Michaël Darques

Controlled changes in the microstructure and magnetic anisotropy in arrays of electrodeposited Co nanowires induced by the solution pH

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.

Electrochemical control and selection of the structural and magnetic properties of cobalt nanowires

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

An unbiased integrated microstrip circulator based on magnetic nanowired substrate

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.

Microwave circulator based on ferromagnetic nanowires in an alumina template

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.

Unbiased microwave circulator based on ferromagnetic nanowires arrays of tunable magnetization state

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.

Configurable multiband microwave absorption states prepared by field cycling in arrays of magnetic nanowires

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.

Permittivity Model for Ferromagnetic Nanowired Substrates

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.

Controlled growth of CoCu nanowires and application to multilayered CoCu/Cu nanowires with selected anisotropy

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.

Influence of the diameter and growth conditions on the magnetic properties of cobalt 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.

Tailoring of the c-axis orientation and magnetic anisotropy in electrodeposited Co nanowires

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.