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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 studied in elliptical dots composed of ferromagnetic Ni80Fe20-antiferromagnetic Ir20Mn80 bilayers. The magnetization reversal mechanisms and magnetic configurations have been investigated by magneto-optic Kerr effect and magnetic force microscopy. Although the obtained bias fields in these dots are relatively small, the magnetization reversal is found to be influenced by the ferromagnetic-antiferromagnetic coupling. Namely, for some off-axis angles of measurement, the magnetization reversal mechanism of the Ni80Fe20-Ir20Mn80 ellipses depends on whether exchange bias is induced along the minor or major axis of the ellipses. Hence, exchange bias is shown to be an effective means for tailoring the magnetization reversal of elliptical dots after sample fabrication.

Perpendicular magnetic anisotropy at metal/oxide interfaces continues to receive great interest, from the viewpoints of both fundamental research and spintronic applications. In a detailed investigation of the magnetic properties of Pt/Co/AlO${}_{x}$ sandwiches, unusual phenomena are observed, including reorientation of anisotropy as a function of temperature, and the onset of exchange bias associated with very large coercivity and anisotropy field at low temperature. These are interpreted in terms of chemical bonding and varying material phases in the trilayers.

International audience; We present a theory of the anisotropy of tunneling magnetoresistance (ATMR) phenomenon in magnetic tunnel junctions (MTJs) attributed to Rashba spin-orbit interaction in the insulating barrier. ATMR represents the difference of tunnel magnetoresistance (TMR) amplitude measured with in-plane and out-of-plane magnetic configurations. It is demonstrated that within the spin-polarized free-electron model the change of conductance associated with the ATMR is exactly twice the change of conductance measured at full saturation (i.e., in parallel configuration of magnetizations) between in-plane and out-of-plane configuration, i.e., the tunneling anisotropic magnetoresistance (TAMR). Both ATMR and TAMR are closely related to the TMR amplitude and spin-orbit constant. The predicted ATMR phenomenon is confirmed experimentally, showing a few percent value in the case of the widely studied CoFeB/MgO/CoFeB based MTJ.

International audience; STT-MRAM are foreseen as the best contender forDRAM replacement. STT-MRAM could also be used for SRAMapplications if switching time below 1ns could be realized in areliable way. In this study, we demonstrate that sub-ns switchingwith final state determined by the current polarity through thestack can be achieved in STT-MRAM cells comprising two spinpolarizinglayers having orthogonal magnetic anisotropies [1],[2].We carried out a thorough experimental and modeling study ofthese ultrafast STT-MRAM. We demonstrated that a quitereliable switching can be achieved by increasing the cell aspectratio (AR) or advantageously by applying an in-plane statictransverse field on the cell. Switching in 200ps could bedemonstrated with write energy less than 100fJ.

We report here the development of Pt and Pd-free perpendicular magnetic tunnel junctions (p-MTJ) for STT-MRAM applications. We start by studying a p-MTJ consisting of a bottom synthetic Co/Pt reference layer and a synthetic FeCoB/Ru/FeCoB storage layer covered with an MgO layer. We first investigate the evolution of RKKY coupling with Ru spacer thickness in such a storage layer. The coupling becomes antiferromagnetic above 0.5 nm and its strength decreases monotonously with increasing Ru thickness. This contrasts with the behavior of Co-based systems for which a maximum in interlayer coupling is generally observed around 0.8 nm. A thin Ta insertion below the Ru spacer considerably decreases the coupling energy, without basically changing its variation with Ru thickness. After optimization of the non-magnetic and magnetic layer thicknesses, it appears that such a FeCoB/Ru/FeCoB synthetic storage layer sandwiched between MgO barriers can be made stable enough to actually be used as hard reference layer in single or double magnetic tunnel junctions, the storage layer being now a single soft FeCoB layer. Finally, we realize Pt- or Pd-free robust perpendicular magnetic tunnel junctions, still keeping the advantage of a synthetic reference layer in terms of reduction of stray fields at small pillar sizes.

