The competition between shape and perpendicular magnetic anisotropies in magnetic thin films give... more The competition between shape and perpendicular magnetic anisotropies in magnetic thin films gives rise to unusual magnetic behaviors. In ferromagnetic thin films, the presence of an out-of-plane component of the magnetic anisotropy may induce a transition from planar to stripe-like magnetic domains above a critical thickness, t c . In this article, we present a detailed study of the magnetization switching mechanism in FePt thin films, where this phenomenon is observed. Using micromagnetic simulations and experiments, we found that below t c the reversal mechanism is well described by the two-phase model while above this thickness the magnetization within each stripe reverses by coherent rotation. We also analyzed the out-of-plane component of the magnetic anisotropy and its temperature dependence, probing that substrate-induced strains are responsible for the abnormal coercive field behavior observed for FePt films with t > t c .
We have studied the thickness dependence of the transport and magnetic properties of strongly tex... more We have studied the thickness dependence of the transport and magnetic properties of strongly textured La 0.6 Sr 0.4 MnO 3 thin films grown on MgO and SrTiO 3 , for thickness between 5 and 500 nm. While the resistivity and the magnetoresistance are strongly affected, the magnetization is only moderately changed. The effect on the metal-insulator transition, the low field magnetoresistance and carrier localization is dependent on the substrate, evidencing the effect of the induced defects and strains.
ABSTRACT Gd2-xCexCuO oxides belong to a family of recently discovered electron doped cuprate supe... more ABSTRACT Gd2-xCexCuO oxides belong to a family of recently discovered electron doped cuprate superconductors. The undoped material Gd2CuO4 shows antiferromagnetic ordering of the Cu moments about room temperature. We report here ESR and static magnetization measurements made in ceramic samples which indicate a depression of the ordering temperature at a rate of 18 K/at%Ce, down to the Ce solubility limit of x∽0.16, where the material orders below 180 K. A similar rate of depression has been observed for a sample with Th substitution for Gd ions.
We calculate electronic transport properties of multilayered granular alloys, composed of discont... more We calculate electronic transport properties of multilayered granular alloys, composed of discontinuous Co layers embedded in Ag alternated with complete Ag layers. We focus our attention on the conductivity dependance on the shape and size of the clusters. The electronic structure is self-consistently calculated using a tight binding hamiltonian which includes a Hubbard term within the unrestricted Hartree–Fock approximation. We obtain different transport regimes depending on the growth conditions and transport direction. r 2002 Elsevier Science B.V. All rights reserved.
Size effects and their consequences in the thermal stability of the magnetization of the micro-si... more Size effects and their consequences in the thermal stability of the magnetization of the micro-sized MnAs/GaAs(100) ribbons were studied by magnetic force microscopy. We found out that the orientation in which size is reduced plays a key role in the magnetic configuration and thermal stability of the micro-sized patterns. On the one hand, when reducing the size in the [0001] a-MnAs direction, the system shows an improvement in the thermal stability of the remanent magnetization. On the other hand, when the size is reduced in the [11-20] a-MnAs direction, the system goes through a magnetic reconfiguration from in-plane magnetized to out-of-plane magnetized, which also improves the thermal stability. V
ABSTRACT We show that ferromagnetic/paramagnetic La0.75Sr0.25MnO3/LaNiO3 multilayers present an u... more ABSTRACT We show that ferromagnetic/paramagnetic La0.75Sr0.25MnO3/LaNiO3 multilayers present an unexpected magnetic exchange-bias effect (EBE), observed in field-cooled magnetization loops. The exchange-bias field and the enhancement of the coercivity vanish around 50 K. We demonstrate that the oxidation state of the Ni and Mn cations changes from Mn3+-Ni3+ to Mn4+-Ni2+ in the layers close to the interface probed by x-ray absorption spectroscopy measurements. The variation of the valence states is accompanied by a change in the magnetic behavior of the cations at the La0.75Sr0.25MnO3/LaNiO3 interface, possibly giving rise to the formation of magnetic or magnetically frustrated regions that may pin the ferromagnetic La0.75Sr0.25MnO3 layers and explain the EBE.
ABSTRACT In this contribution we present a study of the magnetic properties of MnAs thin films gr... more ABSTRACT In this contribution we present a study of the magnetic properties of MnAs thin films grown over GaAs(111) substrates, performed by ferromagnetic resonance experiments. We have calculated the magneto-crystalline anisotropy of the system analyzing the angular dependence of the resonance field with the applied field direction. The anisotropy measured for these films is much smaller than the calculated for MnAs bulk samples and MnAs/GaAs(100) thin films. We propose different microscopic mechanism for addressing the change in the anisotropy.
