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A 5 nm thick ferrimagnetic film made of amorphous rare-earth transition-metal alloys Gdx(Fe90Co10)1-x was grown by physical vapor deposition. Its magnetic properties (coercivity, perpendicular magnetic anisotropy, and compensation composition at room temperature) were investigated for various buffer and capping layers in contact with a ferrimagnetic thin film. While Gdx(Fe90Co10)1-x appears to be amorphous for all the samples, it appears that (111) textured Cu is the best material to promote perpendicular magnetization. The large compensation composition change as a function of the magnetic film interface at room temperature is analyzed in terms of polarizability of the surrounding buffer and capping materials.

Switching of perpendicular magnetization via spin–orbit torque (SOT) is of particular interest in the development of non-volatile magnetic random access memory (MRAM) devices. We studied current-induced magnetization switching of Ir/GdFeCo/Cu/Pt heterostructures in a Hall cross geometry as a function of the in-plane applied magnetic field. Remarkably, magnetization switching is observed at zero applied field. This is shown to result from the competition between SOT, the Oersted field generated by the charge current, and the material's coercivity. Our results show a means of achieving zero-field switching that can impact the design of future spintronics devices, such as SOT-MRAM.

Shape-anisotropy magnetic tunnel junctions (MTJs) are attracting much attention as a high-performance nonvolatile spintronic device in the X/1X nm regime. In this study, we investigate an energy barrier relevant to the retention property in CoFeB/MgO-based shape-anisotropy MTJs with various diameters at high temperatures and compare it with that in conventional interfacial-anisotropy MTJs. We find that the scaling relationship between the energy barrier and the spontaneous magnetization in shape-anisotropy MTJs is well described by a model assuming the dominant contribution of shape anisotropy to the energy barrier. Also, the scaling exponent is much smaller than that for the interfacial-anisotropy MTJs, indicating that the properties of shape-anisotropy MTJs are less sensitive to the temperature. Using the experimentally determined scaling relationship, we discuss the design window of the MTJ dimensions to achieve data retention of 10 years at various temperatures. This study demonstrates that the shape-anisotropy MTJ holds promise of scaling beyond 20 nm for high-temperature applications.

International audience; Combining multiple degrees of freedom in strongly correlated materials such as transition-metal oxides would lead to fascinating magnetic and magnetocaloric features. Herein, the strain effects are used to markedly tailor the magnetic and magnetocaloric properties of PrVO3 thin films. The selection of an appropriate thickness and substrate enables us to dramatically decrease the coercive magnetic field from 2.4 T previously observed in sintered PVO3 bulk to 0.05 T for compressive thin films making from the PrVO3 compound a nearly soft magnet. This is associated with a marked enhancement of the magnetic moment and the magnetocaloric effect that reaches unusual maximum values of roughly 4.86 μB and 56.8 J/kg K with the magnetic field change of 6 T applied in the sample plane in the cryogenic temperature range (3 K), respectively. This work strongly suggests that taking advantage of different degrees of freedom and the exploitation of multiple instabilities in a nanoscale regime is a promising strategy for unveiling unexpected phases accompanied by a large magnetocaloric effect in oxides.

Logic computing in magnetic domain walls is investigated using the interplay of all-optical helicity-dependent switching and current-induced spin–orbit torque switching. By simultaneously controlling current and laser pulses, logic functions of AND, OR, NAND, and NOR are experimentally demonstrated through the anomalous Hall effect and verified by micromagnetic simulations. The optoelectronic domain-wall motion is energy-efficient compared to the traditional all-current approach and provides another degree of freedom for the realization of logic applications.

International audience; Magnetic refrigeration based on the magneto-caloric effect is one of the best alternatives to compete with vapor-compression technology. The viability of a magnetic refrigeration system for magnetic cooling can be tested by exploiting the materials in various forms, ranging from bulk to nanostructured materials. In order to achieve a wide refrigerating temperature range in magnetic refrigeration, we study in this paper a 100 nm-thick Gd-Co alloys-based multilayer stack. The stack is made of four individual Gd-Co alloy layers with different values of concentration and Curie temperature (TC). A magnetic entropy change associated with the second-order magnetic phase transition was determined from the magnetic isotherms. Moreover, the relative cooling power (RCP) of the studied Gd-Co-based multilayer is enhanced compared to the one of bulk Gd, and reaches a value of 200 J/kg. Such an enhancement of the RCP is not due to an enhanced maximum variation of entropy, but this is due to a much broader magnetic entropy peak. This study demonstrates the potential of nanostructured Gd-Co multilayer stack for magnetic cooling applications.

