Electron Microscopy Investigation of Magnetization Process in Thin Foils and Nanostructures

International audienceThis paper presents an investigation of magnetization configuration evolution during insitu magnetic processes, in materials exhibiting planar and perpendicular magnetic anisotropy. Transmission electron microscopy (TEM) has been used to perform magnetic imaging. Fresnel contrast in Lorentz Transmission Electron Microscopy (LTEM), phase retrieval methods such as Transport of Intensity Equation (TIE) solving and electron holography have all been implemented. These techniques are sensitive to magnetic induction perpendicular to the electron beam, allowing the mapping of magnetic induction distribution with a spatial resolution better than 10nm and can be extended to allow dynamical studies during in-situ observation. Thin foils of FePd alloys with a strong perpendicular magnetic anisotropy (PMA) and self-assembled Fe dots have been examined. Both are studied during magnetization processes, exhibiting the capacities of in-situ magnetic imaging in a TEM

The use of Lorentz microscopy for the determination of magnetic reversal mechanism of exchange-biased Co30Fe70/NiMn bilayer

Lorentz transmission electron microscopy (LTEM) combined with in-situ magnetizing experiments is a powerful tool for the investigation of the magnetization of the reversal process at the micron scale. We have implemented this tool on a conventional transmission electron microscope (TEM) to study the exchange anisotropy of a polycrystalline Co35Fe65/NiMn bilayer. Semi-quantitative maps of the magnetic induction were obtained at different field values by the differential phase contrast (DPC) technique adapted for a TEM (SIDPC). The hysteresis loop of the bilayer has been calculated from the relative intensity of magnetic maps. The curve shows the appearance of an exchange-bias field reveals with two distinct reversal modes of the magnetization: the first path corresponds to a reversal by wall propagation when the applied field is parallel to the anisotropy direction whereas the second is a reversal by coherent rotation of magnetic moments when the field is applied antiparallel to unidirectional anisotropy direction

Probing domain walls in cylindrical magnetic nanowires with electron holography

3 pages, 2 figuresInternational audienceWe probe magnetic domain walls in cylindrical soft magnetic nanowires using electron holography. We detail the modelling of expected contrast for both transverse and Bloch point domain walls and provide comparison with experimental observations performed on NiCo nanowires, involving also both magnetic and electrostatic contribution to the electron holography map. This allows the fast determination of the domain wall type without the need for uneasy and time-consuming experimental removal of the electrostatic contribution. Finally, we describe and implement a new efficient algorithm for calculating the magnetic contrast

Exchange bias in Co/CoO core-shell nanowires: Role of the antiferromagnetic superparamagnetic fluctuations

The magnetic properties of Co (=15 nm, =130nm) nanowires are reported. In oxidized wires, we measure large exchange bias fields of the order of 0.1 T below T ~ 100 K. The onset of the exchange bias, between the ferromagnetic core and the anti-ferromagnetic CoO shell, is accompanied by a coercivity drop of 0.2 T which leads to a minimum in coercivity at $\sim100$ K. Magnetization relaxation measurements show a temperature dependence of the magnetic viscosity S which is consistent with a volume distribution of the CoO grains at the surface. We propose that the superparamagnetic fluctuations of the anti-ferromagnetic CoO shell play a key role in the flipping of the nanowire magnetization and explain the coercivity drop. This is supported by micromagnetic simulations. This behavior is specific to the geometry of a 1D system which possesses a large shape anisotropy and was not previously observed in 0D (spheres) or 2D (thin films) systems which have a high degree of symmetry and low coercivities. This study underlines the importance of the AFM super-paramagnetic fluctuations in the exchange bias mechanism.Comment: 10 pages, 10 figures, submitted to Phys. Rev.

