Amalio Fernández-Pacheco

Magnetotransport properties of high-quality cobalt nanowires grown by focused-electron-beam-induced deposition

High-quality cobalt nanowires have been grown by focused-electron-beam-induced deposition (FEBID) and their magnetic and transport properties determined. The nanowires contain up to about 95% Co atomic percentage, as measured by EDX spectroscopy, which remarkably represents a high value compared with other metal deposits grown by the same technique. The Co content has been found to correlate with the beam current used for the growth. The magnetotransport properties have been studied on individual nanowires through 4-probe measurements. For the nanowires with the highest Co content, the resistivity at room temperature is low (~40 µΩ cm), and shows metallic temperature dependence. The magnetotransport properties clearly demonstrate the ferromagnetic nature of the nanowire, with a saturation magnetization of Ms = 1329 ± 20 emu cm−3, very close to the bulk one. Due to the local character of this type of growth at targeted places and its high lateral resolution, these results pave the way for the creation of magnetic nanostructures and devices with the full potentiality of high-quality Co.

Magnetic ratchet for three-dimensional spintronic memory and logic

One of the key challenges for future electronic memory and logic devices is finding viable ways of moving from today’s two-dimensional structures, which hold data in an x–y mesh of cells, to three-dimensional structures in which data are stored in an x–y–z lattice of cells. This could allow a many-fold increase in performance. A suggested solution is the shift register—a digital building block that passes data from cell to cell along a chain. In conventional digital microelectronics, two-dimensional shift registers are routinely constructed from a number of connected transistors. However, for three-dimensional devices the added process complexity and space needed for such transistors would largely cancel out the benefits of moving into the third dimension. ‘Physical’ shift registers, in which an intrinsic physical phenomenon is used to move data near-atomic distances, without requiring conventional transistors, are therefore much preferred. Here we demonstrate a way of implementing a spintronic unidirectional vertical shift register between perpendicularly magnetized ferromagnets of subnanometre thickness, similar to the layers used in non-volatile magnetic random-access memory. By carefully controlling the thickness of each magnetic layer and the exchange coupling between the layers, we form a ratchet that allows information in the form of a sharp magnetic kink soliton to be unidirectionally pumped (or ‘shifted’) from one magnetic layer to another. This simple and efficient shift-register concept suggests a route to the creation of three-dimensional microchips for memory and logic applications.

Three dimensional magnetic nanowires grown by focused electron-beam induced deposition.

Control of the motion of domain walls in magnetic nanowires is at the heart of various recently proposed three-dimensional (3D) memory devices. However, fabricating 3D nanostructures is extremely complicated using standard lithography techniques. Here we show that highly pure 3D magnetic nanowires with aspect-ratios of ~100 can be grown using focused electron-beam-induced-deposition. By combining micromanipulation, Kerr magnetometry and magnetic force microscopy, we determine that the magnetisation reversal of the wires occurs via the nucleation and propagation of domain walls. In addition, we demonstrate that the magnetic switching of individual 3D nanostructures can be directly probed by magneto-optical Kerr effect.

Fast domain wall motion in magnetic comb structures

Modern fabrication technology has enabled the study of submicron ferromagnetic strips with a particularly simple domain structure, allowing single, well-defined domain walls to be isolated and characterized. However, these domain walls have complex field-driven dynamics. The wall velocity initially increases with field, but above a certain threshold the domain wall abruptly slows down, accompanied by periodic transformations of the domain wall structure. This behaviour is potentially detrimental to the speed and proper functioning of proposed domain-wall-based devices, and although methods for suppression of the breakdown have been demonstrated in simulations, a convincing experimental demonstration is lacking. Here, we show experimentally that a series of cross-shaped traps acts to prevent transformations of the domain wall structure and increase the domain wall velocity by a factor of four compared to the maximum velocity on a plain strip. Our results suggest a route to faster and more reliable domain wall devices for memory, logic and sensing.

Domain wall conduit behavior in cobalt nanowires grown by focused electron beam induced deposition

The domain wall nucleation and propagation fields in cobalt nanowires grown by focused electron beam induced deposition are measured using spatially resolved magneto-optical Kerr effect. The study was systematically done for wire widths from 600 to 150 nm, finding significant differences in the value of both fields for the wires, indicating high quality domain wall conduit behavior. The extreme simplicity and flexibility of this technique with respect to the multistep lithographic processes used nowadays opens a different route to create magnetic nanostructures with a good control of the domain wall motion.

