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International audience; Magnetic skyrmions and bubbles, observed in ferromagnetic thin films with perpendicular magnetic anisotropy, are topological solitons which differ by their characteristic size and the balance in the energies at the origin of their stabilisation. However, these two spin textures have the same topology and a continuous transformation between them is allowed. In the present work, we derive an analytical model to explore the skyrmion-bubble transition. We evidence a region in the parameter space where both topological soliton solutions coexist and close to which transformations between skyrmion and bubbles are observed as a function of the magnetic field. Above a critical point, at which the energy barrier separating both solutions vanishes, only one topological soliton solution remains, which size can be continuously tuned from micrometer to nanometer with applied magnetic field.

In this article, we show how the microwave properties of ferromagnetic nanowires electrodeposited into a porous polymer template depend on various parameters such as the geometrical parameters of the nanowires array and elaboration conditions. Our nanowires behave as strong uniaxial nanomagnets and exhibit zero-field resonance frequencies tunable up to 36 GHz. Novel microwave devices such as circulators and filters, using a porous polymer template loaded with metallic nanowires, are discussed. An accurate knowledge of magnetic and dielectric properties of the loaded polymer template is mandatory for the design of such devices. (C) 2008 Elsevier B.V. All rights reserved.

Arrays of electrodeposited Co0.96Cu0.04/Cu nanowires into porous polycarbonate membranes have been characterized by ferromagnetic resonance measurements in order to study the effect of the dipolar interactions on the effective anisotropy field as a function of the magnetic and non magnetic layer thickness. It is found that breaking the infinite cylinder geometry reduces their shape anisotropy field, which can be modified to a large extent from 6.5 kOe for non layered nanowires down to nearly zero for layered nanowires with the thinnest magnetic layers. An analytical model is proposed to describe the magnetostatic interactions between magnetic layers and their effect on the total anisotropy field which shows a good agreement with the experiment.

The effects of the solution pH and deposition potential on the structural and magnetic properties in arrays of electrodeposited CoCu nanowires with low Cu content have been studied combining ferromagnetic resonance, magnetometry and electron transmission microscopy. It is shown that, depending on the synthesis parameters, the average crystallographic structure can be controlled, giving rise to sensible changes in the effective crystal anisotropy field which varies from -4.05 to +2.75 kOe. It is also shown that in CoCu/Cu multilayered nanowires, the preferential structure of the CoCu-magnetic layer can also be controlled by both pH and deposition potential, opening an interesting route for designing multilayered CoCu/Cu nanowires with controlled and alternated crystal anisotropy.

A planar microwave circulator operating without biasing static magnetic field is presented, using bistable ferromagnetic nanowires embedded in a dielectric medium. We show that its circulation mechanism is strongly dependent on the magnetization state at zero external magnetic field. A theoretical model, yielding the permeability components of the magnetic nanowired composite at remanent or partially magnetized state and the transmission characteristics of the circulator, is proposed. It explains the essential role played by the magnetization state on the circulation performances. Composites with very high remanence are necessary for an optimal circulation effect. It is also shown that the insertion losses can be significantly reduced by improving the matching between the access lines and the circulator.

A very compact planar fully integrated circulator operating at millimeter wavelength has been designed using a magnetic substrate combining a polymer membrane with an array of ferromagnetic nanowires. The original feature of this substrate, called magnetic nanowired substrate (MNWS), relies on the fact that the circulation effect is obtained without requiring any biasing dc magnetic field. This leads to a significant reduction of device dimensions since no magnetic field source is needed, and a realistic ability for integration with monolithic microwave integrated circuits. The circulator design is performed by an efficient analytical model including a self design of the impedance matching network. This model also allows a physical understanding of the circulation mechanism through the access to the electromagnetic field patterns inside the circulator substrate. Based on the excellent agreement between the theoretical and experimental results, the model is used to predict the improvement of circulator performances resulting from a reduction of dielectric and conductor losses. Insertion losses lower than 2 dB with an isolation higher than 45 dB are expected for MNWS circulators with a low-loss substrate and thick metallic layers.

