José María de Teresa

Ultrasmall Functional Ferromagnetic Nanostructures Grown by Focused Electron-Beam-Induced Deposition

We have successfully grown ultrasmall cobalt nanostructures (lateral size below 30 nm) by optimization of the growth conditions using focused electron-beam-induced deposition techniques. This direct-write nanolithography technique is thus shown to produce unprecedented resolution in the growth of magnetic nanostructures. The challenging magnetic characterization of such small structures is here carried out by means of electron holography techniques. Apart from growing ultranarrow nanowires, very small Hall sensors have been created and their large response has been unveiled.

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.

Distinguishing magnetic and electrostatic interactions by a Kelvin probe force microscopy–magnetic force microscopy combination

Summary The most outstanding feature of scanning force microscopy (SFM) is its capability to detect various different short and long range interactions. In particular, magnetic force microscopy (MFM) is used to characterize the domain configuration in ferromagnetic materials such as thin films grown by physical techniques or ferromagnetic nanostructures. It is a usual procedure to separate the topography and the magnetic signal by scanning at a lift distance of 25–50 nm such that the long range tip–sample interactions dominate. Nowadays, MFM is becoming a valuable technique to detect weak magnetic fields arising from low dimensional complex systems such as organic nanomagnets, superparamagnetic nanoparticles, carbon-based materials, etc. In all these cases, the magnetic nanocomponents and the substrate supporting them present quite different electronic behavior, i.e., they exhibit large surface potential differences causing heterogeneous electrostatic interaction between the tip and the sample that could be interpreted as a magnetic interaction. To distinguish clearly the origin of the tip–sample forces we propose to use a combination of Kelvin probe force microscopy (KPFM) and MFM. The KPFM technique allows us to compensate in real time the electrostatic forces between the tip and the sample by minimizing the electrostatic contribution to the frequency shift signal. This is a great challenge in samples with low magnetic moment. In this work we studied an array of Co nanostructures that exhibit high electrostatic interaction with the MFM tip. Thanks to the use of the KPFM/MFM system we were able to separate the electric and magnetic interactions between the tip and the sample.

Focused Electron and Ion Beam Induced Deposition on Flexible and Transparent Polycarbonate Substrates

The successful application of focused electron (and ion) beam induced deposition techniques for the growth of nanowires on flexible and transparent polycarbonate films is reported here. After minimization of charging effects in the substrate, sub-100 nm-wide Pt, W, and Co nanowires have been grown and their electrical conduction is similar compared to the use of standard Si-based substrates. Experiments where the substrate is bent in a controlled way indicate that the electrical conduction is stable up to high bending angles, >50°, for low-resistivity Pt nanowires grown by the ion beam. On the other hand, the resistance of Pt nanowires grown by the electron beam changes significantly and reversibly with the bending angle. Aided by the substrate transparency, a diffraction grating in transmission mode has been built based on the growth of an array of Pt nanowires that shows sharp diffraction spots. The set of results supports the large potential of focused beam deposition as a high-resolution nanolithography technique on transparent and flexible substrates. The most promising applications are expected in flexible nano-optics and nanoplasmonics, flexible electronics, and nanosensing.

Hysteresis loops of individual Co nanostripes measured by magnetic force microscopy

High-resolution magnetic imaging is of utmost importance to understand magnetism at the nanoscale. In the present work, we use a magnetic force microscope (MFM) operating under in-plane magnetic field in order to observe with high accuracy the domain configuration changes in Co nanowires as a function of the externally applied magnetic field. The main result is the quantitative evaluation of the coercive field of the individual nanostructures. Such characterization is performed by using an MFM-based technique in which a map of the magnetic signal is obtained as a function of both the lateral displacement and the magnetic field.

Nanoscale chemical and structural study of Co-based FEBID structures by STEM-EELS and HRTEM

Nanolithography techniques in a scanning electron microscope/focused ion beam are very attractive tools for a number of synthetic processes, including the fabrication of ferromagnetic nano-objects, with potential applications in magnetic storage or magnetic sensing. One of the most versatile techniques is the focused electron beam induced deposition, an efficient method for the production of magnetic structures highly resolved at the nanometric scale. In this work, this method has been applied to the controlled growth of magnetic nanostructures using Co2(CO)8. The chemical and structural properties of these deposits have been studied by electron energy loss spectroscopy and high-resolution transmission electron microscopy at the nanometric scale. The obtained results allow us to correlate the chemical and structural properties with the functionality of these magnetic nanostructures.

