Magnetothermopower and magnetoresistance of single Co-Ni/Cu multilayered nanowires

The magnetothermopower and the magnetoresistance of single Co Ni/Cu multilayered nanowires with various thicknesses of the Cu spacer are investigated. Both kinds of measure-ment have been performed as a function of temperature (50 K to 325 K) and under applied mag-netic fields perpendicular to the nanowire axis, with magnitudes up to 15 % at room tempera-ture. A linear relation between thermopower S and electrical conductivity σ of the nanowires is found, with the magnetic field as an implicit variable. Combining the linear behavior of the S vs. σ and the Mott formula, the energy derivative of the resistivity has been determined. In order to extract the true nanowire materials parameters from the measured thermopower, a simple model based on the Mott formula is employed to distinguish the individual thermopower contributions of the sample. By assuming that the non-diffusive thermopower contributions of the nanowire can be neglected, it was found that the magnetic field induced changes of thermopower and re-sistivity are equivalent. The main emphasis in the present paper is put on a comparison of the magnetoresistance and magnetothermopower results and it was found that the same correlation is valid between the two sets of data for all samples, irrespective of the relative importance of the giant magnetoresistance or anisotropic magnetoresistance contributions in the various indi-vidual nanowires

Influence of Anodic Conditions on Self-ordered Growth of Highly Aligned Titanium Oxide Nanopores

Self-aligned nanoporous TiO2templates synthesized via dc current electrochemical anodization have been carefully analyzed. The influence of environmental temperature during the anodization, ranging from 2 °C to ambient, on the structure and morphology of the nanoporous oxide formation has been investigated, as well as that of the HF electrolyte chemical composition, its concentration and their mixtures with other acids employed for the anodization. Arrays of self-assembled titania nanopores with inner pores diameter ranging between 50 and 100 nm, wall thickness around 20–60 nm and 300 nm in length, are grown in amorphous phase, vertical to the Ti substrate, parallel aligned to each other and uniformly disordering distributed over all the sample surface. Additional remarks about the photoluminiscence properties of the titania nanoporous templates and the magnetic behavior of the Ni filled nanoporous semiconductor Ti oxide template are also included

Temperature gradient-induced magnetization reversal of single ferromagnetic nanowires

In this study, we investigate the temperature- and temperature gradient-dependent magnetization reversal process of individual, single-domain Co39Ni61 and Fe15Ni85 ferromagnetic nanowires via the magneto-optical Kerr effect and magnetoresistance measurements. While the coercive fields (HC) and therefore the magnetic switching fields (HSW) generally decrease under isothermal conditions at elevated base temperatures (Tbase), temperature gradients (ΔT) along the nanowires lead to an increased switching field of up to 15% for ΔT  = 300 K in Co39Ni61 nanowires. This enhancement is attributed to a stress-induced, magneto-elastic anisotropy term due to an applied temperature gradient along the nanowire that counteracts the thermally assisted magnetization reversal process. Our results demonstrate that a careful distinction between locally elevated temperatures and temperature gradients has to be made in future heat-assisted magnetic recording devices

Tailoring of magnetocaloric response in nanostructured materials: Role of anisotropy

The magnetocaloric response of an ensemble of oriented uniaxial magnetic objects, perpendicularly magne- tized to their easy axes, for temperatures close to the blocking temperature is calculated with the aim of demonstrating that the control of the sample’s microstructure makes up an effective way to tailor its magne- tocaloric response. Coexisting positive and negative magnetocaloric effect (MCE) is found for a model mate- rial with a single magnetic phase transition. Both MCE regimes are controlled by the magnitude of the applied magnetic field. As a proof of concept, experimental results for arrays of self-assembled ferromagnetic nano- wires embedded into highly ordered nanoporous anodic alumina templates are shown, suggesting the validity of the numerical calculations

Thermoelectric power factor enhancement by spin-polarized currents – a nanowire case study

In this work, thermoelectric (TE) measurements have been performed on the workhorses of today’s data storage devices, namely nanostructured materials exhibiting either the giant or the anisotropic magnetoresistance effect (GMR and AMR). In particular, the temperature-dependent (50 K - 300 K) and magnetic field-dependent (up to 1 T) TE power factor (PF) has been determined for several Co-Ni alloy nanowires with varying Co:Ni ratios as well as for Co-Ni/Cu multilayered nanowires with various Cu layer thicknesses, which were all synthesized via a template-assisted electrodeposition process. A systematic investigation of the resistivity, (rho), as well as the Seebeck coefficient, S, was performed for Co-Ni alloy nanowires exhibiting AMR and Co-Ni/Cu multilayered nanowires exhibiting GMR. At room temperature, measured values of TE PFs up to 3.6 mWK-2m-1 for AMR samples and 2.0 mWK-2m-1 for GMR nanowires were obtained. Furthermore, the TE PF was found to increase by up to 13.1 % for AMR Co-Ni alloy nanowires and by up to 52 % for GMR Co-Ni/Cu samples in an external applied magnetic field. According to these measurements, the magnetic nanowires exhibit TE PFs that are of the same order of magnitude as TE PFs of Bi-Sb-Se-Te based thermoelectric materials and, additionally, give the opportunity to adjust the TE power output to changing loads and hot spots through external magnetic fields