Magneto-optical microscopy and magnetometry have been used to study magnetization reversal in an ultrathin magnetically soft (Pt/Co)2 ferromagnetic film coupled to an array of magnetically harder (Co/Pt)4 nanodots via a predominantly dipolar interaction across a 3 nm Pt spacer. This interaction generates a spatially periodic pinning potential for domain walls propagating through the continuous magnetic film. When reversing the applied field with respect to the static nanodot array magnetization orientation, strong asymmetries in the wall velocity and switching fields are observed. Asymmetric switching fields mean that hysteresis of the film is characterized by a large bias field of dipolar origin which is linked to the wall velocity asymmetry. This latter asymmetry, though large at low fields, vanishes at high fields where the domains become round and compact. A field-polarity-controlled transition from dendritic to compact faceted domain structures is also seen at intermediate fields and a model is proposed to interpret the transition.

We have measured the magnetization and magnetoresistance of as-deposited and annealed CoxAg1−x and NixAg1−x (x=0.2, 0.37) heterogeneous alloys. From the magnetization curves, we have estimated the magnetic diameter of the particles. A structural diameter has been determined from EXAFS and anomalous small-angle X-ray scattering measurements. NiAg alloys exhibit original magnetic properties characterized by magnetic fluctuations both at the scale of the particles as in usual superparamagnetism, and also at the atomic scale, within each particle. The as-deposited Ni20Ag80 and Ni37Ag63 samples show intra-particle magnetic ordering temperature lower than room temperature (70 K for Ni20Ag80 and 200 K C K for Ni37Ag63). Model has been developed to explain the special magnetic behavior of NiAg alloys. The largest amplitude of magnetoresistance (15% at room temperature) is observed for Co37Ag63 annealed at 350°C. In this alloy, the average diameter of the granules is about 43 A. CoAg alloys always present larger MR amplitudes than the NiAg alloys of same volume fraction of magnetic element. In the as-deposited and annealed CoAg series, a maximum is observed in the relative MR amplitude as a function of particle diameter. This indicates a significant contribution of bulk spin-dependent scattering to the magnetoresistance (according to Vedyayev theory [A. Vedyayev et al., J. Magn. Magn. Mater. 164 (1996) 91]).

The structural evolution of co-sputtered Ag–20 (and 35) at.% Co and Ag–20 (and 35) at.% Ni was studied by anomalous small-angle X-ray scattering in the as-deposited state and after different anneals for 10 min at 573, 623 and 723 K. Anomalous scattering was used to separate the part of the scattering signal due to the transition metal particles from the signal of other heterogeneities. Strong segregation, involving about two-thirds of the Co (or Ni) atoms, already exists for the as-deposited state. After a 573 K anneal, the phases (Ag matrix and Co or Ni well defined particles) have almost reached equilibrium,i.e.complete immiscibility. Most of the magnetic particles are three dimensional with an average radius of 5–25 Å and the average distance between the particles varies from 17 to 110 Å, depending on the magnetic element and its concentration, and on the annealing conditions. The size distribution does not correspond to that of usual coarsening, but becomes broader after extended annealing. This is probably due to heterogeneous precipitation at grain boundaries of the Ag matrix.

International audience; Field-current phase diagrams were measured on in-plane anisotropy Co60Fe20B20 magnetic tunnel junctions to obtain the spin transfer torque (STT) field-current switching window. These measurements were used to characterise junctions with varying free layer thicknesses from 2.5 down to 1.1 nm having a reduced effective demagnetizing field due to the perpendicular magnetic anisotropy at CoFeB/MgO interface. Diagrams were obtained with 100 ns current pulses, of either same or alternating polarity. When consecutive pulses have the same polarity, it is possible to realize the STT switching even for conditions having a low switching probability. This was evidenced in diagrams with consecutive pulses of alternating polarity, with 100% switching obtained at 4.7 MA/cm2, compared to the lower 3.4 MA/cm2 value for same polarity pulses. Although the low level of the current density window is higher in alternating polarity diagrams, the field window in both diagrams is the same and therefore independent of the pulse polarity sequence.

