Juan de la Figuera

Atomically Flat Ultrathin Cobalt Ferrite Islands

A route for fabricating structurally perfect cobalt ferrite magnetic nanostructures is demonstrated. Ultrathin islands of up to 100 μm(2) with atomically flat surfaces and free from antiphase boundaries are developed. The extremely low defect concentration leads to a robust magnetic order, even for thicknesses below 1 nm, and exceptionally large magnetic domains. This approach allows the evaluation of the influence of specific extrinsic effects on domain wall pinning.

An ultrahigh vacuum fast-scanning and variable temperature scanning tunneling microscope for large scale imaging

We describe the design and performance of a fast-scanning, variable temperature scanning tunneling microscope (STM) operating from 80 to 700 K in ultrahigh vacuum (UHV), which routinely achieves large scale atomically resolved imaging of compact metallic surfaces. An efficient in-vacuum vibration isolation and cryogenic system allows for no external vibration isolation of the UHV chamber. The design of the sample holder and STM head permits imaging of the same nanometer-size area of the sample before and after sample preparation outside the STM base. Refractory metal samples are frequently annealed up to 2000 K and their cooldown time from room temperature to 80 K is 15 min. The vertical resolution of the instrument was found to be about 2 pm at room temperature. The coarse motor design allows both translation and rotation of the scanner tube. The total scanning area is about 8 x 8 microm(2). The sample temperature can be adjusted by a few tens of degrees while scanning over the same sample area.

Micromagnetism in (001) magnetite by spin-polarized low-energy electron microscopy

Spin-polarized low-energy electron microscopy was used to image a magnetite crystal with (001) surface orientation. Sets of spin-dependent images of magnetic domain patterns observed in this surface were used to map the direction of the magnetization vector with high spatial and angular resolution. We find that domains are magnetized along the surface <110> directions, and domain wall structures include 90° and 180° walls. A type of unusually curved domain walls are interpreted as Néel-capped surface terminations of 180° Bloch walls.

Unconventional properties of nanometric FeO(111) films on Ru(0001): stoichiometry and surface structure

We report on the growth of high quality monocrystalline and stoichiometric FeO(111) films on Ru(0001) by infrared pulsed laser deposition (IR-PLD), in a thickness range from below 1 nm to above 8 nm. The films are characterized by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS). Besides the 1:1 Fe/O ratio, they show some unexpected properties, such as the lack of Fe3+ sites at the surface, the (1 × 1) surface symmetry and the large lattice expansion. First-principles calculations show that these properties can be understood from the existence of a wurtzite-like environment at the surface region that preserves the bulk-like antiferromagnetism. This extends the validity of stacking-faults as efficient mechanisms to compensate surface polarity, and suggests that surface-induced processes can be tailored to design nanoscaled materials beyond the parent bulk phase diagram.

Fourfold in-plane magnetic anisotropy of magnetite thin films grown on TiN buffered Si(001) by ion-assisted sputtering

Highly oriented magnetite thin films showing well-defined fourfold in-plane magnetic anisotropy have been grown on TiN buffered Si(001) substrates by ion beam sputtering assisted by a second ion beam containing a controlled mixture of Ar+ and O2+ ions. The structure and composition of stoichiometric Fe3O4 and non-stoichiometric Fe3−δO4 magnetite thin films have been characterized by X-ray diffraction, Rutherford backscattering spectroscopy and Mossbauer spectroscopy. Magneto-optical Kerr effect measurements show that the maxima of the remanence and coercivity of all these films lie along the Si[010] and [100] directions. The introduction of Fe vacancies in magnetite does not alter the well-defined fourfold in-plane anisotropy but induces a decrease of the coercive field as the number of vacancies increases. Furthermore, the results indicate that a 5 nm TiN thick buffer layer is enough to maintain the Fe3O4[100]/TiN[100]/Si[100] epitaxial relationship.

Co on Fe3O4(001): Towards precise control of surface properties

A novel approach to incorporate cobalt atoms into a magnetite single crystal is demonstrated by a combination of x-ray spectro-microscopy, low-energy electron diffraction, and density-functional theory calculations. Co is deposited at room temperature on the reconstructed magnetite (001) surface filling first the subsurface octahedral vacancies and then occupying adatom sites on the surface. Progressive annealing treatments at temperatures up to 733 K diffuse the Co atoms into deeper crystal positions, mainly into octahedral ones with a marked inversion level. The oxidation state, coordination, and magnetic moments of the cobalt atoms are followed from their adsorption to their final incorporation into the bulk, mostly as octahedral Co(2+). This precise control of the near-surface Co atoms location opens up the way to accurately tune the surface physical and magnetic properties of mixed spinel oxides.

Low-Energy Electron Microscopy

Low-energy electron microscopy (LEEM) images a beam of low-energy electrons that have been reflected from a sample. The technique characterizes the sample’s surface in real-space with nanometer-scale lateral resolution. Through a variety of contrast mechanisms, different aspects of the surface can be imaged, including the distribution of different phases and the location of atomic steps. LEEM instrumentation can also acquire electron diffraction patterns from local regions of the surface. The ability to acquire images quickly during temperature changes, while depositing films and exposing materials to reactive gases makes LEEM extremely useful for studying dynamical processes on surfaces. New developments include aberration correction systems for improved spatial resolution and bright spin-polarized electron sources.

Valence band circular dichroism in non-magnetic Ag/Ru(0001) at normal emission

For the non-magnetic system of Ag films on Ru(0001), we have measured the circular dichroism of photoelectrons emitted along the surface normal, the geometry typically used in photoemission electron microscopy. Photoemission spectra were acquired from micrometer-sized regions having uniformly thick Ag films on a single, atomically flat Ru terrace. For a single Ag layer, we find a circular dichroism that exceeds 6% at the d-derived band region around 4.5 eV binding energy. The dichroism decreases as the Ag film thickness increases to three atomic layers. We discuss the origin of the circular dichroism in terms of the symmetry lowering that can occur even in normal emission.

Memory effect and magnetocrystalline anisotropy impact on the surface magnetic domains of magnetite(001)

The structure of magnetic domains, i.e. regions of uniform magnetization separated by domain walls, depends on the balance of competing interactions present in ferromagnetic (or ferrimagnetic) materials. When these interactions change then domain configurations also change as a result. Magnetite provides a good test bench to study these effects, as its magnetocrystalline anisotropy varies significantly with temperature. Using spin-polarized electron microscopy to map the micromagnetic domain structure in the (001) surface of a macroscopic magnetite crystal (~1u2009cm size) shows complex domain patterns with characteristic length-scales in the micrometer range and highly temperature dependent domain geometries. Although heating above the Curie temperature erases the domain patterns completely, cooling down reproduces domain patterns not only in terms of general characteristics: instead, complex microscopic domain geometries are reproduced in almost perfect fidelity between heating cycles. A possible explanation of the origin of the high-fidelity reproducibility is suggested to be a combination of the presence of hematite inclusions that lock bulk domains, together with the strong effect of the first order magnetocrystalline anisotropy which competes with the shape anisotropy to give rise to the observed complex patterns.

Determination of vibrational energy distribution in excited molecules from experimental data. New program and test

Abstract The vibrational energy distribution can now be determined confidently from experimental data thanks to the new program of the previously published algorithm that has been reprogrammed and improved to an extent that should make it useful for its application in experimental work. The program has been thoroughly tested using as initial energy distributions, functions mathematically well defined. The influence of unavoidable errors in the data have been carefully studied and found that random errors up to 2% in the experimental results introduce practically neglectful changes in the recovered energy distributions. Even data with 5% errors produced valuable approximations to the true function distributions.