We report on magnetic domain wall velocity measurements in a synthetic ferrimagnet made of two perpendicular ferromagnetic layers antiferromagnetically exchange coupled. In this system, two types of transitions may be observed: one from a parallel alignment to an antiparallel alignment of the magnetization of the two layers and the other between the two possible antiparallel alignments. Those transitions are shown to be dominated by domain wall propagation. The domain wall velocity as a function of the applied magnetic field pulse amplitude has been measured. Two remarkable features are observed: first, a drastic breakdown of the domain wall velocity and then an inversion of the domain propagation direction are observed when the field pulses reach values comparable to the exchange field between the two layers. This unexpected behavior can be understood qualitatively using a simple model taking into account the competition between interlayer exchange coupling and the external driving field.

International audience; Stable magnetic states of the MgO/CoFeB/Ta/CoFeB/MgO/Ta spin valve as well as transitions between the states were detected by microwave magnetoresistance (MMR) measured in the cavity of an electron spin resonance spectrometer. Advantages of this experimental technique are the possibility to study the orientation dependence of the MMR, the absence of the additional contact/sample interfaces, the wireless control of the spin valves, and the compatibility of the MMR measurements with ferromagnetic resonance experiments. The magnetic field dependence of the first derivation of the microwave absorption allows one to judge about the negative magnetoresistance of the layers and positive interlayer giant magnetoresistance. The obtained experimental results could be used for engineering of the microwave high sensitive sensors available for remote identification of the stable magnetic and logic states of the spin valves needful in medical spintronics to detect biological objects labeled with nanoparticles.

Magnetic nanowires supporting field- and current-driven domain wall motion are envisioned for new methods of information storage and processing. A major obstacle for their practical use is the domainwall velocity, which is traditionally limited due to the Walker breakdown occurring when the forcing field or current reaches a critical threshold value. We show through numerical and analytical modeling that the Walker breakdown limit can be extended or completely eliminated in antiferromagnetically coupled magnetic nanowires. These coupled nanowires allow for giant domain-wall velocities driven by field and/or current via spin transfer torque as compared to conventional nanowires.

We present an experimental study of all-optical helicity-dependent switching (AO-HDS) of ferromagnetic Pt/Co/Pt heterostructures with perpendicular magnetic anisotropy. The sample is patterned into a Hall cross and the AO-HDS is measured via the anomalous Hall effect. This all-electrical probing of the magnetization during AO-HDS enables a statistical quantification of the switching ratio for different laser parameters, such as the threshold power to achieve AO-HDS and the exposure time needed to reach complete switching at a given laser power. We find that the AO-HDS is a cumulative process, a certain number of optical pulses is needed to obtain a full and reproducible helicity-dependent switching. The deterministic switching of the ferromagnetic Pt/Co/Pt Hall cross provides a full “opto-spintronic device,” where the remanent magnetization can be all-optically and reproducibly written and erased without the need of an external magnetic field.

We present positive and negative exchange bias (EB) phenomena observed in two ferrimagnetic-based amorphous bilayers systems: FeSn/FeGd and FeGd/TbFe. In both cases, magnetization measurements exhibit a shift of the loop of one of the ferrimagnetic layer towards a positive or negative field. In FeSn/FeGd, the coupling at the interface between the net magnetization of the two layers is antiferromagnetic, which leads to a positive shift. However in FeGd/TbFe bilayers, the coupling is ferromagnetic and the shift is negative. The EB phenomenon is attributed to the occurrence of a magnetic domain wall present at the interface, which can be “compressed” or “decompressed” by an external applied field. A simple model based on the competition between the Zeeman energy and the interface exchange interaction energy permits a good quantitative evaluation of the observed EB fields.