We report experimental data on the crystal field interaction of Gd ions with their environment in... more We report experimental data on the crystal field interaction of Gd ions with their environment in Gd/sub x/Eu/1-x/BaâCuâOâ. The ESR spectrum and a Schotiky anomaly in the specific heat of dilute samples indicate the existence of a crystal field splitting of the â¸Sââ ground state of the Gd/sup 3 +/ ions of about 1.5K. Since the single ion energies involved are of the same order of magnitude as the energies associated with the magnetic ordering of the Gd moments in GdBaâCuâOâ (T/sub N/ = 2.24K) we analyze the effects of the crystal field interaction on the magnetic transition. 10 refs., 4 figs.
We have studied the change in transport and magnetic properties of Co90Fe10/Ag multilayers with t... more We have studied the change in transport and magnetic properties of Co90Fe10/Ag multilayers with the magnetic layer and spacer thickness. By annealing or increasing the magnetic layer thickness, the character of the Co90Fe10 layers changes from a clustered-like to a continuous one.
The electronic conductivity of multilayered granular alloys of Co clusters embedded in Ag is calc... more The electronic conductivity of multilayered granular alloys of Co clusters embedded in Ag is calculated and analyzed around the topological percolation limit. The obtained results are in agreement with the main features of the available experimental data. In CIP geometry the conductivity increases when the percolated situation is reached as it does in the experiments, while in CPP we predict that it does not change.
We report normal and superconducting properties of Fe-doped YBa2Cu3O7−δ. Results from x-ray powde... more We report normal and superconducting properties of Fe-doped YBa2Cu3O7−δ. Results from x-ray powder diffraction, electrical resistivity, magnetic susceptibility, and ESR measurements are presented. The structural effects of doping are described and their correlation with the change in superconducting properties is discussed. Similarities with the superconducting properties of oxygen deficient YBa2Cu3O7−δ are analyzed. The comparison of the average distance between impurities
ABSTRACT We studied the transport and magnetic properties of low-doped manganite films after diff... more ABSTRACT We studied the transport and magnetic properties of low-doped manganite films after different oxygenation processes. The oxygen content was adjusted by postdeposition annealing at different oxygen pressures and annealing times. For all the samples we observed an increase in the Curie temperature and the remnant magnetization with the oxygen content. In general, for decreasing number of oxygen vacancies, samples under expansive strain become more homogeneous and their electrical resistivity decreases. A metal-insulator transition is induced in highly oxygenated films grown on SrTiO 3 , probably related to a shift of the mobility edge crossing below the Fermi energy. We found that the oxygenation dynamics depend critically on the strain field induced by the substrates and also on the Sr doping concentration.
Thin films of CoFe 2 O 4 were deposited on (1 0 0) Si and (1 0 0) MgO substrates by pulsed laser ... more Thin films of CoFe 2 O 4 were deposited on (1 0 0) Si and (1 0 0) MgO substrates by pulsed laser deposition (PLD). The X-ray analysis shows the existence of single-phase spinel structure. Thin films deposited on (1 0 0) MgO substrates are epitaxial and completely oriented in-plane and out of plane due to the small lattice mismatch between Co ferrite and MgO. The surface microstructure was probed by atomic force microscopy and we can describe it like a tidy mosaic of monocrystals. Surprisingly, the films grew on (1 0 0) Si using 355 nm, reveal a complete (1 1 1) orientation in spite of the native oxide of the substrate when deposited. The films deposited with 266 nm also were textured in the (1 1 1) but with less particulate on the surface. (1 0 0) films show at 35 K a perpendicular coercive field H c as high as 12.9 kOe, meanwhile for the (1 1 1) films H c was around 9 kOe. However, at room temperature, the (1 1 1) films deposited with 266 nm show a perpendicular coercive field of 5.1 kOe and a squareness of 0.86 which make them attractive for magneto-optic recording applications.
Nanoscale variation of antiferromagnetic and charge anisotropies has been found at manganite inte... more Nanoscale variation of antiferromagnetic and charge anisotropies has been found at manganite interfaces with an artificially created Sr-doping.
We report here experimental data on the crystal-field interaction of gadolinium ions with their e... more We report here experimental data on the crystal-field interaction of gadolinium ions with their environment in Gd Eul-, Ba2Cu307-&. The ESR spectrum and a Schottky anomaly in the specific heat of dilute samples (x«1) indicate the existence of a crystal-field splitting of the S7/2 ground state of Gd + ions of about 1.5 K. We discuss the parametrization of the measured splitting in terms of an effective Hamiltonian, and the microscopic origin of the interaction within the framework of a superposition model. Since the single-ion energies involved are of the same order of magnitude as the energies associated with the magnetic ordering of the Gd + moments in GdBa2Cu&07-s (T&~2.24 K), we analyze the effects of the crystal-field interaction on the magnetic transition.