3D magnetic induction maps of nanoscale materials revealed by electron holographic tomography

This is an open access article published under a Creative Commons Attribution (CC-BY) License.-- et al.The investigation of three-dimensional (3D) ferromagnetic nanoscale materials constitutes one of the key research areas of the current magnetism roadmap and carries great potential to impact areas such as data storage, sensing, and biomagnetism. The properties of such nanostructures are closely connected with their 3D magnetic nanostructure, making their determination highly valuable. Up to now, quantitative 3D maps providing both the internal magnetic and electric configuration of the same specimen with high spatial resolution are missing. Here, we demonstrate the quantitative 3D reconstruction of the dominant axial component of the magnetic induction and electrostatic potential within a cobalt nanowire (NW) of 100 nm in diameter with spatial resolution below 10 nm by applying electron holographic tomography. The tomogram was obtained using a dedicated TEM sample holder for acquisition, in combination with advanced alignment and tomographic reconstruction routines. The powerful approach presented here is widely applicable to a broad range of 3D magnetic nanostructures and may trigger the progress of novel spintronic nonplanar nanodevices.This work was supported by the European Union under the Seventh Framework Program under a contract for an Integrated Infrastructure Initiative Reference 312483-ESTEEM2. S.B. and A.B. gratefully acknowledge funding by ERC Starting grants number 335078 COLOURATOMS and number 278510 VORTEX. A.F.-P. acknowledges an EPSRC Early Career fellowship and support from the Winton Foundation. E.S., C.G., and L.A.R. acknowledge the French ANR program for support though the project EMMA. J.M.D.T. and C. M. acknowledge the Spanish MINECO projects MAT2014-51982- C2-1-R and MAT2014-51982-C2-2-R, respectively.Peer Reviewe

Synthesis and electrical characterization of monocrystalline nickel nanorods and Ni-CNT composites

Aerospace vessels require electrically conductive, light weight frames to minimize damage from electromagnetic radiation, electrostatic discharge and lightning strikes while economizing fuel. Nickel nanowires and hybrid nickel-carbon nanotube materials are suitable nanostructures to ensure high conductivity at low mass loading. Monocrystalline nickel structures have even better conduction properties than the polycrystalline equivalent due to possessing less particle-particle junctions. We have developed a solutionbased method that produces monocrystalline nickel nanowires via the decomposition of metalorganic precursors in the presence of self-assembled surfactants. The resulting wires are approximately 20 nm wide by 1.5 µm in length. These wires have a morphology consisting of semi-flattened rods with pyramidal ends. Despite the changing dimensions between the nanorod body and its head, there was no disruption in the crystallographic orientation, as observed with HRTEM and diffraction patterns. The nickel nanostructures were exposed to air for several weeks, but no oxidation was detectable by magnetic measurement, i.e. the saturation magnetization corresponds to Ni0 and no bias is observed in the hysteresis loops. It seems that the long alkyl chain amine surfactant, in addition to being a structuration agent, remains at the surface of the Ni wires after washing and acts as a protective layer. The magnetic field around Ni nanowires was imaged using electron holography. Each Ni wire is a magnetic monodomain. Routes to prepare hybrid nickel-CNT materials were explored using chemical vapor deposition in a fluidized bed, solution chemistry and dry preparation in a Fisher-Porter reactor. Different nickel compositions and material morphologies resulted, depending on the preparation technique. The nickel nanorods and hybrid materials were incorporated into carbon fiber-reinforced polymer composites. The electrical conductivity as a function of wt% loading was measured, showing promise for these materials in discharging electrostatic charges

Customized MFM probes based on magnetic nanorods

International audienc

Magnetic field strength and orientation effects on co-fe discontinuous multilayers close to percolation

International audienceMagnetization and magnetoresistance in function of the magnitude and orientation of applied magnetic field were studied in Co-Fe discontinuous multilayers close to their structural percolation. The high pulsed magnetic fields up to 33 T were used in the 120–310 K temperature range. Comparison between longitudinal and transverse with respect to the film plane field configurations was made in the low-field and high-field regimes in order to clarify the nature of the measured negative magnetoresistance. Coexistence of two distinct magnetic fractions, superparamagnetic SPM, consisting of small spherical Co-Fe granules and superferromagnetic SFM, by bigger Co-Fe clusters, was established in this system. These fractions were shown to have different relevance for the system magnetization and magnetotransport. While the magnetization is almost completely up to 97% defined by the SFM contribution and practically independent of temperature in this range, the magnetoresistance experiences a crossover from a regime dominated by Langevin correlations suppressed with temperature between neighbor SPM and SFM moments at low fields, to that dominated by spin scattering enhanced with temperature of charge carriers within SFM clusters at high fields. Also, the demagnetizing effects, sensitive to the field orientation, were found to essentially define the low-field behavior and characteristic crossover field