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 figures

The indirect exchange coupling of Co through pure and hydrogenated Nb spacer layers has been investigated as a function of the spacer layer thickness and temperature. It is found that these systems exhibit significant biquadratic and bilinear coupling in the temperature range studied between 10 and 300 K. The biquadratic term appears to decay exponentially with spacer layer thickness, increases rapidly with decreasing temperature and is insensitive to the addition of hydrogen. For both sets of samples a peak in the antiferromagnetic bilinear term is observed, with the hydrogenated samples displaying a larger maximum coupling strength at a smaller spacer layer thickness. The data are consistent with Slonczewski's ‘loose spins’ model for biquadratic coupling. Resistance, magnetoresistance, magnetization and X-ray diffraction data suggest that the multilayers have a highly disordered structure.

This work reports on advances in MRAM cells aiming at sub-nanosecond switching and for sub-20nm technology nodes. Ultrafast precessional spin-transfer switching in elliptical magnetic tunnel junction nanopillars is possible to obtain in samples integrating a perpendicular polarizer and a tunnel junction with in-plane magnetized electrodes. We show that spin transfer torque (STT) switching in less than 500ps can be achieved in these structures with corresponding write energy less than 100fJ. For high density integration and possibly sub-20nm diameter cells the use of a thermally assisted concept for perpendicular anisotropy cells, where the intrinsic heating is used to simultaneously achieve high thermal stability and low current switching.

The magnetoresistance of microscopic strips of (NiFe/Ag) multilayers with large bi-quadratic coupling has been measured. When the field is applied along the length of the strip, the magnetoresistance has an almost linear R(H) characteristic. On the other hand, when the field is applied transversely i.e. in the plane of the film but perpendicular to its length, a plateau in the resistivity is observed up to a critical field, Hcrit, which depends on the shape anisotropy of the strip. Above this field, R(H) decreases as in the longitudinal case. Computer simulations have been undertaken to explain this behaviour. The plateau observed with a transverse applied field is due to the effect of the shape anisotropy on the remanent magnetization of the multilayers. This remanent magnetization can be quite large because of the significant bi-quadratic coupling between the layers.

The thermal stability of pinned synthetic ferrimagnets with perpendicular anisotropy is investigated. These structures consist of two [Pt/Co] mutilayers (MLs) antiferromagnetically coupled through a Ru spacer, in which one of them is, in turn, exchange biased with FeMn. The switching fields of both MLs can be tuned, at a given temperature, by varying the relative number of (Pt/Co) repeats comprised in each ML. These fields are determined by the values of the exchange constant through the Ru, ARu, and the exchange constant with FeMn, Apinning. Interestingly, for some compositions, it is found that the order in which the two MLs switch during magnetization reversal changes depending on temperature. This effect is due to the different thermal evolution of ARu and Apinning.

In this work the thermal heating of tunnel junctions is investigated doing one-dimensional numerical simulations by solving the heat equation. The temperature increase is proportional to the power density jV, which is confirmed by transport measurements on patterned μm size junctions. The observed temperature increase was 2.3 K/(mW μm−2) compared to a simulation result of 0.9 K/(mW μm−2). Tunneling AFM measurements on 4, 5.4, and 6.7 A barrier half-junctions, show hot spots with current densities up to 13 times the average density value. The current concentration could explain the higher heating observed experimentally compared to the simulated value. The hot spot density is 600–900/μm2 occupying roughly 5% of the barrier surface.

Magnetoresistance and magnetization measurements were performed on sputtered Ag/CoFe multilayers, with various CoFe and Ag thicknesses. For thicknesses larger than 6 A, CoFe layers are continuous. The amplitude Δ ϱ/ϱ 0 of the giant magnetoresistance (GMR) is maximum around 7 A of CoFe and 12 A of Ag, reaching 30% in 700 Oe at room temperature and 100% at 4.2 K. For lower CoFe thicknesses, the magnetic layers break into oblate particles. These particles exhibit a superparamagnetic behaviour and the system a ‘granular-like’ magnetoresistance. Magnetization and magnetoresistance behaviours have been modelized in each case, introducing bilinear and biquadratic exchange couplings in multilayer systems, or the size distribution and interactions of the particles in granular multilayers.