Mechanical magnetometry of Cobalt nanospheres deposited by focused electron beam at the tip of ultra-soft cantilevers

Abstract Using focused-electron-beam-induced deposition, Cobalt magnetic nanospheres with diameter ranging between 100 nm and 300 nm are grown at the tip of ultra-soft cantilevers. By monitoring the mechanical resonance frequency of the cantilever as a function of the applied magnetic field, the hysteresis curve of these individual nanospheres are measured. This enables the evaluation of their saturation magnetization, found to be around 430 emu/cm3 independent of the size of the particle, and to infer that the magnetic vortex state is the equilibrium configuration of these nanospheres at remanence. SEM image of a 200 nm Co nanosphere grown at the tip of an ultra-soft cantilever by focus electron beam induced deposition.

Arrays of Densely Packed Isolated Nanowires by Focused Beam Induced Deposition Plus Ar+ Milling

One of the main features of any lithography technique is its resolution, generally maximized for a single isolated object. However, in most cases, functional devices call for highly dense arrays of nanostructures, the fabrication of which is generally challenging. Here, we show the growth of arrays of densely packed isolated nanowires based on the use of focused beam induced deposition plus Ar(+) milling. The growth strategy presented herein allows the creation of films showing thickness modulation with periodicity determined by the beam scan pitch. The subsequent Ar(+) milling translates such modulation into an array of isolated nanowires. This approach has been applied to grow arrays of W-based nanowires by focused ion beam induced deposition and Co nanowires by focused electron beam induced deposition, achieving linear densities up to 2.5 × 10(7) nanowires/cm (one nanowire every 40 nm). These results open the route for specific applications in nanomagnetism, nanosuperconductivity, and nanophotonics, where arrays of densely packed isolated nanowires grown by focused beam deposition are required.

High Conductivity in Hydrothermally Grown AgCuO2 Single Crystals Verified Using Focused-Ion-Beam-Deposited Nanocontacts

The silver-copper mixed oxide AgCuO(2) (also formulated as Ag(2)Cu(2)O(4)) possesses a peculiar electronic structure in which both Ag and Cu are partially oxidized, with the charge being delocalized among the three elements in the oxide. Accordingly, a quasi-metallic behavior should be expected for this oxide, and indeed bulk transport measurements show conductivity values that are orders of magnitude higher than for other members of this novel oxide family. The presence of silver makes thermal sintering an inadequate method to evaluate true conductivity, and thus such measurements were performed on low density pellets, giving an underestimated value for the conductivity. In the present work we present a new synthetic route for AgCuO(2) based on mild hydrothermal reactions that has yielded unprecedented large AgCuO(2) single-crystals well over 1 μm in size using temperatures as low as 88 °C. We have used a dual beam instrument to apply nanocontacts to those crystals, allowing the in situ measurement of transport properties of AgCuO(2) single crystals. The results show a linear relationship between applied current and measured voltage. The conductivity values obtained are 50 to 300 times higher than those obtained for bulk low density AgCuO(2) pellets, thus confirming the high conductivity of this oxide and therefore supporting the delocalized charge observed by spectroscopic techniques.

Observation of the strain induced magnetic phase segregation in manganite thin films.

Epitaxial strain alters the physical properties of thin films grown on single crystal substrates. Thin film oxides are particularly apt for strain engineering new functionalities in ferroic materials. In the case of La(2/3)Ca(1/3)MnO(3) (LCMO) thin films, here we show the first experimental images obtained by electron holography demonstrating that epitaxial strain induces the segregation of a flat and uniform nonferromagnetic layer with antiferromagnetic (AFM) character at the top surface of a ferromagnetic (FM) layer, the whole film being chemical and structurally homogeneous at room temperature. For different substrates and growth conditions the tetragonality of LCMO at room temperature, defined as τ = |c - a|/a, is the driving force for a phase coexistence above an approximate critical value of τC ≈ 0.024. Theoretical calculations prove that the increased tetragonality changes the energy balance of the FM and AFM ground states in strained LCMO, enabling the formation of magnetically inhomogeneous states. This work gives the key evidence that opens a new route to synthesize strain-induced exchanged-biased FM-AFM bilayers in single thin films, which could serve as building blocks of future spintronic devices.