M. E. Gómez

Magnetoelectric coefficient in BiFeO3 compounds

In this work, we report magnetoelectric coefficient measurements in BiFeO3 (BFO) compounds using dynamic lock-in technique to evidence magnetoelectric coupling behavior. The response of the magnetoelectric coefficient shows rapid increase from low frequencies (0.1kHz) to around 5kHz, reaching a maximum, and then decreasing monotonously to 100kHz. For a constant frequency at 7kHz, the maximum magnetoelectric coefficient was close to 7mV∕cmOe, obtained at a bias magnetic field of H=120Oe. Thus, we demonstrated the feasibility of this technique for characterizing multiferroic materials.

First-order reversal curve measurements of the metal-insulator transition in VO 2 : Signatures of persistent metallic domains

We have performed first order reversal curve measurements of the temperature-driven metalinsulator transition in VO2 thin films, which enable quantitative analysis of the hysteresis behavior. An unexpected tail-like feature in the contour plot of the reversal curve distribution indicates the existence of metallic domains, even at temperatures below the closing of the hysteresis. These domains interact with the surrounding medium and change the reversal path relative to a path from a fully insulating state. With this in mind, and assuming that such interaction persist through the entire phase transition, we develop a model where the driving force (or energy barrier) in charge of opening a hysteresis in VO2 are inter-domain interactions. This model is intrinsically different from the Preisach model usually used to describe hysteresis; given that it looks for the microscopic origin of the hysteresis, and provides physical parameters to characterize it.

Formation of highly textured BiSrCaCuO superconducting thick films by a rapid melting process

Abstract We have prepared highly textured Biue5f8Srue5f8Caue5f8Cuue5f8O thick films from mixtures of the oxides by a rapid melting process. Sample powders on (100)-MgO-substrates were melted within seconds and kept between 910 °C and 1050 °C for typically several minutes. The samples were then directly cooled to room temperature. The crystallization of Bi2Sr2CaCu2Ox and the formation of a (001) - texture was observed in situ with a high-temperature X-ray diffractometer. Resistivity measurements showed the film to exhibit a Tc of 70 K. Subsequent annealing steps in air between 850 °C and 885 °C raised Tc to 85 K. The critical current density as inferred from magnetization measurements is at least 103 A/cm2. Morphology and composition of the films were investigated by scanning electron microscopy and electron diffraction.

In situ production of sputtered epitaxial HoBa2Cu3O7-x thin films

Abstract Epitaxial HoBa 2 Cu 3 O 7-x thin films (100–200 nm) were successfully prepared on SrTiO 3 (100), MgO (100), and LaAlO 3 substrates by single target DC sputtering. Pure oxygen gas at high pressure was used. Without any post-deposition annealing superconducting films were produced. The films deposited at substrate temperatures of about 750 C had a good crystalline structure with superconducting transitions above 90 K on all substrates. A critical current density above 10 5 A/cm 2 at 77 K was measured. The HoBa 2 Cu 3 O 7-x thin were characterized by X-ray diffraction, SEM and EDX measurements.

Fabrication and characterization of a diluted magnetic semiconducting TM co-doped Al:ZnO (TM=Co, Ni) thin films by sol-gel spin coating method.

Effect of transition metal oxides (TM=Co and Ni) co-doping on the crystallinity, surface morphology, grain growth and magnetic properties of nanostructure Al:ZnO thin films has been studied for diluted magnetic semiconductor applications. Al:ZnO thin films were fabricated by sol-gel spin coating on p-type Si (100) substrates. Fabrication of hexagonal wurtzite TM co-doped Al:ZnO thin films having thickness 2μm was successfully achieved. The Raman spectra of the TM co-doped Al:ZnO thin films showed a broad vibrational mode in the range 520-540cm(-1) due to crystal defects created co-doping elements in the ZnO host lattice. Scanning electron microscopy (SEM) revealed that the films are composed of uniform size, polycrystalline dense ZnO particles with defect free, smooth surfaces. The surface roughness was further verified with atomic force microscopy (AFM). The energy dispersive X-ray spectroscopic analysis (EDX) confirmed the stoichiometric compositions of the TM co-doped Al:ZnO films. The magnetic measurements exhibited that the Co, Al:ZnO and Ni, Al:ZnO thin films were ferromagnetic at room temperature.