X-ray resonant magnetic scattering experiments using linear and circular polarized light were performed at the Ni ${L}_{2,3}$ absorption edges on a Ag $/$Ni multilayer. A superlattice magnetic peak, due to the antiferromagnetic coupling between Ni layers, is evidenced. In the case of a ferromagnetic coupling, large changes in the charge peak (up to 15%) are observed upon reversal of the direction of the magnetic field. The magnetic scattering amplitude is evaluated to ${8r}_{0}$ per nickel atom. Sum rules have been applied for the first time to the energy-dependent magnetic amplitude.

X-ray resonant magnetic scattering experiments using linear and circular polarised light have been caried out at the Ni and Fe L 2,3 absorption edges on magnetically coupled Ag/Ni and Ag/NiFe multilayers. The strong enhancement of the X-ray magnetic resonant scattering, especially at the Fe L edges, allows to evidence a superlattice magnetic peak due to the antiferromagnetic coupling between Ni or NiFe layers. Magnetic reflections represent several % of the intensity of the charge peak. With the magnetic moments aligned between layers, large changes (up to 75%) in the intensity of the chemical modulation peak have been observed upon reversal of the direction of the magnetic field. For the Ag/Ni, multilayer a detailed analysis gives a magnetic scattering amplitude equal to 8 r 0 per nickel atom.

Pt/Co/M sandwiches (M=Al, Ta, Cr) were prepared by sputtering. The M capping layer was oxidized either by natural oxidation in air or by exposure to an oxygen plasma. By measuring the extraordinary Hall effect in these systems, we observed that the magnetic anisotropy of the Co electrode is extremely sensitive to the degree of oxidation of the capping layer. In particular, when M=Al, the magnetic anisotropy is found in plane when the AlOx layer is under or overoxidized whereas it is out of plane when this oxide layer is stochiometric alumina. The conditions of oxidation which give the perpendicular anisotropy in Pt/Co/AlOx are found to be the same than those which lead to a maximum of specular reflection at Co/AlOx interface in Co/Ru/Co/Cu/Co/AlOx synthetic spin-valves. This crossover of anisotropy therefore provides a very useful way for controlling the oxidation of tunnel barriers. Furthermore, this phenomenon can be used to study the aging of naturally oxidized tunnel junctions exposed to air, and it i...

The effect of annealing on the structural stability of (Pt2nm/Co0.4 nm)3/IrMn7nm multilayers has been investigated using atom probe tomography. The composition of individual layers was measured after annealing at 300, 400, 500, and 700 °C. While results show that the (Pt/Co)3/IrMn stacking sequence is preserved up to 400 °C, there is an almost complete destruction of the multilayered structure when annealing at higher temperatures (500 and 700 °C). Co layers no more alternate with Pt-rich layers. The whole stack is transformed into an IrCo/PtMn bilayer. These results are interpreted on the basis of atomic mobilities and chemical affinities. Diffusion of Co and Mn is shown to become important when annealing temperature approaches 500 °C. Results are well accounted for by thermodynamics arguments considering the Co-Ir and Pt-Mn phase diagrams.

Synthetic antiferromagnets are of great interest as reference layers in magnetic tunnel junctions since they allow decreasing the dipolar coupling between the two magnetic electrodes and exhibit larger pinning fields than single reference layers. In this letter, we investigate the effect of the insertion of an ultrathin Pt layer in contact with the Ru spacer in synthetic antiferromagnets with perpendicular magnetic anisotropy. Surprisingly, for Ru thickness below 0.75 nm, the antiferromagnetic coupling amplitude through Ru first increases upon Pt insertion up to a critical Pt thickness (∼0.25 nm) above which coupling decreases. In addition, the corresponding increase of perpendicular magnetic anisotropy enhances the thermal stability of the structure.