Electroplating and magnetostructural characterization of multisegmented Co54

Highly hexagonally ordered hard anodic aluminum oxide membranes, which have been modified by a thin cover layer of SiO2 deposited by atomic layer deposition method, were used as templates for the synthesis of electrodeposited magnetic Co-Ni nanowire arrays having diameters of around 180 to 200 nm and made of tens of segments with alternating compositions of Co54Ni46 and Co85Ni15. Each Co-Ni single segment has a mean length of around 290 nm for the Co54Ni46 alloy, whereas the length of the Co85Ni15 segments was around 430 nm. The composition and crystalline structure of each Co-Ni nanowire segment were determined by transmission electron microscopy and selected area electron diffraction techniques. The employed single-bath electrochemical nanowire growth method allows for tuning both the composition and crystalline structure of each individual Co-Ni segment. The room temperature magnetic behavior of the multisegmented Co-Ni nanowire arrays is also studied and correlated with their structural and morphological properties

Effect of Porosity and Concentration Polarization on Electrolyte Diffusive Transport Parameters through Ceramic Membranes with Similar Nanopore Size

Diffusive transport through nanoporous alumina membranes (NPAMs) produced by the two-step anodization method, with similar pore size but different porosity, is studied by analyzing membrane potential measured with NaCl solutions at different concentrations. Donnan exclusion of co-ions at the solution/membrane interface seem to exert a certain control on the diffusive transport of ions through NPAMs with low porosity, which might be reduced by coating the membrane surface with appropriated materials, as it is the case of SiO2. Our results also show the effect of concentration polarization at the membrane surface on ionic transport numbers (or diffusion coefficients) for low-porosity and high electrolyte affinity membranes, which could mask values of those characteristic electrochemical parameters

Effect of Chromium-Doping on the Crystallization and Phase Stability in Anodized TiO2 Nanotubes

Production of limitless hydrogen fuel by visible light splitting of water using the photo-electrochemical technology is cost-effective and sustainable. To make this an attractive viable technology will require the design of TiO2 photocatalyst capable of harnessing the energy of visible light. One possible solution is the doping of TiO2 to reduce its band gap. In this paper, the effect of Cr-doping by ion-implantation on the crystallisation and phase stability of TiO2 nanotubes at elevated temperature is described. The effect of Cr-doping on the resultant microstructures, phase changes and composition depth profiles are discussed in terms of synchrotron radiation diffraction, scanning electron microscopy, and ion-beam analysis by Rutherford backscattering spectrometry

Tailoring of Perpendicular Magnetic Anisotropy in Dy13Fe87 Thin Films with Hexagonal Antidot Lattice Nanostructure

In this article, the magnetic properties of hexagonally ordered antidot arrays made of Dy13Fe87 alloy are studied and compared with corresponding ones of continuous thin films with the same compositions and thicknesses, varying between 20 nm and 50 nm. Both samples, the continuous thin films and antidot arrays, were prepared by high vacuum e-beam evaporation of the alloy on the top-surface of glass and hexagonally self-ordered nanoporous alumina templates, which serve as substrates, respectively. By using a highly sensitive magneto-optical Kerr effect (MOKE) and vibrating sample magnetometer (VSM) measurements an interesting phenomenon has been observed, consisting in the easy magnetization axis transfer from a purely in-plane (INP) magnetic anisotropy to out-of-plane (OOP) magnetization. For the 30 nm film thickness we have measured the volume hysteresis loops by VSM with the easy magnetization axis lying along the OOP direction. Using magnetic force microscopy measurements (MFM), there is strong evidence to suggest that the formation of magnetic domains with OOP magnetization occurs in this sample. This phenomenon can be of high interest for the development of novel magnetic and magneto-optic perpendicular recording patterned media based on template-assisted deposition techniques

Wide-angle Magnetoimpedance field sensor based on two crossed amorphous ribbons

[EN] Soft magnetic amorphous ribbons have been exhaustively studied in order to use them as magnetic field sensitive elements based on the magnetoimpedance effect, MI. In comparison with other materials they exhibit some disadvantages like double-peak response for small values of the applied magnetic field due to the peculiarities of the surface anisotropy. In this work, magnetic properties, MI effect, and the angular dependence of the impedance on the applied magnetic field direction have been studied for CoFeMoSiB amorphous ribbon based sensitive elements in two configurations. The first one is a classic MI prototype with one amorphous ribbon sensitive element installed on a printed circuit board. In the second configuration two amorphous ribbons with the same size are placed in the board forming an angle of 15 degrees between them. MI effect is measured in a frequency range from 0.4 to 10 MHz for exciting current amplitudes ranging from 2 to 20 mA. For this prototype of sensor device formed by two crossed amorphous ribbons, it is possible to obtain single-peak MI response and independent on the orientation of the external field output signal in a wide range of the angles from 0 to 45 degrees, in contrast with the double peak and angle dependent response obtained for single ribbon sensitive element. (C) 2007 Elsevier B.V. All rights reserved.This work has been supported by “Ramon y Cajal” Fellowship of Spanish MEC and Basque Country University UPV-EHU. Dr. G.V. Kurlyandskaya wants to acknowledge Prof. Blanca Hernando for thorough support during her stay at the University of Oviedo. Spanish FICyT research project no. PCTI06-041 is also acknowledged.CDe La Prida, VM.; García Miquel, ÁH.; Kurlyandskaya, G. (2008). Wide-angle Magnetoimpedance field sensor based on two crossed amorphous ribbons. Sensors and Actuators A Physical. 142(2):496-502. doi:10.1016/j.sna.2007.08.026S496502142