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International audience; The study of the new forms of magnetic states in magnetic cylinders has recently attracted much attention [1]. However, multi-layered structures in cylindrical geometry, which are basic blocks for spin torque nano-oscillators for example, have been poorly studied up to now. Among others, one of the main challenge for such studies is the experimental observation of magnetic configurations below 100 nm. In this work, we used off-axis electron holography to observe the remnant magnetic states of Co/Cu multi-layered nano-cylinders with a spatial resolution down to 2 nm. Nanocylinders are grown by electrodeposition in commercial polycarbonate membranes. Their diameters range between 70 and 90 nm. In order to study the effect of the aspect ratio and dipolar coupling on the magnetic states, four series of samples with different nominal Co and Cu thicknesses have been realized, namely Co25 nm/Cu15 nm, Co25 nm/Cu45 nm , Co50 nm/Cu50 nm and Co100 nm/Cu100 nm. The magnetic configurations are recovered from direct qualitative and quantitative comparison between the experimental magnetic phase shift image obtained by holography and the one calculated from micromagnetic simulations (OOMMF). These ones are performed including the geometrical properties of each individual layers deduced from EFTEM experiments. With our method, we have observed in the different wires either uniform magnetic states with antiparallel coupling or vortex states with different orientations of the core with respect to the wire axis [2,3]. In addition, different magnetic states can coexist within the same nanowire. Such a wide variety of magnetic configuration arises from the low values of the Co magnetic constants with respect to bulk, typical of electrodeposition process in single bath. To explain our results, we have calculated the phase diagram of the remnant states in a single disc for different orientations of the crystal anisotropy and saturation field. We show that the occurrence of each configuration depends on the aspect ratio of the layers, the direction of magneto crystalline anisotropy, and, in some cases, the interlayer dipolar coupling. In particular case, we will show that information on the preferential orientation of the crystalline anisotropy can be recovered from the statistics of occurrence of each magnetic configurations in the multilayer. Finally, we discuss the interest of 3D holographic vector field electron tomography for nanomagnetism studies. Preliminary results performed on the Co25 nm/Cu15 nm cylinders demonstrates that it allows getting the exact shape of each layer and the 3D components of the magnetic induction in each layer. This avoids geometrical uncertainties in micromagnetic simulations and solves the problem of 2D projection in off-axis electron holography experiments, leading to a much better determination of the magnetic constant of the layers.

Monitoring a magnetic state using thermal or electrical activation is mandatory for the development of new magnetic devices, for instance in heat or electrically assisted magnetic recording or room-temperature memory resistor. Compounds such as FeRh, which undergoes a magnetic transition from an antiferromagnetic state to a ferromagnetic state around 100 °C, are thus highly desirable. However, the mechanisms involved in the transition are still under debate. Here we use in situ heating and cooling electron holography to quantitatively map at the nanometre scale the magnetization of a cross-sectional FeRh thin film through the antiferromagnetic–ferromagnetic transition. Our results provide a direct observation of an inhomogeneous spatial distribution of the transition temperature along the growth direction. Most interestingly, a regular spacing of the ferromagnetic domains nucleated upon monitoring of the transition is also observed. Beyond these findings on the fundamental transition mechanisms, our work also brings insights for in operando analysis of magnetic devices., Films of iron-rhodium alloy undergo a magnetic transition at 100°C, and so are attractive for applications, but a detailed understanding of the transition mechanism has not been achieved. Here, the authors use electron holography to quantitatively map the transition's progress through the film depth.

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.

The transition from a film to a full magnonic crystal is studied by sequentially ion-milling a periodic stripe pattern into a 40 nm thick Ni$_{80}$Fe$_{20}$ film. The spin-wave resonances of each milling stage are detected by ferromagnetic resonance for both in-plain main field axes, i.e. parallel and perpendicular to the stripe pattern. Theoretical calculations and micromagnetic simulations yield the individual mode profiles, which are analyzed in order to track changes of the mode character. The latter is strongly linked to the evolution of the internal demagnetizing field. It’s role is further studied and imaged by electron holography measurements on a hybrid magnonic crystal, which is made with a 10 nm deep surface modulation. The complex effects of mode coupling, mode localization, and anisotropy-like contributions by the internal field are unraveled. Simple transition rules from the $𝑛^_\mathrm{th}$ film mode to the $𝑚\mathrm{th}$ mode of the full magnonic crystal are formulated. This work has been supported by DFG grant KL2443/5-1.

The transition from a film to a full magnonic crystal is studied by sequentially ion-milling a 40 nm Ni80Fe20 film. The spin-wave resonances of each stage are detected by ferromagnetic resonance for both in-plain field main axes. Theoretical calculations and micromagnetic simulations yield the individual mode profiles, which are analyzed in order to track changes of the mode character. The latter is strongly linked to the evolution of the internal demagnetizing field. It’s role is further studied by electron holography measurements of a hybrid magnonic crystal with 10 nm deep surface modulation. The complex effects of mode coupling, mode localization and anisotropy-like contributions by the internal field are unraveled. Simple transition rules from the 𝑛th film mode to the 𝑚th mode of the full magnonic crystal are formulated.

Magnonic crystals with locally alternating properties and specific periodicities exhibit interesting effects, such as a multitude of different spin-wave states with adjustable band gaps. This work aims for demonstrating and understanding the key role, which local demagnetizing fields play in such systems. In order to get direct access to this matter, the spin-dynamics of a magnonic crystal were reconstructed from ‘bottom-up’, i.e. the structural shape as well as the internal field landscape of the structure were experimentally reconstructed on the nanoscale using electron holography. Subsequently, both properties were utilized to perform dynamic response calculations. The simulations yield the frequency-field dependence as well as the angular dependence of spin waves in a magnonic crystal and reveal the governing role of the internal field landscape around the backward-volume geometry. Connecting the internal field landscape with the individual spin-wave mode localization, a comprehensive model is presented describing the complex angle-dependent spin-wave behavior around 360◦ rotation of an in-plane external field. A significant spin-wave propagation is reported for a broad angular range around the Damon-Eshbach geometry.