The effects of sputter-deposition-induced and post-deposition thermally activated layer intermixing on the exchange bias properties of [Pt/Co] x3/Pt(t Pt)/IrMn films with out-of-plane anisotropy and deposited under a perpendicular magnetic field are investigated. The consequences of the intermixing on the magnetic properties are correlated with atom probe structural investigations. We observe that Co–Mn and Co–Pt intermixing are already present in the as-deposited state. The intermixing is more pronounced for Pt on Co (∼50%) than for Mn on Co (∼10%). It is observed that annealing up to 200 °C does not result in noticeable further diffusion of Mn in Co. In contrast, it significantly accentuates the Co–Pt initial intermixing. This enhanced intermixing leads to a 40% reduction in the exchange bias field. We measured that this reduction is not primarily due to changes in effective anisotropy or in saturation magnetization. This is possibly ascribed to a decrease in the mean interfacial moment and exchange stiffness both due to the observed reduction in Co content within the Co sublayers.

This work reports sub-nanosecond precessional spin-transfer switching in elliptical magnetic tunnel junction nanopillars. This result is obtained in samples integrating a perpendicular polarizer and a tunnel junction with in-plane magnetized electrodes. We have performed statistics on time-resolved electrical measurements showing oscillations of the switching probability as a function of write voltage pulse width. This behavior is characteristic of the free layer precessional motion induced by the perpendicular polarizer. Ultrafast STT switching in less than 300ps could be achieved in these structures with corresponding write energy less than 100fJ.

This letter presents a study of perpendicular anisotropy in Co/Ni multilayers, which could constitute a thick polarizer in spin torque oscillators or a magnetic electrode in magnetic tunnel junctions (MTJ) with perpendicular anisotropy. Perfectly square perpendicular loops are observed for as-deposited Co/Ni multilayers with various sublayer thicknesses and bilayer repetition numbers using a Pt buffer layer. An anisotropy energy of 1.0 × 106 erg·cm−3 is obtained for 9 nm thick Co/Ni multilayers. For Co/Ni multilayers deposited on MgO, no perpendicular magnetization component is observed in the as-deposited state, but it develops (even in 2.1 nm Co/Ni multilayers) after annealing at 250 °C.

International audience; We present results from low-angle neutron and x-ray scattering experiments on hydrogen and deuterium loaded (001)-oriented single-crystal Mo/V superlattices. The individual Mo and V layer thicknesses in the samples were determined by simulations of the x-ray reflectivity curves. The intensity of the first-order neutron-diffraction peak for the hydrogenated sample is 31% larger than from the dehydrogenated sample. When deuterated, the intensity of the first-order peak is extremely small, only 0.5% of the dedeuterated sample. The distribution of hydrogen in the layers was derived by simulations of the neutron and the x-ray diffraction patterns. The results unambiguously demonstrate that the hydrogen isotopes are exclusively located in the V layers, but at the interfaces there is a region with reduced hydrogen concentration.

Resistance and length changes were measured during thermal treatments on freestanding AgNi multilayers. The experiments allowed a clear separation of the successive stages of the microstructural evolution of these stratified structures: interfacial relaxation, destratification and grain growth. The kinetics of these processes were extracted by monitoring resistivity and length during thermal cycles and isotherms. A substantial densification was revealed during the interface relaxation, that results in a slowing down of the local reorganization process. An early diffusional creep was also shown; its occurrence is restricted to the destratification region.

By measuring the extraordinary Hall effect on a series of naturally oxidized Pt3 nm/Co90Fe10 0.6 nm/Al tAl samples with 0

A method to switch the magnetization of the free layer in magnetic tunnel junctions with perpendicular anisotropy is demonstrated. It consists in assisting the spin transfer switching of the magnetization by a thermally induced reorientation of the free layer magnetic anisotropy from out-of-plane to in-plane. The junction temperature increase is due to the Joule dissipation around the tunnel barrier produced by the same pulse of current which generates the spin transfer torque. This magnetic reorientation allows the spin transfer torque efficiency to be maximal since the spin polarization of the current is perpendicular to the magnetization of the free layer. Such a thermally assisted switching allows designing highly down-size scalable magnetoresistive random access memory cells with improved write efficiency.