In situ growth of superconducting Bi-Sr-Ca-Cu-O thin films by a high pressure sputtering process

Abstract High quality superconducting Bi-Sr-Ca-Cu-O thin films have successfully been prepared in situ on MgO(100) and SrTiO3(100), substrates using a high oxygen pressure (3–4 mbar) DC sputtering system. We used Bi2Sr2Ca1Cu2Ox and Bi1.7Pb0.3Sr2 single targets. The as-sputtered films showed a high degree of c-axis orientation on MgO substrates and an epitaxial growth on SrTiO3 substrates. Without any post-annealing treatment the main composition of the films corresponded to the 2212 phase with a Tconset of 85 K. The films were characterized by X-ray diffraction, SEM, EDX and RBS analysis.

Exchange bias and magnetic structure in modulation-doped manganite superlattices.

Superlattices of antiferromagnetic (AF) La1∕3Ca2∕3MnO3 and ferromagnetic (F) La2∕3Ca1∕3MnO3 were grown using sputtering technique. The AF layer thickness ranged between 3.9nm⩽tAF⩽15.6nm while the F layer thickness was kept constant (7.8 nm). Hysteresis measurements after field cooling, show an exchange bias up to a temperature of ∼100K. Surprisingly, the saturation magnetization increases with antiferromagnetic layer thickness and can exceed the expected bulk value for the La2∕3Ca1∕3MnO3 composition, whereas the interface coupling energy between the AF and F layers derived from the exchange bias is constant. This suggests that magnetization depth profile extends beyond the chemical doping interface.

Influence of ferromagnetic thickness on structural and magnetic properties of exchange-biased manganite superlattices

The structural and magnetic properties of superlattice structures of alternating ferromagnetic La2∕3Ca1∕3MnO3 (F-LCMO) and antiferromagnetic La1∕3Ca2∕3MnO3 (AF-LCMO) layers were systematically studied as functions of F-LCMO layer thickness, tF. Samples were grown via a high-oxygen pressure sputtering process. Magnetic hysteresis measurements after field cooling revealed an exchange bias, Hex, at low temperatures in such superlattices. We found a correlation of the structural and magnetic properties with tF. In particular, we observed diminished resistance, increased metal-insulator transition temperature, TMI, as well as increased Curie temperature with increasing tF. Additionally, we found that the temperature dependence of Hex*tF for superlattices with the same antiferromagnetic layer thickness, tAF, is a unique function and independent of tF. We also find that the low-temperature saturation magnetization, MS, follows a power-law dependence with temperature, according to M0(1−BTα) with an exponent of α=...

Superconducting properties of HoBa2Cu3O7/SrTiO3/HoBa2Cu3O7 heterostructures

Abstract We have produced HoBa 2 Cu 3 O 7 - x /SrTiO 3 /HoBa 2 Cu 3 O 7 - x heterostructures by a high pressure oxygen dc/rf sputtering technique on (001) SrTiO 3 substrates. The deposition conditions for HoBa 2 Cu 3 O 7 - x and SrTiO 3 are completely compatible. Cross-sectional transmission electron microscopy observations indicate abrupt HoBa 2 Cu 3 O 7 - x /SrTiO 3 interfaces with a 4.5 nm thick intermediate layer and virtually no disruption of the layered structure at the interface. X-ray diffraction measurements show that the HoBa 2 Cu 3 O 7 films grow with a well pronounced orientation (001) parallel to (001) SrTiO 3 . Both the top and the bottom HoBa 2 Cu 3 O 7 layers are superconductors above 90 K. These results indicate the potential for the growth of excellent SIS structures based on this heteroepitaxial system.

Surface Morphology Analysis of GaInAsSb Films Grown by Liquid Phase Epitaxy

We studied growth mechanisms in semiconducting Ga1 - xInxAsySb1 - y films grown by liquid phase epitaxy on (100) GaSb:Te (1017 cm - 3) substrates at 600 oC solution-substrate temperature. Atomic Force Microscopy (AFM) images of these films show step-like corrugations, undulations, grooves, and terraces. The terraces are a few nm in height and hundreds of nanometers in length. Epitaxial Ga1 - xInxAsySb1 - y films grow lattice-matched to the (100) GaSb substrates. These narrow-gap quaternary Ga1 - xInxAsySb1 - y compounds are suitable materials for heterostructure devices operating in the infrared wavelength range. The estimated x and y values are in the GaSb-rich zone. The step-like corrugations (bunching steps) are formed by different mechanisms, such as pinning effect probably due to impurities on the substrate surface.