This article is focused on the use of electrodeposition and of various nanoporous templates for the fabrication of metallic nanowires made from single metals (Ni, Co, Pb, Sn), alloys (NiFe, CoFe, CoPt), and multilayers (Co/Cu, NiFe/Cu). An overview is given of our recent studies performed on both magnetic and superconducting nanowires. Using different approaches entailing measurements on both single wires and arrays, numerous interesting physical properties have been identified in relation to the nanoscopic dimensions of these materials. Finally, various novel applications of the nanowires are also discussed.

The magnetic properties of arrays of electrodeposited Co nanowires have been studied by ferromagnetic resonance as a function of the electrolytic bath acidity and the plating current intensity. It is observed that by adjusting the pH or the plating current it is possible to find appropriate electrolyte-deposition conditions which lead to the deposition of Co nanowires with a c-axis orientation either parallel or perpendicular to the wires. At relatively high plating currents, systems containing a dominant fraction of grains having the c-axis oriented perpendicular to the wires are favoured. Such wires show a significant decrease of the effective anisotropy due to the competition between the magnetocrystalline and the shape anisotropy that can be as low as 4.3 kOe. In contrast, at low plating currents the c-axis is aligned parallel to the wires and an effective anisotropy field, that can be as high as 12 kOe, is observed.

The effects of the electrolytic bath acidity, or pH, on the magnetic properties in arrays of electrodeposited Co nanowires and their correlation with the crystalline properties have been studied using ferromagnetic resonance. The results show that, depending on the value of the pH of the electrolyte, appreciable changes in the effective anisotropy can be induced. These changes are attributed to modifications in the microstructure of the Co nanowires. In particular, quantification of the effective anisotropy field shows that the microstructure of the deposited Co wires can be set to contain a dominant fraction of the Co-hcp phase with the c-axis oriented perpendicular to the wires, for pH values of 3.8-4.0, or parallel to the wires, for pH values >or=6.0. This results in a competitive or additive magnetocrystalline contribution to the total anisotropy field. Furthermore, at a pH value of 2.0, no contribution from the magnetocrystalline anisotropy is present, indicating a lack of texture in the Co microstructure. As a result, the effective anisotropy can be controlled over a field range of 5 kOe.

International audience; Topological protection is an elegant way of warranting the integrity of quantum and nanosized systems. In magnetism one example is the Bloch-point, a peculiar object implying the local vanishing of magnetization within a ferromagnet. Its existence had been postulated and described theoretically since several decades, however it has never been observed. We con rm experimentally the existence of Bloch points, imaged within domain walls in cylindrical magnetic nanowires, combining surface and transmission XMCD-PEEM magnetic microscopy. This opens the way to the experimental search for peculiar phenomena predicted during the motion of Bloch-point-based domain walls.

This letter presents the influence of ferromagnetic nanowires on permittivity and losses on a so-filled membrane. For this purpose a theoretical permittivity model has been developed by considering geometrical parameters of the substrate such as the porosity of the membrane and the height of the nanowires. It has been validated by microstrip line measurements on a composite substrate filled with cobalt nanowires. The ferromagnetic nanowires enhance the permittivity, but give no further contribution to the losses beyond the ferromagnetic resonance. This is confirmed by measurements on a similar topology including non ferromagnetic copper nanowires.

Arrays of cobalt nanowires of diameter 23-70 nm were grown by electrodeposition. Their magnetic properties are probed using ferromagnetic resonance. Depending on the solution pH and deposition rate, the orientation of the dense planes of the hexagonal structure can be controlled so that its c-axis is oriented either parallel or perpendicular to the wires axis. In particular, the effects of the pH and deposition current on the structural and magnetic properties are considered and it is shown that the wire diameter plays an important role in the final preferred structure by inducing local changes of the pH within the pores.

We report on microwave oscillations induced by spin-transfer torque in metallic spin valves obtained by electrodeposition of Co-Cu-Co trilayer structures in nanoporous alumina templates. Using micromagnetic calculations performed on similar spin-valve structures it was possible to identify the magnetization dynamics associated with the experimentally determined microwave emission. Furthermore it appears that in our particular geometry the microwave emission is generated by the vortex gyrotropic motion which occurs in, at least, one of the two magnetic layers of our spin-valve structures. Microwave emission was obtained in the absence of any external magnetic field with the appropriate magnetization configuration. © 2012 American Physical Society.