The competition between shape and perpendicular magnetic anisotropies in magnetic thin films give... more The competition between shape and perpendicular magnetic anisotropies in magnetic thin films gives rise to unusual magnetic behaviors. In ferromagnetic thin films, the presence of an out-of-plane component of the magnetic anisotropy may induce a transition from planar to stripe-like magnetic domains above a critical thickness, t c . In this article, we present a detailed study of the magnetization switching mechanism in FePt thin films, where this phenomenon is observed. Using micromagnetic simulations and experiments, we found that below t c the reversal mechanism is well described by the two-phase model while above this thickness the magnetization within each stripe reverses by coherent rotation. We also analyzed the out-of-plane component of the magnetic anisotropy and its temperature dependence, probing that substrate-induced strains are responsible for the abnormal coercive field behavior observed for FePt films with t > t c .
We have studied the thickness dependence of the transport and magnetic properties of strongly tex... more We have studied the thickness dependence of the transport and magnetic properties of strongly textured La 0.6 Sr 0.4 MnO 3 thin films grown on MgO and SrTiO 3 , for thickness between 5 and 500 nm. While the resistivity and the magnetoresistance are strongly affected, the magnetization is only moderately changed. The effect on the metal-insulator transition, the low field magnetoresistance and carrier localization is dependent on the substrate, evidencing the effect of the induced defects and strains.
ABSTRACT Gd2-xCexCuO oxides belong to a family of recently discovered electron doped cuprate supe... more ABSTRACT Gd2-xCexCuO oxides belong to a family of recently discovered electron doped cuprate superconductors. The undoped material Gd2CuO4 shows antiferromagnetic ordering of the Cu moments about room temperature. We report here ESR and static magnetization measurements made in ceramic samples which indicate a depression of the ordering temperature at a rate of 18 K/at%Ce, down to the Ce solubility limit of x∽0.16, where the material orders below 180 K. A similar rate of depression has been observed for a sample with Th substitution for Gd ions.
We calculate electronic transport properties of multilayered granular alloys, composed of discont... more We calculate electronic transport properties of multilayered granular alloys, composed of discontinuous Co layers embedded in Ag alternated with complete Ag layers. We focus our attention on the conductivity dependance on the shape and size of the clusters. The electronic structure is self-consistently calculated using a tight binding hamiltonian which includes a Hubbard term within the unrestricted Hartree–Fock approximation. We obtain different transport regimes depending on the growth conditions and transport direction. r 2002 Elsevier Science B.V. All rights reserved.
Size effects and their consequences in the thermal stability of the magnetization of the micro-si... more Size effects and their consequences in the thermal stability of the magnetization of the micro-sized MnAs/GaAs(100) ribbons were studied by magnetic force microscopy. We found out that the orientation in which size is reduced plays a key role in the magnetic configuration and thermal stability of the micro-sized patterns. On the one hand, when reducing the size in the [0001] a-MnAs direction, the system shows an improvement in the thermal stability of the remanent magnetization. On the other hand, when the size is reduced in the [11-20] a-MnAs direction, the system goes through a magnetic reconfiguration from in-plane magnetized to out-of-plane magnetized, which also improves the thermal stability. V
ABSTRACT We show that ferromagnetic/paramagnetic La0.75Sr0.25MnO3/LaNiO3 multilayers present an u... more ABSTRACT We show that ferromagnetic/paramagnetic La0.75Sr0.25MnO3/LaNiO3 multilayers present an unexpected magnetic exchange-bias effect (EBE), observed in field-cooled magnetization loops. The exchange-bias field and the enhancement of the coercivity vanish around 50 K. We demonstrate that the oxidation state of the Ni and Mn cations changes from Mn3+-Ni3+ to Mn4+-Ni2+ in the layers close to the interface probed by x-ray absorption spectroscopy measurements. The variation of the valence states is accompanied by a change in the magnetic behavior of the cations at the La0.75Sr0.25MnO3/LaNiO3 interface, possibly giving rise to the formation of magnetic or magnetically frustrated regions that may pin the ferromagnetic La0.75Sr0.25MnO3 layers and explain the EBE.
ABSTRACT In this contribution we present a study of the magnetic properties of MnAs thin films gr... more ABSTRACT In this contribution we present a study of the magnetic properties of MnAs thin films grown over GaAs(111) substrates, performed by ferromagnetic resonance experiments. We have calculated the magneto-crystalline anisotropy of the system analyzing the angular dependence of the resonance field with the applied field direction. The anisotropy measured for these films is much smaller than the calculated for MnAs bulk samples and MnAs/GaAs(100) thin films. We propose different microscopic mechanism for addressing the change in the anisotropy.
