J. A. Pardo

Microstructure and physical properties of some oxide eutectic composites processed by directional solidification

Abstract Eutectic composites of lamellar ZrO 2 –CaO and ZrO 2 –NiO and fibrous Al 2 O 3 –ZrO 2 , Al 2 O 3 –ZrO 2 (Y 2 O 3 ), ZrO 2 –MgO and CaF 2 –MgO wide gap materials have been grown from the melt by unidirectional solidification using laser floating zone and Bridgman techniques. The unique microstructure and interface morphology of these composites led to some remarkable mechanical (strength and toughness), optical (light guiding) and transport (ionic conduction) properties. The underlying relationships between microstructure and properties were briefly discussed in each case in the light of some possible applications of these eutectics.

Spectroscopic properties of Er3+ and Nd3+ doped glasses with the 0.8CaSiO3–0.2Ca3(PO4)2 eutectic composition

Abstract Rods of glass with the composition 56CaO–35SiO 2 –9P 2 O 5 (in mol%) were produced by the fast solidification of laser float zone melted precursors with the wollastonite (CS) and tricalcium phosphate (TCP) eutectic composition. This inverted glass with a high content of CaO modifier presents a high transparency optical window from 4 to 0.35 μm and is not hygroscopic. Its refractive index is n =1.65. The glass can be doped with rare-earth oxides up to relatively high doping levels without impurity crystallisation or aggregation effects. The Er 3+ and Nd 3+ absorption and emission spectra were measured at temperatures from 10 to 300 K for different doping concentrations between 0.07 and 4 wt%. The energy of the 4f levels was determined from the absorption and emission spectra. Judd–Ofelt intensity parameters were calculated and compared with those of other glass systems. The Nd 3+ emissions from the 4 F 3/2 level were studied in detail. The lifetime in the less concentrated sample at 300 K is 330 μs and the peak cross-section of the 1.062 μm emission is 2.2×10 −20 cm 2 . The decay time of the Er 3+ emission at 1.539 μm is 7.75 ms and the emission cross-section 0.6×10 −20 cm 2 . These values are well retained up to the highest doping levels.

Magnetic properties of Fe/MgO granular multilayers prepared by pulsed laser deposition

Granular multilayers [Fe(tnm)∕MgO(3nm)]N with 0.4nm⩽t⩽1.5nm were prepared by sequential pulsed laser deposition. Transmission electron microscopy (TEM) images show that increasing t causes the growth of the sizes of Fe nanoparticles and broadening of the particle size distribution. For t>0.81nm, continuous Fe layers are formed. The evolution of the shapes and sizes of the particles is reflected in the magnetic properties of the investigated films. A crossover from superparamagnetic to ferromagnetic behavior upon formation of a continuous Fe layer is observed. The fit of zero field cooled and field cooled susceptibility measurements and magnetization curves using Curie–Weiss law and a weighted sum of Langevin functions, respectively, allows the estimation of the average granule size for the films with t<0.61nm. The results of the estimations correlate with the data obtained from TEM images. Reduction of saturation magnetization for Fe nanoparticles and an increase of the coercivity up to 1200Oe at low temp...

High-field Hall effect and magnetoresistance in Fe3O4 epitaxial thin films up to 30 Tesla

We have measured the Hall effect and the magnetoresistance of epitaxial Fe3O4 thin films grown on MgO (001) in magnetic fields up to 30 T. Using such high fields, it is possible to magnetically saturate films thicker than 40 nm, providing access to intrinsic conduction properties. We find an effective electron density corresponding to 1 electron per f.u. A smaller value is obtained for thinner films, caused by the increasing density of antiphase boundaries defects. The magnetoresistance is not saturating at 30 T, showing linear dependence at high fields, and peaks at the Verwey transition.

Tunneling magnetoresistance in Fe/MgO granular multilayers

We have investigated the transport and magnetotransport properties in Fe/MgO multilayers around the Fe percolation threshold as a function of the temperature and the nominal thickness of iron layer (t). Electrical resistivity measurements allowed us to disclose the charge transport mechanisms involved, which are closely related to the degree of discontinuity in the Fe layers. The samples with Fe thickness below percolation threshold (t∼0.8 nm) exhibit isotropic magnetoresistance (MR), which can be understood considering spin-polarized electron tunneling between nanometer-sized, superparamagnetic Fe grains. The MR ratio increases with decreasing temperature from ∼3% at room temperature to ∼10% at 30 K. The temperature dependence of MR can be explained satisfactorily in terms of a modified Mitani’s model.

