Periodic chiral magnetic domains in single-crystal nickel nanowires

We report on experimental and computational investigations of the domain structure of ~0.2 x 0.2 x 8 {\mu}m single-crystal Ni nanowires (NWs). The Ni NWs were grown by a thermal chemical vapor deposition technique that results in highly-oriented single-crystal structures on amorphous SiOx coated Si substrates. Magnetoresistance measurements of the Ni NWs suggest the average magnetization points largely off the NW long axis at zero field. X-ray photoemission electron microscopy images show a well-defined periodic magnetization pattern along the surface of the nanowires with a period of {\lambda} = 250 nm. Finite element micromagnetic simulations reveal that an oscillatory magnetization configuration with a period closely matching experimental observation ({\lambda} = 240 nm) is obtainable at remanence. This magnetization configuration involves a periodic array of alternating chirality vortex domains distributed along the length of the NW. Vortex formation is attributable to the cubic anisotropy of the single crystal Ni NW system and its reduced structural dimensions. The periodic alternating chirality vortex state is a topologically protected metastable state, analogous to an array of 360{\deg} domain walls in a thin strip. Simulations show that other remanent states are also possible, depending on the field history. Effects of material properties and strain on the vortex pattern are investigated. It is shown that at reduced cubic anisotropy vortices are no longer stable, while negative uniaxial anisotropy and magnetoelastic effects in the presence of compressive biaxial strain contribute to vortex formation.Comment: 15 pages, 11 figure

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

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

Microscale metasurfaces for on-chip magnetic flux concentration

Magnetic metamaterials have demonstrated promising perspectives to improve the efficiency of magnetic flux concentrators. In this work, we investigate the effects of downscaling these devices for on-chip integration. We scrutinize the influence of the non-linear magnetic response of the ferromagnetic components, their magnetic irreversibility, the formation of magnetic domains, as well as the effects of geometry and size of the devices. Our results demonstrate that the implementation of metasurfaces at the microscale opens up new technological possibilities for enhancing the performance of magnetic field detectors and remotely charging small electric devices, thus paving the way towards new approaches in information and communication technologies

Current-induced domain wall motion and magnetization dynamics in CoFeB/Cu/Co nanostripes

Article in a special number of Journal of Physics : Condensed Matter on 'domain wall dynamics in nanostructures'. See http://iopscience.iop.org/0953-8984/24/2International audienceCurrent-induced domain wall motion and magnetization dynamics in the CoFeB layer of CoFeB/Cu/Co nanostripes were studied using photoemission electron microscopy combined with x-ray magnetic circular dichroism (XMCD-PEEM). Quasi-static measurements show that current-induced domain wall motion in the CoFeB layer is similar to the one observed in the NiFe layer of NiFe/Cu/Co trilayers, although the threshold current densities for domain wall depinning are lower. Time-resolved XMCD-PEEM measurements are used as an efficient probe of domain wall depinning statistics. They also reveal that, during the application of current pulses, the CoFeB magnetization rotates in the direction transverse to the nanostripe. The corresponding tilt angles have been quantified and compared to analytical and micromagnetic calculations, highlighting the influence of magnetostatic interactions between the two magnetic layers on the magnetization rotation

X-ray analysis of the magnetic influence of oxygen in Pt/Co/AlOx trilayers

International audienceX-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

Dynamique de l'aimantation étudiée par rayonnement synchrotron

National audienceLa course à la miniaturisation pour augmenter la densité de stockage de données en utilisant des nanostructures magnétiques, nécessite de maîtriser les processus de retournement de l'aimantation dans les matériaux. Grâce aux propriétés du rayonnement synchrotron, nous pouvons effectuer des images des domaines magnétiques dans des fils de 100 nm d'épaisseur, avec sensibilité à l'élément chimique et une résolution temporelle de quelques dizaines de picosecondes. Les processus dynamiques de l'aimantation ont été étudiés pendant l'injection d'un courant suffisant pour déplacer les parois des domaines magnétiques par transfert de spin

Current-induced motion and pinning of domain walls in spin-valve nanowires studied by XMCD-PEEM

International audienceVery large average velocities, up to 600 m/s, have been found for domain-wall motion driven by 3-ns-long pulses of electric current in zero magnetic field in the NiFe layer of 200-nm-wide NiFe/Cu/Co nanowires. For longer pulses, the domain-wall motion is strongly hindered by pinning potentials. Dipolar interactions between the NiFe and Co layers caused by anisotropy inhomogeneities have been identified as the most important among the different potential sources of DW pinning. The domain-wall velocities increase with current density, but a substantial drop is observed at current densities above 4×10^11 A/m

Subpicosecond metamagnetic phase transition driven by non-equilibrium electron dynamics

Femtosecond light-induced phase transitions between different macroscopic orders provide the possibility to tune the functional properties of condensed matter on ultrafast timescales. In first-order phase transitions, transient non-equilibrium phases and inherent phase coexistence often preclude a non-ambiguous detection of the transition precursors and their temporal onset.Here, we present a study combining time-resolved photoelectron spectroscopy and ab-initio electron dynamics calculations elucidating the transient subpicosecond processes governing the photoinduced generation of ferromagnetic order in antiferromagnetic {F}e{R}h. The transient photoemission spectra are accounted for by assuming that not only the occupation of electronic states is modified during the photoexcitation process.Instead, the photo-generated non-thermal distribution of electrons modifies the electronic band structure. The ferromagnetic phase of FeRh, characterized by a minority band near the Fermi energy, is established $350 \pm 30$ fs after the laser excitation. Ab-initio calculations indicate that the phase transition is initiated by a photoinduced charge transfer from Rh to Fe