Cristina Bran

Tuning the magnetization reversal process of FeCoCu nanowire arrays by thermal annealing

Arrays of hexagonally ordered Fe28Co67Cu5 nanowires with tailored diameter from 18 to 27 nm were prepared by electroplating into anodic alumina templates and annealed in the temperature range of 300–600 °C, preserving but refining their bcc crystal structure. Despite the partial reduction of saturation magnetization and corresponding shape anisotropy after annealing at 500 °C, larger coercivity, 0.36 T, and squareness ratio, Mr/Ms = 0.98, were obtained. This unexpected behavior is interpreted through micromagnetic simulations where the magnetic hardening is associated with the transition from vortex to transverse domain-wall reversal modes connected with the reduction of saturation magnetization. Simulations also predict a significant coercivity increase with decreasing nanowires diameter which agrees with experimental data in the overlapping diameter range.

Tailoring the magnetic anisotropy of CoFeB/MgO stacks onto W with a Ta buffer layer

The emergence of perpendicular magnetic anisotropy (PMA) in CoFeB/MgO stacks deposited on W using a Ta buffer layer is studied as a function of Ta and CoFeB layer thickness and annealing temperature. It is shown that very thin Ta “dusting” layers (thickness between 0.3 and 1 nm) enhance PMA of CoFeB layers grown on top of W. We find that Ta thickness is a crucial factor affecting magnetic anisotropy and it needs to be scaled proportionally to CoFeB thickness for obtaining PMA. Stacks without Ta have in-plane anisotropy, verifying the “PMA-enhancing” role of Ta. The maximum effective PMA energy ( 3.6×106 erg/cm3) is obtained for a stack with 1.4 nm of CoFeB and 1 nm of Ta and after annealing at 350 °C. Besides, PMA can be obtained even at the as-deposited state for certain thicknesses. This W-based CoFeB/MgO system could enable the development of low power consumption, high density, and non-volatile magnetic memories.

Structural Dependence of Magnetic Properties in Co-Based Nanowires: Experiments and Micromagnetic Simulations

The magnetic properties of several series of cylindrical Co and CoFe nanowires with tailored hexagonal or cubic crystalline symmetry are reviewed. Nanowires are prepared by electroplating filling the self-assembled nanopores of anodic alumina membranes. Their structure is tailored through the electroplating conditions to present cubic (fcc Co and bcc CoFe) or hcp hexagonal ([101], [110] and [002] textures) symmetry. Hysteresis parameters (i.e., coercivity and remanence) are measured in parallel and perpendicular to nanowires magnetic field configurations. Micromagnetic simulations have been performed taking into account the different crystalline anisotropy of nanowires. They show that the magnetization reversal process takes place in hcp symmetry crystal phase wires by vortex-like domain wall and quasi-curling mechanisms, while for fcc and bcc symmetries crystal phases only a vortex domain wall is involved resulting in high-squareness hysteresis loops.

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.

Quantitative Nanoscale Magnetic Study of Isolated Diameter-Modulated FeCoCu Nanowires

The comprehension of the magnetic configuration in FeCoCu nanowires with a diameter-modulated cylindrical geometry will allow controlling the domain wall motion in this low-dimensional system under the application of magnetic fields and/or the injection of current pulses. Here we perform a quantitative magnetic characterization of isolated diameter-modulated FeCoCu nanowires by combining nanoscale magnetic characterization techniques such as electron holography, magnetic force microscopy, and micromagnetic simulations. Local reconstructions of the magnetic distribution show the diameter-modulated geometry of the wires induces the formation of vortex-like structures and magnetic charges in the regions where the diameter is varied. Vortex-like structures modify the axial alignment of the magnetization in large-diameter segments. Moreover, the magnetic charges control the demagnetizing field distribution, promoting a flux-closure stray field configuration around large-diameter segments and keeping the demagnetizing field parallel to the NWs magnetization around small diameter segments. The detailed description of the remanent state in diameter-modulated cylindrical FeCoCu nanowires allows us to provide a clear explanation of the origin of bright and dark contrast observed in magnetic force microscopy images, which have the same feature of magnetic domain walls. This work establishes the primary knowledge required for future magnetization reversal studies with the aim of searching efficient modulated geometries that allow an optimum and controlled domain wall propagation.

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.

Structural and magnetic characterization of as-prepared and annealed FeCoCu nanowire arrays in ordered anodic aluminum oxide templates

Herein, we report on the preparation, structure, and magnetic characterization of FeCoCu nanowire arrays grown by DC electrodeposition inside self-assembled ordered nanopores of anodic aluminum oxide templates. A systematic study of their structure has been performed both in as-prepared samples and after annealing in the temperature range up to 800 °C, although particular attention has been paid to annealing at 700 °C after which maximum magnetic hardening is achieved. The obtained nanowires have a diameter of 40 nm and their Fe0.28Co0.67Cu0.05 composition was confirmed by energy dispersive X-ray spectroscopy (EDS). Focused ion-beam lamellas of two samples (as-prepared and annealed at 700 °C) were prepared for their imaging in the high-resolution transmission electron microscopy (HRTEM) perpendicularly to the electron beam, where the obtained EDS compositional mappings show a homogeneous distribution of the elements. X-ray diffraction analysis, and selected area electron diffraction (SAED) patterns confir...

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.

Spin configuration in isolated FeCoCu nanowires modulated in diameter.

Cylindrical Fe28Co67Cu5 nanowires modulated in diameter between 22 and 35 nm are synthesized by electroplating into the nanopores of alumina membranes. High-sensitivity MFM imaging (with a detection noise of 1 μN m(-1)) reveals the presence of single-domain structures in remanence with strong contrast at the ends of the nanowires, as well as at the transition regions where the diameter is modulated. Micromagnetic simulations suggest that curling of the magnetization takes place at these transition sites, extending over 10-20 nm and giving rise to stray fields measurable with our MFM. An additional weaker contrast is imaged, which is interpreted to arise from inhomogeneities in the nanowire diameter.