J. De La Torre Medina

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.

Differential phase shift in nonreciprocal microstrip lines on magnetic nanowired substrates

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.

Self consistent measurement and removal of the dipolar interaction field in magnetic particle assemblies and the determination of their intrinsic switching field distribution

Using low density arrays of bistable magnetic nanowires as a model dipolar system, it is shown that the dipolar interaction field coefficient can be measured from the remanence curves as well as from other functions of the isothermal remanent magnetization and the DC demagnetization remanence obtained as an affine transformation of the Wohlfarth relation. Based on mean field arguments, these measurements are used to subtract and remove the contribution of the configuration dependent dipolar interaction field from the major loop and remanence curves. The corrected remanence curves are first used to obtain the intrinsic switching field distribution of the nanowire array and then to validate this approach showing that they yield results consistent with the Wohlfarth relation for an assembly of noninteracting particles, thus providing a self-consistent procedure to verify the measured values of the interaction field and its removal from the measurements.

Dipolar interaction in arrays of magnetic?nanotubes

The dipolar interaction field in arrays of nickel nanotubes has been investigated on the basis of expressions derived from the effective demagnetizing field of the assembly as well as magnetometry measurements. The model incorporates explicitly the wall thickness and aspect ratio, as well as the spatial order of the nanotubes. The model and experiment show that the interaction field in nanotubes is smaller than that in solid nanowires due to the packing fraction reduction in tubes related to their inner cavity. Finally, good agreement between the model and experiment is found for the variation of the interaction field as a function of the tube wall thickness.

Magnetic force microscopy investigation of arrays of nickel nanowires and nanotubes

The magnetic properties of arrays of nanowires (NWs) and nanotubes (NTs), 150 nm in diameter, electrodeposited inside nanoporous polycarbonate membranes are investigated. The comparison of the nanoscopic magnetic force microscopy (MFM) imaging and the macroscopic behavior as measured by alternating gradient force magnetometry (AGFM) is made. It is shown that MFM is a complementary technique that provides an understanding of the magnetization reversal characteristics at the microscopic scale of individual nanostructures. The local hysteresis loops have been extracted by MFM measurements. The influence of the shape of such elongated nanostructures on the dipolar coupling and consequently on the squareness of the hysteresis curves is demonstrated. It is shown that the nanowires exhibit stronger magnetic interactions than nanotubes. The non-uniformity of the magnetization states is also revealed by combining the MFM and AGFM measurements.

Magnetic force microscopy study of the switching field distribution of low density arrays of single domain magnetic nanowires

In the present work, we report on the in situ magnetic force microscopy (MFM) study of the magnetization reversal in two-dimensional arrays of ferromagnetic Ni80Fe20 and Co55Fe45 nanowires (NW) with different diameters (40, 50, 70, and 100 nm) deposited inside low porosity (P<1%) nanoporous polycarbonate membranes. In such arrays, the nanowires are sufficiently isolated from each other so that long range dipolar interactions can be neglected. The MFM experiments performed for different magnetization states at the same spot of the samples are analysed to determine the switching field distribution (SFD). The magnetization curves obtained from the MFM images are relatively square shaped. The SFD widths are narrower compared to those obtained for high density arrays. The weak broadening of the curves may be ascribed to the NW intrinsic SFD. The influence of diameter and composition of the ferromagnetic NW is also investigated. VC 2013 AIP Publishing LLC.

Application of the anisotropy field distribution method to arrays of magnetic nanowires

The applicability of the anisotropy field distribution method and the conditions required for an accurate determination of the effective anisotropy field in arrays of magnetic nanowires have been evaluated. In arrays of magnetic nanowires that behave as ideal uniaxial systems having only magnetostatic contributions to the effective anisotropy field, i.e., shape anisotropy and magnetostatic coupling, the method yields accurate values of the average anisotropy field at low-moderate dipolar coupling and accuracy decreases as wire packing increases. When an additional non-negligible magnetocrystalline anisotropy is present, the method is less accurate, as shown for the case of hcp Co nanowires.

Self-Biased Nonreciprocal Microstrip Phase Shifter on Magnetic Nanowired Substrate Suitable for Gyrator Applications

Magnetic nanowired substrates (MNWS) have been used for the fabrication of a planar nonreciprocal microstrip device. It shows a differential phase shift of 300 degrees cm-1 at K a -band without requiring the application of a dc bias magnetic field, and making it suitable for miniaturized gyrator applications. The nonreciprocal operation is achieved by loading the device with nanowires of different ferromagnetic materials. This allows to control the phase velocity of the microwave signal passing through the device by virtue of the spatial variation of the MNWS permeability. The measured microwave performances of the device have been reproduced with excellent accuracy using a proposed analytical model based on an effective medium theory and useful for the prediction of further tunable capabilities.

Bottom-up approach for the fabrication of spin torque nano-oscillators

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.

Large magnetic anisotropy enhancement in size controlled Ni nanowires electrodeposited into nanoporous alumina templates.

A large enhancement of the magnetic anisotropy of Ni nanowires (NWs) embedded in anodic aluminium oxide porous membranes is obtained as a result of an induced magnetoelastic (ME) anisotropy contribution. This unusual large anisotropy enhancement depends on the diameter of the NWs and exceeds the magnetostatic (MS) contribution. As a consequence, it leads to effective magnetic anisotropy energies as large as 1.4 × 10(6) erg cm(-3), which are of the same order of magnitude and comparable to the MS energies of harder magnetic materials like Co NWs. Specifically, from ferromagnetic resonance experiments, the magnetic anisotropy of the NWs has been observed to increase as its diameter is decreased, leading to values that are about four times larger than the corresponding value when only the MS anisotropy is present. Our results are consistent with the recently proposed growth mechanism of Ni NWs that proceeds via a poly-crystalline stage at the bottom followed by a single-crystalline stage with texture [110] parallel to the axis of the NWs. A strong correlation between reducing the diameter of the NWs with the decrease of the length of the poly-crystalline segment and the enhancement of the effective magnetic anisotropy has been shown. Magnetization curves obtained from alternating gradient magnetometry experiments show that the average ME anisotropy results from the competition between the magnetic anisotropies of both crystalline segments of the NWs. Understanding the influence of size and confinement effects on the magnetic properties of nanocomposites is of prime interest for the development of novel and agile devices.