Direct observation of melting in a two-dimensional superconducting vortex lattice

A two-dimensional lattice of vortices melts into an isotropic liquid with increasing temperature. A microscopic view of the melting transition reveals that this actually occurs in three steps, one of which is an unusual liquid-crystal-like vortex phase.

Tuning the interlayer exchange coupling between single perpendicularly magnetized CoFeB layers

We experimentally study the tunability of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interlayer exchange coupling (IEC) in Pt/CoFeB/Pt/Ru/Pt/CoFeB/Pt stacks with perpendicular magnetic anisotropy (PMA). The perpendicular magnetization of a single Pt/Co60Fe20B20/Pt (at. %) shows full remanence and square hysteresis loops for a CoFeB thickness range of 0.60–1.0 nm. By inserting a Pt layer between the Ru and CoFeB, the PMA of the ultrathin CoFeB layers is stabilized and the IEC can be tuned. In particular, we show that the IEC versus Pt thickness exhibits a simple exponential decay with a decay length of 0.16 nm.

Magnetization reversal in individual cobalt micro- and nanowires grown by focused-electron-beam-induced-deposition.

We systematically study individual micro- and nanometric polycrystalline cobalt wires grown by focused-electron-beam-induced-deposition. The deposits were grown in a range of aspect ratios varying from 1 up to 26. The minimum lateral dimension of the nanowires was 150 nm, for a thickness of 40 nm. Atomic force microscopy images show beam-current-dependent profiles, associated with different regimes of deposition. The magnetization reversal of individual nanowires is studied by means of the spatially resolved magneto-optical Kerr effect. Abrupt switching is observed, with a systematic dependence on the wires dimensions. This dependence of the coercive field is understood in magnetostatic terms, and agrees well with previous results on cobalt wires grown with different techniques. The influence of compositional gradients along the structural profile on the magnetic reversal is studied by using micromagnetic simulations. This work demonstrates the feasibility of using this technique to fabricate highly pure magnetic nanostructures, and highlights the advantages and disadvantages of the technique with respect to more conventional ones.

Origin of the difference in the resistivity of as-grown focused-ion- and focused-electron-beam-induced Pt nanodeposits

We study the origin of the strong difference in the resistivity of focused-electron- and focused-Ga-ion-beam-induced deposition (FEBID and FIBID, resp.) of Pt performed in a dual beam equipment using (CH3)3Pt(CpCH3) as the precursor gas. We have performed in-situ and ex-situ resistance measurements in both types of nanodeposits, finding that the resistivity of Pt by FEBID is typically four orders of magnitude higher than Pt by FIBID. In the case of Pt by FEBID, the current-versus-voltage dependence is nonlinear and the resistance-versus-temperature behavior is strongly semiconducting, whereas Pt by FIBID shows linear current-versus-voltage dependence and only slight temperature dependence. The microstructure, as investigated by high-resolution transmission electron microscopy, consists in all cases of Pt single crystals with size about 3nm embedded in an amorphous carbonaceous matrix. Due to the semiconducting character of the carbon matrix, which is the main component of the deposit, we propose that the transport results can be mapped onto those obtained in semiconducting materials with different degrees of doping. The different transport properties of Pt by FEBID and FIBID are attributed to the higher doping level in the case of FIBID, as given by composition measurements obtained with energy-dispersive X-ray microanalysis.

Nanoscale superconducting properties of amorphous W-based deposits grown with a focused-ion-beam

We present very low temperature scanning tunneling microscopy and spectroscopy (STM/S) measurements in W-based amorphous superconducting nanodeposits grown using a metal–organic precursor and a focused-ion-beam. The superconducting gap closely follows s-wave Bardeen–Cooper– Schrieffer theory, and STS images under magnetic fields show a hexagonal vortex lattice whose orientation is related to features observed in the topography through STM. Our results demonstrate that the superconducting properties at the surface of these deposits are very homogeneous, down to atomic scale.