Polar-Graded Multiferroic SrMnO3 Thin Films

Engineering defects and strains in oxides provides a promising route for the quest of thin film materials with coexisting ferroic orders, multiferroics, with efficient magnetoelectric coupling at room temperature. Precise control of the strain gradient would enable custom tailoring of the multiferroic properties but presently remains challenging. Here we explore the existence of a polar-graded state in epitaxially strained antiferromagnetic SrMnO3 thin films, whose polar nature was predicted theoretically and recently demonstrated experimentally. By means of aberration-corrected scanning transmission electron microscopy we map the polar rotation of the ferroelectric polarization with atomic resolution, both far from and near the domain walls, and find flexoelectricity resulting from vertical strain gradients. The origin of this particular strain state is a gradual distribution of oxygen vacancies across the film thickness, according to electron energy loss spectroscopy. Herein we present a chemistry-mediated route to induce polar rotations in oxygen-deficient multiferroic films, resulting in flexoelectric polar rotations and with potentially enhanced piezoelectricity.

Determination of the percolation threshold in Fe/MgO magnetic granular multilayers.

The evolution of the morphology, magnetic and transport properties of Fe(t nm)/MgO(3.0 nm) multilayers with respect to the nominal metallic layer thickness was investigated. A comparison with existing experimental data on discontinuous metal-insulator multilayers, ultrathin epitaxial Fe films on MgO substrates and granular cermet films is made. It is confirmed that the deposition conditions and the material composition play a crucial role in the percolation process. Nominal thicknesses of Fe layers at which an infinite metallic cluster is formed and the conditions for continuous Fe coverage were determined. Different methods of percolation threshold detection were analysed. We show that investigation of the temperature dependence of resistance in nanostructures could lead to an overestimation of the percolation threshold value, while magnetic measurements alone could lead to its underestimation.

Microstructure-size dependence of the 1.520 μm Er3+ luminescence lifetime in Al2O3-ZrO2 eutectic melt growth composites

The radiative lifetime of the 4I13/2 level of Er3+ was modified up to 15% by changing the microstructure in laser-floating-zone-grown Er3+-doped Al2O3–ZrO2 eutectic melt growth composites. Erbium enters only the higher-refractive-index ZrO2 phase, which occupies 30% of the volume of the sample. The lifetimes of 4S3/2 (545 nm) and 4I13/2 (1520 nm) Er3+ levels were measured at 10 K and room temperature in different microstructures. The variation in the lifetime of this last level with interphase spacing is due to the change of the available electromagnetic modes at 1520 nm due to the change in the optical environment of the luminescent ion.

Epitaxial stabilization of the perovskite phase in (Sr1-xBax)MnO3 thin films

A novel mechanism of ferroelectricity driven by off-centering magnetic Mn(4+) ions was proposed in (Sr1-xBax)MnO3, in its ideal perovskite phase, which yields enormous expectations in the search for strong magnetoelectric materials. Still, the desired perovskite phase has never been stabilized in thin films due to its extremely metastable character. Here, we report on a thorough study of the perovskite phase stabilization of (Sr1-xBax)MnO3 thin films, 0.2 ≤ x ≤ 0.5, grown by pulsed laser deposition onto (001)-oriented perovskite substrates. X-ray diffraction measurements and scanning transmission electron microscopy reveal that, under appropriate deposition conditions, the perovskite phase is fully stabilized over the nonferroelectric hexagonal phase, despite the latter being increasingly favored on increasing Ba-content. Moreover, we have managed to grow epitaxial coherent cube-on-cube (Sr1-xBax)MnO3 films upon strains ranging from 0% to 4%. Our results become a milestone in further studying perovskite (Sr1-xBax)MnO3 thin films and pave the way for tailoring ferroic and magnetoelectric properties either by strain engineering or Ba-doping.

Tunneling magnetoresistance in epitaxial discontinuous Fe/MgO multilayers

Epitaxial discontinuous Fe/MgO multilayers have been grown by pulsed laser deposition on MgO(001) single-crystal substrates. The multilayers with 0.6 nm nominal Fe layers thickness are superparamagnetic and demonstrate tunneling magnetoresistance (TMR) in the current-in-plane configuration. Increasing deposition temperature causes an improvement in crystal quality and is accompanied by higher TMR ratios. The maximum value (9.2% TMR at room temperature and 18 kOe magnetic field) trebles that of polycrystalline samples deposited simultaneously on glass substrates. A model formula for TMR ratio that includes both spin-dependent tunneling and spin-filtering effect is proposed to explain these results.