International audience; Ordered arrays of magnetic nanowires are commonly synthesized by electrodeposition in nanoporous alumina templates. Due to their dense packing, strong magnetostatic interactions prevent the manipulation of wires individually. Using atomic layer deposition we reduce the diameter of the pores prior to electrodeposition. This reduces magnetostatic interactions, yielding fully remanent hysteresis loops. This is a first step towards the use of such arrays for magnetic racetrack memories.

International audience; Unbiased planar microwave circulators were fabricated by electrodeposition of NiFe nanowires into porous alumina templates. Microwave properties of the devices are seen to depend drastically on the height of the nanowires and the newly developed devices exhibit improved features, compared to existing nanowire-based designs. Thanks to the high anisotropy of the nanowires, zero-field circulation modes may be observed in a frequency range from 10 to 30 GHz, with isolation as large as 30 dB, as well as low insertion losses - 5 dB, making it compatible with industrial needs for device applications.

We present magnetotransport measurements as well as micromagnetic simulations of magnetization reversal in magnetic nanowires having strong transverse magnetocrystalline anisotropy. The interplay between exchange, demagnetizing, and magnetocrystalline energies gives rise to a micromagnetic configuration involving vortices with alternative chirality along the wire. Despite the complexity of the field angle-dependent magnetization reversal process, a very good agreement is obtained between experiments and simulations. This provides evidence that the reported magnetization process appears generally in nanowires with strong transverse magnetocrystalline anisotropy. Moreover an analytical expression is established for the angular dependency of the core switching field. The simulations indicate the occurrence of Bloch points during the core reversal.

The phase locking behavior of spin transfer nano-oscillators (STNOs) to an external microwave signal is experimentally studied as a function of the STNO intrinsic parameters. We extract the coupling strength from our data using the derived phase dynamics of a forced STNO. The predicted trends on the coupling strength for phase locking as a function of intrinsic features of the oscillators, i.e., power, linewidth, agility in current, are central to optimize the emitted power in arrays of mutually coupled STNOs.

Arrays of electrodeposited CoCu/Cu multilayered nanowires have been characterized by ferromagnetic resonance and magnetometry measurements in order to study the effect of the dipolar interactions on the effective anisotropy field as a function of the magnetic and nonmagnetic layer thicknesses. Breaking the continuous cylinder geometry results in a reduction of the effective anisotropy field, which can be modified over a large range of values starting from 7 kOe in the case of nonlayered continuous nanowires down to nearly zero for the thinnest magnetic layers. An analytical model is presented to describe the magnetostatic interactions between magnetic layers and their effect on the total anisotropy field which shows a very good agreement with the experiments. Moreover, the model allows generalizing the description of the effective magnetostatic field as a function of the aspect ratio of both the magnetic and the nonmagnetic layers for multilayered nanowires of any combination of materials. A general anisotropy diagram is presented that describes the geometrical conditions required to obtain an easy axis parallel or perpendicular to the wire axis, thus providing a guide for the engineering and fine-tuning of the magnetic properties of these systems.

Arrays of nickel nanowires embedded into polycarbonate membranes were studied by means of ferromagnetic resonance. Large magnetoelastic effects were evidenced in the entire temperature range 4.3-300 K, which were attributed to the thermal expansion coefficients mismatch between the Ni nanowires and the polycarbonate membrane. The resulting magnetoelastic energy was measured as a function of both membrane porosity and diameter of the nanowires. For low volume fractions of Ni in Ni/polycarbonate samples, the magnitude of the magnetoelastic anisotropy may be as high as the shape anisotropy. Conversely, no magnetoelastic effects were observed in permalloy nanowires embedded in the same membranes.

Using appropriate electrodeposition conditions, it is shown that the structural and magnetic properties of arrays of Co and CoCu/Co nanowires can be controlled. The hcp c axis orientation can be oriented parallel or perpendicular to the wire axis simply by changing the pH of the electrolytic solution and/or deposition rate. This selected orientation of the c axis leads to a drastic change in overall magnetic anisotropy and is promising for the fabrication of spin valves structures by electrodeposition.

