Joaquin De La Torre Medina

Electrodeposition Growth of Nanowire Arrays with Height Gradient Profiles for Microwave Device Applications

A simple and nonexpensive adapted dip-coating technique is presented and used to fabricate arrays of magnetic nanowires with a linear varying height profile. This approach allows controlling the wire height from tenths of nanometers up to several micrometers. Furthermore, the main parameters of this height gradient can be controlled, such as the maximum wire height and the lateral span of the wire array, which can be predicted with excellent accuracy using a proposed analytical model. Moreover, we show that by sequential electrodeposition with dip-coating, arrays of these height varying wires can be grown. This technique represents a novel method to fill porous templates with controlled spatial growth, leading to the fabrication of novel structures and providing control over structural features on the nanoscale level. In particular, the use of these asymmetrically loaded magnetic nanowired substrates to obtain improved microwave nonreciprocal behavior is shown for a microwave phase shifter.

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.

Tunable zero field ferromagnetic resonance in arrays of bistable magnetic nanowires

The tunability of the zero field resonance frequency in arrays of bistable nanowires is demonstrated. Analysis of the resonance spectra for different remanent states show that the signal results from the superposition of the double absorption related to wires magnetized in both the positive and negative direction. An analytical model is proposed and validated which depends of the configuration dependent dipolar interaction and the peak amplitude of the superposing signals. The range over which the zero field resonance frequency can be tuned has a lower bound at remanence equal to one and a maxima when remanence vanishes.

Strong low temperature magnetoelastic effects in template grown Ni nanowires

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.

Configuration dependent demagnetizing field in assemblies of interacting magnetic particles

A mean field model is presented for the configuration dependent effective demagnetizing and anisotropy fields in assemblies of exchange decoupled magnetic particles of arbitrary shape which are expressed in terms of the demagnetizing factors of the particles and the volumetric shape containing the assembly. Perpendicularly magnetized two-dimensional (2D) assemblies have been considered, for which it is shown that the demagnetizing field is lower than the continuous thin film. As an example of these 2D systems, arrays of bistable cylindrical nanowires have been characterized by remanence curves as well as ferromagnetic resonance, serving to show the correspondence of these measurements with the model and also to validate the mean field approach. Linear chains of cylinders and spheres have been analyzed, leading to simple expressions to describe the easy axis rotation induced by the interaction field in chains of low aspect ratio cylindrical particles, and the dipolar magnetic anisotropy observed in the linear chain of spheres. These examples serve to underline the dependence on the dipolar interaction field and effective demagnetizing factor of the contributions that arise from the shape of the outer volume.

Template Approach for Novel Magnetic–Ferroelectric Nanocomposites

Ni nanowires were electrodeposited into track-etched ferroelectric poly(vinylidene difluoride) (PVDF) polymer nanoporous templates to make multiferroic nanostructured composites. Using ferromagnetic resonance measurements under static voltage bias, we demonstrate a magnetoelectric effect arising from a mechanical coupling between the magnetostrictive and piezoelectric phases. The calculated electric field lines and intensity indicate that PVDF matrix surrounding the surface of the Ni nanowires experiences shear stress. The competing magnetoelastic anisotropy originating from the piezoelectric effects leads to a reduced magnetic anisotropy field along the wire axis. The nanowires packing in the array are found to play a dominant role in the magnetoelectric effect.

Magnetic and Magnetoresistive Properties of 3D Interconnected NiCo Nanowire Networks

Track-etched polymer membranes with crossed nanochannels have been revealed to be most suitable as templates to produce large surface area and mechanically stable 3D interconnected nanowire (NW) networks by electrodeposition. Geometrically controlled NW superstructures made of NiCo ferromagnetic alloys exhibit appealing magnetoresistive properties. The combination of exact alloy compositions with the spatial arrangement of NWs in the 3D network is decisive to obtain specific magnetic and magneto-transport behavior. A proposed simple model based on topological aspects of the 3D NW networks is used to accurately determine the anisotropic magnetoresistance ratios. Despite of their complex topology, the microstructure of Co-rich NiCo NW networks display mixed fcc-hcp phases with the c-axis of the hcp phase oriented perpendicular to their axis. These interconnected NW networks have high potential as reliable and stable magnetic field sensors.

Interplay between the magnetic and magneto-transport properties of 3D interconnected nanowire networks

We have explored the interplay between the magnetic and magneto-transport properties of 3D interconnected nanowire networks made of various magnetic metals by electrodeposition into nanoporous membranes with crossed channels and controlled topology. The close relationship between their magnetic and structural properties has a direct impact on their magneto-transport behavior. In order to accurately and reliably describe the effective magnetic anisotropy and anisotropic magnetoresistance, an analytical model inherent to the topology of 3D nanowire networks is proposed and validated. The feasibility to obtain magneto-transport responses in nanowire network films based on interconnected nanowires makes them very attractive for the development of mechanically stable superstructures that are suitable for potential technological applications.

Influence of the packing fraction and host matrix on the magnetoelastic anisotropy in Ni nanowire composite arrays

The influence of the packing fraction on thermally induced magnetoelastic effects has been studied in Ni nanowires embedded in polycarbonate, poly(vinylidene difluoride), and alumina nanoporous membranes of different porosities for temperatures between 77 K and 345 K. For nanowires embedded in polymer membranes, the contrasting shift in the ferromagnetic resonance frequency when the temperature is either above or below ambient temperature is consistent with the occurrence of uniaxial magnetoelastic anisotropy effects due to the large thermal expansion coefficient mismatch between the metal nanowires and the membrane. A model which considers the influence of the nanowires packing fraction and the membrane material on the magnetoelastic effects, arising from the matrix-assisted deformation process, is proposed. The model is able to successfully explain the experimentally observed effects for the Ni nanowire arrays embedded in the different porous membranes and their variation with the packing fraction. The possibility to modulate the magnetic anisotropy of such nanocomposites by an appropriate choice of membrane material, packing fraction, and sample temperature is of considerable importance to achieve magnetically tunable devices.

3-D Interconnected Magnetic Nanofiber Networks With Multifunctional Properties

3-D alloyed and multilayered interconnected nanofiber networks have been fabricated by electrodeposition techniques, allowing a controlled composition and 3-D structural topology. These features have been found crucial to tailor their magnetic and magneto-transport properties. Their interplay along with the use of a simple analytical model based on the particular interconnected topology of the networks has allowed to accurately determine the anisotropic magnetoresistance (AMR) ratio. The as-obtained AMR ratio for interconnected nanofiber networks is consistent with an average that results from all the nanowires orientations in the membrane. The careful choice of magnetic and non-magnetic layer thicknesses has been decisive for the fabrication of Co/Cu multilayered interconnected nanofiber networks with giant magnetoresistive response as high as 19%. Interconnected nanofiber networks with controlled material composition and specific structural features are very attractive for the development of mechanically stable superstructures suitable for potential technological device applications.