3 pages; International audience; Two key parameters were analyzed in Si/SiO/Pt/Co/AlOx: the oxidation time of the Al layer resulting in AlOx, and the ex situ annealing temperatures varied in the 15 and 55 s and 20, 300, and 450 °C ranges, respectively. For intermediate annealing temperatures (∼300 °C), the quantitative analysis of specular reflectometry data shows that the progressive oxidation of layers by increasing the oxidation time goes along with an improvement of the homogeneity of the alumina layer. This outcome casts new light on the temperature dependence of magnetic properties of the samples. The remarkable temperature variation of the coercive field, extracted from extraordinary Hall effects in the 5-300 K range, is associated with structural change due to Co-oxygen bondings, which leads to strong pinning of Co spins in the low temperature regime.

Resonant enhancement of the X-ray magnetic scattering allowed to evidence the antiferromagnetic coupling between the Ni layers in an Ag/Ni multilayer through the appearance at small angles of a magnetic superlattice diffraction peak. The magnetic moments are found to be oriented perpendicular to an applied magnetic field weaker than the saturation field. Moreover, we observed a large magnetization dependent amplitude of 2.5 electrons.

The magnetoresistive properties of sputtered Ni-Fe/Ag multilayers have been investigated as a function of thermal treatments. For the virgin sample, a maximum magnetoresistance ratio of 17% with a saturation field as low as 300 Oe is obtained at room temperature. In addition, the R(H) curve is found to be perfectly linear and reversible. Moreover, these properties improve with annealing treatments. For annealing temperatures of 280°C, the magnetoresistance slope dR/dH increases up to 165% per kOe, which is the largest value ever reported in magnetically coupled multilayers.

Arrays of Si dots 400 nm squares with a spacing of 100 nm and a height of 200 nm were patterned on silicon substrates by electron beam lithography and reactive ion etching, and a Co/Pt multilayer was sputter-deposited on these patterned silicon substrates. The magnetic film covers the top of the dots, the bottom of the grooves and to a much less extent the side-walls of the dots. The magnetic properties were studied by magnetic force microscopy (MFM). The Co/Pt multilayer exhibits perpendicular magnetic anisotropy and single-domain magnetic dots are obtained. Our results indicate that the width of the switching field distribution of the dots is not due to magnetostatic interactions but can be ascribed to a distribution of defects in the dots. A statistical analysis of MFM images shows that no significant direct coupling is mediated by the magnetic deposit on the side-walls of the dots.

The elastic properties of sputter‐deposited Ag/Ni superlattices have been studied by means of the Brillouin light scattering technique. Measurements have been taken on both free‐standing and glass‐supported superlattices, the former deposited at 100 K, the latter at 300 K. The two effective elastic constants c11 and c44 have been determined on specimens 5 μm thick for different superlattice periods; c44 decreases by almost 40% as the bilayer periodicity decreases from 19 to 1.3 nm, while c11 shows no appreciable variation. Measurements performed on supported specimens, 0.5 μm thick, enabled us to get information also on the constant c33, which exhibits a monotonic increase with period. The elastic anomalies found can be related to the appreciable lattice distortion observed by x‐ray diffraction experiments. Both Brillouin scattering and x‐ray analysis have shown that the interface quality improves appreciably when the temperature of deposition is lowered from 300 to 100 K.

The magnetic phase diagram of antiferromagnetically coupled Ni layers in Ni/Ag superlattices is presented from low temperature up to the ordering temperature. Relations between the neutron scattered intensity (linked to the antiferromagnetic order parameter), the magnetization and the absolute magnetoresistance effect are obtained. The transport properties are observed to be driven by the relative orientations and amplitudes of the magnetizations in successive Ni layers both below and above the ordering line H*(T).

The first selective determination of the effective elastic constant c11 in thick, unsupported metallic superlattice samples has been achieved using Brillouin light scattering. This enabled Young's modulus to be estimated. Measurements of the macroscopic Young's modulus were also performed on identical samples using a tensil micromachine. The two approaches, surface phonons and static elasticity, give results mutually consistent in the Ag/Ni superlattices studied. In addition, no significant dependence on period was found in either case.