The influence of magnetic domain walls (DWs) on magnetoresistance in various exchange-bias ferrimagnetic/(ferrimagnetic or ferromagnetic) bilayer systems (Gd68Fe32/Tb55Fe45 and Fe60Gd40/Fe55Sn45) is presented. These systems allow one to create well-defined DWs of various size and shape. The DW morphology is controlled by the applied magnetic field in the film plane. By changing the amplitude of the field, the DW thickness can be controlled, whereas the rotation of the field in plane allows one to create a θ° Bloch DW (90° DW, 180° DW,…). The electrical resistivity of the sample was measured relative to the applied magnetic field (amplitude and direction). The influence of the shape and size of the DW on magnetoresistance was followed. For the two amorphous samples, it made it possible to fit the variation of resistivity relative to the DW thickness and shape using a model based on anisotropic magnetoresistance.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; We present a study of the temperature dependence of the switching fields in Co/Ni-based perpendicularly magnetized spin-valves. While magnetization reversal of all-perpendicular Co/Ni spin valves at ambient temperatures is typically marked by a single sharp step change in resistance, low temperature measurements can reveal a series of resistance steps, consistent with non-uniform magnetization configurations. We propose a model that consists of domain nucleation, propagation, and annihilation to explain the temperature dependence of the switching fields. Interestingly, low temperature (

Equipe 101 : Nanomagnétisme et électronique de spin; International audience; Single-crystalline V/Fe(0.7 nm)/MgO(1.2nm)/Fe(20 nm) magnetic tunnel junctions are studied to quantify the influence of an electric field on the Fe/MgO interface magnetic anisotropy. The thinnest Fe soft layer has a perpendicular magnetic anisotropy (PMA), whereas the thickest Fe layer acts as sensor for magnetic anisotropy changes. When electrons are added at the PMA Fe/MgO interface (negative voltage), no anisotropy changes are observed. For positive voltage, the anisotropy constant decreases with increasing bias voltage. A huge 1150 fJV(-1) m(-1) anisotropy variation with field is observed and the magnetization is found to turn from out-of-plane to in-plane of the sample with the applied voltage.

International audience; Tb-Co ferrimagnetic alloy thin films and spin valves have been grown to study their magnetoresistance response in various geometries. The studied Tb-Co alloys show strong perpendicular anisotropy and tunable magnetization by several orders of magnitude. Magnetoresistance signals such as giant magnetoresistance (GMR), anisotropic magnetoresistance (AMR), extraordinary Hall effect (EHE), and magnon magnetoresistance (MMR) have been studied. The angular dependence of those magnetoresistive effects is also investigated. Finally we demonstrate that by adjusting the Tb-Co layer composition in a spin valve structure, the sign and the amplitude of the GMR and EHE signal can be tuned.

We report the free layer switching field distributions of spin-valve nanopillars with perpendicular magnetization. While the distributions are consistent with a thermal activation model, they show a strong asymmetry between the parallel to antiparallel and the reverse transition, with energy barriers more than 50% higher for the parallel to antiparallel transitions. The inhomogeneous dipolar field from the polarizer is demonstrated to be at the origin of this symmetry breaking. Interestingly, the symmetry is restored for devices with a lithographically defined notch pair removed from the midpoint of the pillar cross-section along the ellipse long axis. These results have important implications for the thermal stability of perpendicular magnetized MRAM bit cells., Comment: Submitted to Applied Physics Letters on November 4, 2011. Consists of 4 pages, 3 figures

International audience; In order to tune the exchange bias training effect, a torque approach is proposed. Torque loops are performed between two consecutive hysteresis loops by clockwise and counterclockwise rotation of the sample within an angular range under an in-plane external magnetic field. For the angular range from 0° to 180°, the coercivity H C and the exchange field H E are both reduced and the pinning direction PD orientation is changed, exhibiting the first type training effect. For the angular range from 20° to 380°, H C and the PD orientation are almost unchanged although H E is reduced, exhibiting the second type training effect.

