loop measurements on the soft layer. The minor curves consist of two loops, which magnetization amplitudes and field shifts can be quantitatively related to the domains sizes, shapes, and up/down relative proportion in the hard layer.

The hysteresis loop shift ${H}_{E}$ of sub-$100\text{\ensuremath{-}}\mathrm{nm}$ ferromagnetic- (FM-) antiferromagnetic (AFM) nanostructures is found to be strongly influenced by thermal activation effects. These effects, which tend to reduce ${H}_{E}$, are more pronounced in the nanostructures than in continuous films with the same composition, particularly for thin AFM layers. In addition, the reduced dimensions of the nanostructures also impose spatial constraints to the AFM domain size, particularly for thick AFM layers. This favors an enhancement of ${H}_{E}$. Due to the interplay between these two competing effects, the loop shift in the dots can be either larger or smaller than in the continuous films with the same composition, depending on both the AFM thickness and temperature. A temperature-AFM thickness phase diagram, separating the conditions resulting in larger or smaller ${H}_{E}$ in the nanostructures with respect to continuous film is derived.