Elaboration et étude d'un système hybride "Oxyde ferrimagnétique / Métal non magnétique / Oxyde ferrimagnétique"

Jury: Martine GAUTIER-SOYER (Présidente), Claude HENRY (Rapporteur), Josep FONTCUBERTA (Rapporteur), André FERT (Examinateur), Pascale BAYLE-GUILLEMAUD (Examinateur), Jean-Claude OUSSET (Invité) et Etienne SNOECK (Directeur de thèse)This work is a contribution to the active research on new magnetic and electric properties of artificial heterostructures. “Ferrimagnetic oxide / Non magnetic metal / Ferrimagnetic oxide” systems have been deposited. In these systems, electrons are confined in the 2D metal layer and many electron reflections occur at the “metal / magnetic insulator”. Two different magnetisation alignments of the magnetic oxides (parallel and antiparallel) are required to obtain suitable magnetic properties, i.e. giant magnetoresistance (GMR). The growth control and the flatness of the different layers are needed to deposit epitaxial layers, in which electron scattering at grain boundaries is limited. Films are deposited by sputtering in a UHV chamber, structural studies have been made using mainly transmission electron microscopy and X-ray diffraction.An epitaxial growth of single Fe3O4 and CoFe2O4 films has been obtained on Al2O3(0001) with a [111] growth axis and on MgO(001) with a [001] growth axis. We have also studied magnetic properties (exchange anisotropy) of epitaxial Fe3O4(x nm)/NiO(66 nm) bilayers , x varying from 5nm to 50nm, grown along a [111] and a [001] axis. Non oxydable metal growth (Pt, Au and Ag) on (001) and (111) Fe3O4 crystals has also been investigated.We have achieved epitaxial growth of Fe3O4/M(M=Au,Pt)/CoFe2O4 layers on Al2O3(0001) with flat interfaces and suitable magnetic properties. Electric investigations have shown that the electrons are confined in the metallic layer. We have measured a 1.8% GMR ratio at 10K resulting from electron reflections at the metal/oxide interfaces with a part of specular reflexions.Ce travail s'inscrit dans les recherches actives des nouvelles propriétés magnétiques et électriques dans les hétérostructures artificielles. Nous avons élaboré et étudié un système du type « Oxyde ferrimagnétique / Métal non magnétique / Oxyde ferrimagnétique » dans lequel les électrons sont confinés dans la couche métallique 2D et subissent de nombreuses réflexions aux interfaces « métal / isolant magnétique ». L'élaboration de ce système impose une maîtrise de la croissance des différentes couches et de la planéité des interfaces. Les dépôts sont épitaxiés afin de limiter les diffusions des électrons aux joints de grains par pulvérisation cathodique dans un bâti UHV, les caractérisations structurales sont essentiellement réalisées par microscopie électronique haute résolution et diffraction des rayons X.Nous avons étudié la croissance épitaxiale de couches simples de Fe3O4 et de CoFe2O4 sur Al2O3(0001) et MgO(001) afin d'obtenir respectivement une direction de croissance [111] et [001]. Nous nous sommes également intéressés à la croissance épitaxiale et à l'anisotropie d'échange de bicouches Fe3O4(5nm à 50nm)/NiO(66nm) dans ces deux mêmes directions de croissance. Nous avons ensuite étudié la croissance de métaux non oxydables (Pt, Au et Ag) sur les surfaces (001) et (111) de Fe3O4.Ces résultats ont permis d'élaborer les systèmes épitaxiés Fe3O4/M(M=Au,Pt)/ CoFe2O4 sur Al2O3(0001). Les propriétés électriques montrent que les électrons sont confinés dans la couche métallique et qu'apparaît une GMR atteignant près de 1,8% à 10K due exclusivement aux réflexions électroniques sur les interfaces métal/oxyde avec certainement une contribution des réflexions spéculaires