Spin-torque switching of perpendicular anisotropy nanopillar spin valve devices with synthetic antiferromagnetic reference layers is presented. The use of composite reference layer reduces the dipolar interaction with the free layer yielding two remanent states. We derive a complete current-field phase diagram including minor loops of the free layer where we determine the relative dipolar fields and effective spin polarization of the reference layer in both the parallel and antiparallel configurations. We find the magnetic response of the reference layer relatively insensitive to the spin polarized current which may prove advantageous for spin-torque applications.

Spin-transfer switching with short current pulses has been studied in spin-valve nanopillars with perpendicularly magnetized free and reference layers. Magnetization switching with current pulses as short as 300 ps is demonstrated. The pulse amplitude needed to reverse the magnetization is shown to be inversely proportional to the pulse duration, consistent with a macrospin spin-transfer model. However, the pulse amplitude duration switching boundary depends on the applied field much more strongly than predicted by the zero temperature macrospin model. The results also demonstrate that there is an optimal pulse length that minimizes the energy required to reverse the magnetization.

[Co/Ni] multilayer films are grown in various conditions to study the influence of growth parameters on the anisotropy of the films in order to optimize the perpendicular uniaxial anisotropy. These multilayers are expected to be useful for current induced magnetization switching experiments due to its softmagnetic behavior by which the critical current density can be reduced dramatically. The polycrystalline films are prepared by electron beam evaporation with varying buffer layer, cap layer, as well as Co layer thickness and number of repetitions of Co and Ni layers. Measurements of the magneto-optic Kerr effect, ferromagnetic resonance, and superconducting quantum interference device magnetometry are used to analyze the magnetic anisotropy and magnetization. Depending on the magnitude of the uniaxial anisotropy, the overall easy direction of magnetization is orientated either in plane or out of plane depending on the amplitude of the demagnetization field. Moreover, we present the results for the temper...

Single crystal Ni/Co(111) superlattices have been grown by molecular beam epitaxy. The Ni thickness is 3 ML whereas the Co thickness varies from 0.2 to 4 ML. The superlattices were studied using magnetometry and ferromagnetic resonance spectroscopy and they all exhibit strong perpendicular to the plane magnetic anisotropy. The maximum magnetocrystalline anisotropy is obtained for one cobalt monolayer. Kerr microscopy measurements show the variation of domain pattern as the Co layer thickness changes.

We describe nanopillar spin valves with perpendicular anisotropy designed to reduce the critical current needed for spin transfer magnetization reversal while maintaining thermal stability. By adjusting the perpendicular anisotropy and volume of the free element consisting of a [Co/Ni] multilayer, we observe that the critical current scales with the height of the anisotropy energy barrier and we achieve critical currents as low as 120 μA in quasistatic room-temperature measurements of a 45 nm diameter device. The field-current phase diagram of such a device is presented.

Spatial and time resolved giant magnetoresistance measurements are used to detect thermally activated domain wall (DW) propagation along wires formed from spin valves having perpendicular magnetic anisotropy. We show that field- and current-induced DW propagation are both strongly influenced by the presence of pinning defects. As a result, the creep motion of DWs over macroscopic length scales is found to be stochastic. Only the combination of a high polarized current density and/or magnetic field allows DWs to propagate freely. These results may help explain low current-driven DW velocities observed in previous experiments.

International audience; Using magnetometry and polarized neutron reflectometry ͑PNR͒, we have mapped the reversal processes of the antiferromagnetically coupled, hard/soft system, TbFeCo/ ͓Co/ Pd͑t Pd ͔͒ 15 , with perpendicular anisotropy. The magnitude of the exchange coupling within the ͓Co/ Pd͔ layer can be tuned by varying Pd thickness. Since PNR is insensitive to moments perpendicular to the plane, a scattering geometry with polarization parallel to the scattering vector was used to isolate in-plane magnetization components and characterize the behavior of a domain wall near the bilayer interface. Consistent with predictions from a micromagnetic calculation, the characteristics of the domain wall vary with field and exchange stiffness in the Co/ Pd. Today, perpendicular recording, in which the magnetic anisotropy axis is normal to the media film plane ͓perpen-dicular magnetic anisotropy ͑PMA͔͒, has effectively replaced traditional in-plane storage media in the hard disk industry. 1 Various exchange-coupled composite structures have been recently proposed to reduce the switching field amplitude of the media without reducing its thermal stability. 1–3 In such soft/hard systems, the magnetization reversal is thought to occur by domain nucleation by the soft layer that propagates into the hard layer. Optimizing these structures requires controlling the vertical exchange and understanding the magnetic reversal pathway. To date, most of the experimental studies report macroscopic magnetization measurements or recording performances, 1,4 providing only indirect information about the shape and the location of PMA domain walls ͑DWs͒. For related multilayer films with in-plane anisotropy, polarized neutron reflectivity ͑PNR͒ has been used in the past to resolve the depth dependence of interfacial magnetic DW. 5,6 However, the application of this powerful technique to PMA media is nontrivial as the perpendicular moments are parallel to the scattering vector and do not give rise to scattering via the neutron selection rules. 7