We report experimental data on the crystal field interaction of Gd ions with their environment in... more We report experimental data on the crystal field interaction of Gd ions with their environment in Gd/sub x/Eu/1-x/BaâCuâOâ. The ESR spectrum and a Schotiky anomaly in the specific heat of dilute samples indicate the existence of a crystal field splitting of the â¸Sââ ground state of the Gd/sup 3 +/ ions of about 1.5K. Since the single ion energies involved are of the same order of magnitude as the energies associated with the magnetic ordering of the Gd moments in GdBaâCuâOâ (T/sub N/ = 2.24K) we analyze the effects of the crystal field interaction on the magnetic transition. 10 refs., 4 figs.
We have studied the change in transport and magnetic properties of Co90Fe10/Ag multilayers with t... more We have studied the change in transport and magnetic properties of Co90Fe10/Ag multilayers with the magnetic layer and spacer thickness. By annealing or increasing the magnetic layer thickness, the character of the Co90Fe10 layers changes from a clustered-like to a continuous one.
The electronic conductivity of multilayered granular alloys of Co clusters embedded in Ag is calc... more The electronic conductivity of multilayered granular alloys of Co clusters embedded in Ag is calculated and analyzed around the topological percolation limit. The obtained results are in agreement with the main features of the available experimental data. In CIP geometry the conductivity increases when the percolated situation is reached as it does in the experiments, while in CPP we predict that it does not change.
We report normal and superconducting properties of Fe-doped YBa2Cu3O7−δ. Results from x-ray powde... more We report normal and superconducting properties of Fe-doped YBa2Cu3O7−δ. Results from x-ray powder diffraction, electrical resistivity, magnetic susceptibility, and ESR measurements are presented. The structural effects of doping are described and their correlation with the change in superconducting properties is discussed. Similarities with the superconducting properties of oxygen deficient YBa2Cu3O7−δ are analyzed. The comparison of the average distance between impurities
ABSTRACT We studied the transport and magnetic properties of low-doped manganite films after diff... more ABSTRACT We studied the transport and magnetic properties of low-doped manganite films after different oxygenation processes. The oxygen content was adjusted by postdeposition annealing at different oxygen pressures and annealing times. For all the samples we observed an increase in the Curie temperature and the remnant magnetization with the oxygen content. In general, for decreasing number of oxygen vacancies, samples under expansive strain become more homogeneous and their electrical resistivity decreases. A metal-insulator transition is induced in highly oxygenated films grown on SrTiO 3 , probably related to a shift of the mobility edge crossing below the Fermi energy. We found that the oxygenation dynamics depend critically on the strain field induced by the substrates and also on the Sr doping concentration.
Thin films of CoFe 2 O 4 were deposited on (1 0 0) Si and (1 0 0) MgO substrates by pulsed laser ... more Thin films of CoFe 2 O 4 were deposited on (1 0 0) Si and (1 0 0) MgO substrates by pulsed laser deposition (PLD). The X-ray analysis shows the existence of single-phase spinel structure. Thin films deposited on (1 0 0) MgO substrates are epitaxial and completely oriented in-plane and out of plane due to the small lattice mismatch between Co ferrite and MgO. The surface microstructure was probed by atomic force microscopy and we can describe it like a tidy mosaic of monocrystals. Surprisingly, the films grew on (1 0 0) Si using 355 nm, reveal a complete (1 1 1) orientation in spite of the native oxide of the substrate when deposited. The films deposited with 266 nm also were textured in the (1 1 1) but with less particulate on the surface. (1 0 0) films show at 35 K a perpendicular coercive field H c as high as 12.9 kOe, meanwhile for the (1 1 1) films H c was around 9 kOe. However, at room temperature, the (1 1 1) films deposited with 266 nm show a perpendicular coercive field of 5.1 kOe and a squareness of 0.86 which make them attractive for magneto-optic recording applications.
Nanoscale variation of antiferromagnetic and charge anisotropies has been found at manganite inte... more Nanoscale variation of antiferromagnetic and charge anisotropies has been found at manganite interfaces with an artificially created Sr-doping.
We report here experimental data on the crystal-field interaction of gadolinium ions with their e... more We report here experimental data on the crystal-field interaction of gadolinium ions with their environment in Gd Eul-, Ba2Cu307-&. The ESR spectrum and a Schottky anomaly in the specific heat of dilute samples (x«1) indicate the existence of a crystal-field splitting of the S7/2 ground state of Gd + ions of about 1.5 K. We discuss the parametrization of the measured splitting in terms of an effective Hamiltonian, and the microscopic origin of the interaction within the framework of a superposition model. Since the single-ion energies involved are of the same order of magnitude as the energies associated with the magnetic ordering of the Gd + moments in GdBa2Cu&07-s (T&~2.24 K), we analyze the effects of the crystal-field interaction on the magnetic transition.
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Papers by Laura Steren