International audience; Nanostructures consisting of a stack of two magnetic multilayers with out-of-plane anisotropy and distinct coercivities hold great promises in spintronics devices. Yet their miniaturization leads to interlayer dipolar coupling, which results in detrimental asymmetrical behaviors of magnetization reversal or even in the loss of intermediate antiparallel configuration. In this letter, we take advantage of Ruderman-Kittel-Kasuya-Yosida interactions in order to compensate for this coupling and restore symmetry. The study has been performed on an array of square dots of 200 nm lateral size, each dot consisting of two Co/Pt multilayers antiferromagnetically coupled through a thin Ru spacer.

Multilayers of gold and titanium have been prepared by sequential sputtering. They are composed of equally thick strata of f.c.c. gold and h.c.p. titanium with respective orientations 〈111〉 and 〈0002〉 perpendicular to the layer planes. X-ray diffraction spectra revealed that for all the periods examined, i.e. 2, 3, 7 and 20 nm, superlattices are formed. The electrical characteristics and the elastic modulus of these superlattices were determined. It appears that the measured values for Young's modulus are centred on 115 GPa with a typical scatter of ± 15%. No systematic dependence on the bilayer period appears to exist within these limits. In addition the temperature interval and time scale of the structural evolution which takes place during annealing sequences were studied. For the lower periods a narrow resistivity stage shows up at about 550 K. The resistivity is then increased by 50%. The kinetic study indicates that the corresponding activition enthalpy is 1.31 ± 0.09 eV. It is constant during the whole reaction, which results in the loss of composition modulation and the formation of a metastable Au 2 Ti tetragonal phase. For the longer periods, evolution towards the equilibrium phases is more complex. Significant dimensional variations were found to accompany these transformations.

Low-angle neutron scattering experiments in reflection geometry have been performed on Ag/Ni superlattices prepared by sputtering. The low-temperature diffraction diagram shows the appearance of a supplementary low-angle peak attributed to the doubling of the chemical period, as a consequence of the antiparallel alignment of the magnetic moments in successive nickel layers. The variation of the intensity of this antiferromagnetic peak with applied magnetic field is in accordance with a classical model of antiferromagnetism in which the magnetic moments, at first perpendicular to the field, rotate progressively to the direction of the field. A good agreement is found with magnetization and magnetoresistance results obtained on the same samples.

Arrays of Si square dots down to 200 nm in size were patterned on silicon substrates, and thin films of different magnetic materials were sputter deposited on these patterned substrates. The magnetic film covers the top of the dots, the bottom of the grooves and to much less extent the sidewalls of the dots. Single domain magnetic dots were obtained for Co/NiO bilayers and Co/Pt multilayers, without significant direct coupling mediated by the magnetic deposit on the sidewalls of the dots. Our results indicate that, in these arrays, the magnetic pinning forces are stronger than the estimated value of the largest demagnetizing magnetostatic field on each individual dot. As a result, any magnetic configuration could in principle be stored in such arrays. This approach seems therefore very promising for the preparation of magnetic storage media with ultrahigh density.

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 at the Co/AlOx interface. It is observed that varying the degree of oxidation modifies the magnetic properties of the Co layer, inducing a magnetic anisotropy crossover from in plane to out of plane. The microscopic structural properties are analyzed via x-ray photoelectron spectroscopy measurements. It is shown that increasing the oxidation time enhances the amount of interfacial oxide, which may be at the origin of perpendicular magnetic anisotropy.

Multilayers of aluminium and tin were prepared by DC sputtering. Their structural evolution was studied by means of X-ray scattering experiments at both small and large angles, and electrical resistivity measurements during or after thermal treatments. Since aluminum and tin are not mutually soluble, this type of multilayer must evolve towards long-range segregation when the temperature is increased. The present results show that this evolution takes place in three steps. The first one is revealed by a slight decrease of the resistivity without loss of the periodicity. It is attributed to the growth of the crystallites in the plane of the layers. Second, a large decrease of the resistivity is observed at a temperature as low as 460 K. It reflects the disappearance of the periodic structure, through long-range diffusion. As a consequence, classical three-dimensional melting of Sn crystallites occurs around 505 K. This is inferred from the reversible resistivity variation detected on thermal cycling.