A convenient way of controlling the magnetic anisotropy of electrodeposited cobalt nanowires was studied. The nanowires were grown by electrodeposition into the cylindrical pores of a polycarbonate membrane. Using ferromagnetic resonance and transmission electron microscopy, it was demonstrated that changes in the two main electrodeposition parameters (electrolyte pH and deposition rate/current density) leads to modifications of the crystallographic structure of the nanowires that can be grown in cubic (FCC) or hexagonal (HCP) phases.

The static magnetization experimental behavior of cobalt (Co), Permalloy (Py), and nickel (Ni) nanocylinders is obtained from vibrating sample magnetometry while the dynamic behavior for the Co and Py ones is analyzed by means of Brillouin light scattering spectroscopy. Assuming the presence at remanence of two populations of cylinders with up and down magnetizations and including the dipolar coupling between the cylinders, a single analytical model based on a mean-field approach allowed us to satisfactorily analyze both series of experimental results. The model requires three physical parameters, allowing us to derive the in-plane saturation field, the eigenfrequency in the absence of applied field, and the eigenfrequency at the in-plane saturation field; these parameters enable us to adjust the whole variation of the eigenfrequency versus the applied field. Moreover, the effect of the magnetocrystalline anisotropy on the softening of the frequency in the nonsaturated state is clearly evidenced: it is more pronounced when the magnetocrystalline anisotropy is not vanishing and adds to the shape anisotropy (Co c-axis parallel to the cylinder axis); the softening being weak in the other cases (Co c-axis perpendicular to the cylinder axis or Permalloy).

We report on a bottom-up approach for the fabrication of spin-transfer nano-oscillators (STNOs). Porous alumina is used as a template for the growth by electrodeposition of metallic spin valves in series. Under specific magnetic field and injected current conditions, emission of microwave current is detected with frequency in the 1.5 GHz range and linewidth as low as 8 MHz. We find strong indications that the microwave emission is due to spin-transfer-driven vortex oscillations. This technique is promising for the fabrication of dense arrays of STNOs in view of device synchronization.

Nonreciprocal microstrip lines based on magnetic nanowired substrates were fabricated for the characterization of microwave differential phase shifts. We provide a fully integrated solution for the nonreciprocal edge guided mode, which is achieved by asymmetrically loading the width of the microstrip with nanowires of different heights. An analytical model explaining the microwave nonreciprocal propagation, which takes into account the substrate permittivity and the microstrip memductance, is validated with the experiment.

We report on exchange bias effects and time evolution in Co nanowires electrodeposited into the pores of track etched polycarbonate membranes. The slow variation of the exchange bias anisotropy, as extracted from field cooled hysteresis loops, is due to the delayed in time oxidation of the nanowires embedded in the polymer template. A model has been proposed to account for the drop of magnetization due to the presence of an oxide layer at the periphery of the nanowires, within the nanowires/polymer interface. The measured exchange bias energy is about four times smaller than the energy in planar bilayers because of the cylindrical geometry of the wires. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3183949]

Arrays of low diameter bi-stable electrodeposited magnetic nanowires demagnetized in the direction parallel to the wires are used to prepare non-saturated stable magnetic states formed by groups of wires magnetized positive and negative with respect to the applied field. Exploiting the coercive field distribution, both the number of wires in each group as well as the number of different groups can be varied by changing the demagnetizing cycle parameters. The ferromagnetic resonance field and peak intensity are shown to be different for each of these magnetic states. By applying demagnetizing cycles, it is possible to induce multiple absorption peaks, and thus show field-programmable multiband absorption properties.

In this letter we present a convenient way of controlling the direction of the uniaxial magnetocrystalline anisotropy in arrays of electrodeposited hcp Co nanowires. Combining electron microscopy and ferromagnetic resonance measurements, it is shown that using an appropriate pH of the electrolytic solution, the hcp c axis can be oriented parallel or perpendicular to the wires axes simply by changing the deposition current density or deposition rate. This reorientation of the c axis leads to a drastic change in overall magnetic anisotropy as the crystal anisotropy either competes for perpendicular oriented c axis or adds to the shape anisotropy for parallel oriented c axis. (C) 2005 American Institute of Physics.