Ester M. Palmero

Vortex domain wall propagation in periodically modulated diameter FeCoCu nanowire as determined by the magneto-optical Kerr effect

Control over the magnetization reversal process of nanowires is essential to current advances in modern spintronic media and magnetic data storage. Much effort has been devoted to permalloy nanostrips with rectangular cross section and vanishing crystalline anisotropy. Our aim was to unveil and control the reversal process in FeCoCu nanowires with significant anisotropy and circular cross section with tailored periodical modulations in diameter. Magneto-optical Kerr effect measurements and their angular dependence performed on individual nanowires together with their analysis allow us to conclude that the demagnetization process takes place due to the propagation of a single vortex domain wall which is eventually pinned at given modulations with slightly higher energy barrier. In addition these results create new expectations for further controlling of the propagation of single and multiple domain walls.

Magnetic hardening of Fe30Co70 nanowires.

3d transition metal-based magnetic nanowires (NWs) are currently considered as potential candidates for alternative rare-earth-free alloys as novel permanent magnets. Here, we report on the magnetic hardening of Fe30Co70 nanowires in anodic aluminium oxide templates with diameters of 20 nm and 40 nm (length 6 μm and 7.5 μm, respectively) by means of magnetic pinning at the tips of the NWs. We observe that a 3-4 nm naturally formed ferrimagnetic FeCo oxide layer covering the tip of the FeCo NW increases the coercive field by 20%, indicating that domain wall nucleation starts at the tip of the magnetic NW. Ferromagnetic resonance (FMR) measurements were used to quantify the magnetic uniaxial anisotropy energy of the samples. Micromagnetic simulations support our experimental findings, showing that the increase of the coercive field can be achieved by controlling domain wall nucleation using magnetic materials with antiferromagnetic exchange coupling, i.e. antiferromagnets or ferrimagnets, as a capping layer at the nanowire tips.

Magnetic behavior of NiCu nanowire arrays: Compositional, geometry and temperature dependence

Arrays of Ni100−xCux nanowires ranging in composition 0 ≤ x ≤ 75, diameter from 35 to 80 nm, and length from 150 nm to 28 μm have been fabricated by electrochemical co-deposition of Ni and Cu into self-ordered anodic aluminum oxide membranes. As determined by X-ray diffraction and Transmission Electron Microscopy, the crystalline structure shows fcc cubic symmetry with [111] preferred texture and preferential Ni or Cu lattice depending on the composition. Their magnetic properties such as coercivity and squareness have been determined as a function of composition and geometry in a Vibrating Sample Magnetometer in the temperature range from 10 to 290 K for applied magnetic fields parallel and perpendicular to the nanowires axis. Addition of Cu into the NiCu alloy up to 50% enhances both parallel coercivity and squareness. For the higher Cu content, these properties decrease and the magnetization easy axis becomes oriented perpendicular to the wires. In addition, coercivity and squareness increase by decrea...

Spin configuration of cylindrical bamboo-like magnetic nanowires

The surface and the internal magnetic structure of bamboo-like cylindrical nanowires with tailored diameter modulations have been determined exploiting the direct photoemission and transmission contrasts using photoemission electron microscopy combined with X-ray magnetic circular dichroism, as well as complementary magnetic force microscopy and micromagnetic simulations. Bamboo-like cylindrical nanowires with diameters of 130 and 140 nm, and a modulation periodicity of 400 nm were electrochemically grown into the pores of alumina templates. FeCoCu and Co nanowires were selected to offer parallel and perpendicular magnetization easy axis, respectively. For FeCoCu nanowires, a main longitudinal magnetization configuration is found consistent with the predominant shape anisotropy. In addition, a weaker modulated contrast along the wires’ axis is observed that matches the position of each diameter modulation: vortex-like structures are observed at the ends of the wires and at the surface around the modulations. In Co nanowires, a multi-segmented vortex-like structure with alternating opposite chirality is found not matching the periodicity of the diameter modulations. Such a spin configuration is interpreted considering that Co nanowires exhibit hexagonal symmetry with c axis nearly perpendicular to the nanowires defining strong uniaxial transverse magnetocrystalline anisotropy.

Magnetic interactions in compositionally modulated nanowire arrays

Series of high hexagonally ordered compositionally modulated nanowire arrays, with different Cu layer and FeCoCu segment thicknesses and a constant diameter of 35 nm, were fabricated by electroplating from a single electrolytic bath into anodic aluminum oxide membranes. The objective of the study was to determine the influence of ferromagnetic (FM) segment and non-ferromagnetic (NFM) layer thickness on the magnetic properties, particularly coercivity and magnetic interactions. First-order reversal curve (FORC) measurements and simulations were performed to quantify the effect of the inter-/intra-nanowire magnetostatic interactions on the coercivity and interaction field distributions. The FORC coercivity increases for a thick NFM layer and long FM segments due to decoupling of the the FM segments and the increased shape anisotropy, respectively. On the other hand, the interaction field presents a parallel strong reduction for a thick NFM layer and thin FM segments, which is ascribed to a similar NFM/FM thickness ratio and degree of FM segment decoupling along the nanowire.

