Laura Martín-García

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.

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.

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 (~1 cm 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.

AMPHIBIAN CSIC experimentaldata N RuizGomez 2018 XPEEM 20180312 75144 (v1)

Los datos se componen de grupos de 3 imagenes de microscopia XPEEM correspondientes a 5 islas nanometricas de magnetita. Cada isla esta identificada en el nombre de los datos como i1, i2….i5, y las tres imagenes de cada isla con el identificador R0, R60 y R120.

Multifunctional core–shell Co–SiO2 nanowires via electrodeposition and sol–gel techniques

In this work, we propose a new strategy for the synthesis of multifunctional nanowires using a combination of sol–gel and electrodeposition techniques, based on a two-step procedure. First of all, nanotubes of SiO2 are synthesized via a sol–gel technique using polycarbonate membranes as templates. Homogenous nanotubes are obtained after centrifugation and thermal annealing. Afterwards, a ferromagnetic cobalt core is grown using potentiostatic electrodeposition. Finally, the core–shell Co–SiO2 nanowires are released by dissolving the template using wet-etching. These nanodevices can be used for many detection and sensing purposes. As a proof of concept, we have developed a pH nanosensor by including a pH-sensitive organic dye in the SiO2 shell. The sensing principle is based on the optical response of the organic dye towards pH when added to a solution. The magnetic core allows the recovery of the nanosensors after use. These nanowires can therefore be used as recoverable pH nanosensors. By changing the dye molecule to another molecule or receptor, the procedure described in the paper can be used to synthesize nanodevices for many different applications.