International audience; The effect of temperature on exchange-bias properties of an antiferromagnetically coupled hard/soft bilayer ͑Tb 12 Fe 88 /Gd 40 Fe 60 ͒ is studied. In a similar manner to its cooling field dependence, a continuous transition from a negative to a positive exchange-bias field is observed with increasing temperature. The changes of magnetic configuration responsible for this effect are studied, combining both magnetization and polarized neutron reflectivity measurements. The temperature is found to enhance the exchange-bias training effect as a result of the relaxation of an interface domain wall. The present study demonstrates that both temperature and cooling field may be used to tune the exchange field. For the last 15 years, the exchange-bias effect in antiferromagnetic/ferromagnetic ͑AF/FM͒ bilayers have been widely studied due to its technological applications and the specific physical mechanisms involved. 1 When an exchange-biased system is cooled below a certain temperature known as the blocking temperature ͑T B ͒, the hysteresis loop of the FM layer is shifted toward a field H E , named the exchange-bias field. Bilayers showing an antiferromagnetic interfacial exchange coupling are particularly interesting since H E shows a continuous transition from negative to positive with increasing cooling field ͑H cf ͒. 2,3 Recently, a similar transition has also been observed as a function of temperature in an AF/FM system FeZnF 2 /Co ͑Ref. 4͒ and in an AF/ferrimagnetic bilayer NiCoO / GdFe. 5 In both cases, the given explanation involves a reorientation of the magnetic configuration inside the AF layer as the temperature changes. However, the type of reorientation mechanism proposed is quite different in the two cases. For the first one, it was assumed that an unstable domain structure is formed in the AF, which then relaxes. For NiCoO / GdFe, a memory effect is proposed, 6 which takes into account the evolution of an in-plane domain wall localized in the AF. The main difficulty in validating either one of these theories is that the interface magnetic configurations in AF layers are very difficult to probe mainly because the antiferromagnetic layer has almost no net magnetization. To go beyond this problem, we have studied a ferrimagnetic/ferrimagnetic, hard/soft bi-layer, namely, TbFe/ GdFe, which mimics antiferromagneti-cally coupled AF/FM systems. Such a system is a model as its magnetic configuration is easy to probe with conventional means. Recently, we were able to prove that during field cooling, a partial interfacial domain wall is frozen in the TbFe layer. The resulting exchange-bias field is then determined only by the orientation of the quenched TbFe magne-tization at the interface, which rotates continuously from an-tiparallel to the cooling field direction to parallel as H cf increases. 7,8 We have used this system to study the exchange-bias training effect. This effect is characterized by the evolution of the hysteresis loop with the number of field cycles. 9,10 Using polarized neutron reflectivity ͑PNR͒, we demonstrated that these changes in H E are caused by irreversible modifications of the magnetic configuration in TbFe.

The authors quantify the impact of the pinning potential on the current driven domain wall depinning process in wires with spin valves having perpendicular magnetic anisotropy. By artificially controlling the strength of the pinning field Hp, the threshold current Jth is found to decrease when reducing Hp. In addition, the efficiency of the depinning process is observed to correlate with the polarization and the damping parameter of the magnetic layer. This suggests that the engineering of pinning sites together with well suited materials provides a pathway for efficient current-induced domain wall manipulation in films with perpendicular anisotropy.