Direct observation of transverse and vortex metastable magnetic domains in cylindrical nanowires

The study has been performed under the framework of the Projects No. MAT2013-48054-C2-1-R, MAT2016-76824-C3-1-R, MAT-2015-68772-P, MAT2015-64110-C2-2-P supported by the MINECO from Spain and DURSI 2014SGR220 supported by the Catalan Government. J.A.F.-R. acknowledges financial support from MINECO and the ESF through the “Ayudas para contratos predoctorales para la formacion de doctores 2014.”

Nanometer Scale Hard/Soft Bilayer Magnetic Antidots.

The effect of arrays of nanometer scale pores on the magnetic properties of thin films has been analyzed. Particularly, we investigated the influence of the out-of-plane magnetization component created by the nanopores on the in-plane magnetic behavior of patterned hard/soft magnetic thin films in antidot morphology. Its influence on the coupling in Co/Py bilayers of few tens of nanometer thick is compared for disordered and ordered antidots of 35-nm diameter. The combination of magneto-optical Kerr effect (MOKE) and first-order reversal curve (FORC) technique allows probing the effects of the induced perpendicular magnetization component on the bilayer magnetic behavior, while magnetic force microscopy (MFM) is used to image it. We found that ordered antidots yield a stronger out-of-plane component than disordered ones, influencing in a similar manner the hard layer global in-plane magnetic behavior if with a thin or without soft layer. However, its influence changes with a thicker soft layer, which may be an indication of a weaker coupling.

Magnetization Ratchet in Cylindrical Nanowires

The unidirectional motion of information carriers such as domain walls in magnetic nanostrips is a key feature for many future spintronic applications based on shift registers. This magnetic ratchet effect has so far been achieved in a limited number of complex nanomagnetic structures, for example, by lithographically engineered pinning sites. Here we report on a simple remagnetization ratchet originated in the asymmetric potential from the designed increasing lengths of magnetostatically coupled ferromagnetic segments in FeCo/Cu cylindrical nanowires. The magnetization reversal in neighboring segments propagates sequentially in steps starting from the shorter segments, irrespective of the applied field direction. This natural and efficient ratchet offers alternatives for the design of three-dimensional advanced storage and logic devices.

Structural and Magnetic Characterization of FeCoCu/Cu Multilayer Nanowire Arrays

A series of [FeCoCu/Cu(x)]10 (7 ≤ × ≤ 40 nm with FeCoCu layer thickness of 300 nm) and [FeCoCu(y)/Cu]10 (120 ≤ y ≤ 900 nm with Cu layer thickness of 15 nm) arrays of multilayer nanowires, 35 nm in diameter, were fabricated by electrodeposition into self-assembled pores of anodic alumina membranes. High-resolution transmission electron microscopy and X-ray diffraction analysis confirm the segregation of layered structures, with well-defined Cu layers (fcc cubic structure) separating FeCoCu-alloy segments (bcc cubic structure). Hysteresis loop measurements indicate an overall magnetization easy axis parallel to the nanowires in all the samples. For constant FeCoCu segment length, the coercivity, the remanence, and especially, the susceptibility increase with the Cu layer thickness, whereas for the series with constant Cu layer thickness, the susceptibility significantly decreases with FeCoCu segment length. Complementary Henkel curves indicate that the net inter/intrananowires magnetostatic interactions always contribute to the demagnetization of the nanowires. The variation of the susceptibility with FeCoCu and Cu layers thickness together with the Henkel plots data indicate that a reduced demagnetizing effect is achieved for multilayer nanowires with the thicker Cu layer and the shorter FeCoCu segment, for which a moderated reduction in saturation magnetization of around 11% is estimated compared to a continuous FeCoCu alloy nanowire array.

Electrochemical nucleation and growth of Fe, Pt and Fe–Pt on n-type Si (001)

Electrodeposition of Fe, Pt and Fe–Pt on n-type Si (001) wafer was studied. The electrochemical nucleation and growth mechanism was investigated using conventional electrochemical techniques and morphological examinations. The current transients recorded during potentiostatic deposition were compared with the non-dimensional plots predicted by the Scharifker-Hills theory indicating that there exists a progressive nucleation mechanism for the electrodeposition of iron and platinum on the silicon substrate. Using different methods it was shown that the diffusion coefficient of iron ions is about three times larger than that of platinum ions. All single and multi-component system studied reveal 3D multiple nucleation with diffusion controlled growth.