G. Campillo

Magnetism and superconductivity in La0.7Ca0.3MnO3/YBa2Cu3O7−δ superlattices

We report on the magnetic and superconducting properties of La0.7Ca0.3MnO3/YBa2Cu3O7 (LCMO/YBCO) superlattices. For a constant LCMO layer thickness of 6 unit cells (u.c.), resistance and susceptibility measurements show superconductivity for YBCO layer thickness in excess of 4 unit cells. The critical temperature increases with YBCO thickness, and a Tc of 58 K is found for a YBCO thickness of 10 unit cells. Magnetization measurements show a ferromagnetic transition at 100 K in a (LCMO6 u.c./YBCO5 u.c.)15 bilayer superlattice, and a depressed value of the saturation magnetization of 20 emu cm−3. These results are discussed in terms of interface disorder (analyzed by x-ray diffraction and transmission electron microscopy) and of the possible interaction between magnetism and superconductivity.

Quasiparticle-injection effects in YBa2Cu3O7-δ/La1/3Ca2/3MnO3/La2/3Ca1/3MnO3 heterostructures

We have grown YBa2Cu3O7-δ (40 nm)/La1/3Ca2/3MnO3 (2 nm)/La2/3Ca1/3MnO3 (30 nm) junctions for quasiparticle-injection measurements. Ferromagnetic (F), antiferromagnetic (AF), and superconducting (S) thin films were sequentially grown “in situ” using a dc-sputtering technique at 850 °C in pure oxygen at pressures around 3.5 mbar. The temperature dependence of the resistivity of the junction, without any current injection, presented coexistence between magnetism and superconductivity with a metallic insulating transition at 186 K, and zero resistance at 52 K. When current was injected from the F layer into the S layer, superconductivity suppression was obtained at temperatures below 52 K; furthermore, small values of current injection were necessary to change the electrical behavior up to the critical temperature. A negative current gain (G) has been observed and described by the relation G∝ατeff, where τeff is the relaxation time of the quasiparticle. The use of an AF layer as an insulating barrier is also ...

Structure of high-Tc/manganite perovskite superlattices

We study the crystalline and interface structure of RBa2Cu3O7/La1−xAxMnO3 superlattices (R=Y, Gd; A=Ca, Sr) grown on mismatched MgO substrates by means of nondestructive x-ray diffraction. The diffraction patterns are adjusted to a model of the structure which includes interface roughness and interdiffusion. Our results show that these disorder mechanisms cannot be neglected and should be taken into account to understand the physical properties of these superlattices.

Magnetotransport Properties in Epitaxial Exchange-Biased La

Superlattices of ferromagnetic (F) and antiferromagnetic (AF) oxide materials have attracted increased attention given the exchange bias or exchange anisotropy phenomenon. Research on magnetic oxide superlattices has focused toward the technological goals of increasing magnetoresistance in lower applied magnetic fields and more useful temperature ranges for various applications. Superlattices of F-LCMO and AF-LCMO layers were grown on (001)-oriented SrTiO3 substrates. We studied the temperature dependence of magnetotransport properties for a series of [AF-LCMO (7.6 nm)/F-LCMO (tF)]N superlattices as a function of the ferromagnetically doped layer thickness, tF, while the antiferromagnetic (AF) layer thickness, tAF, was kept constant. Magnetotransport measurements at different temperatures were done on a series of [AF(7.6 nm)/F(tF)]N superlattices. The ZFC resistance has a maximum at negative fields, for all multilayers, with descending fields (from 1 to -1 kOe). In all samples a shift along the field axis of the FC loop with respect to that of the ZFC loop is observed. The Hex values determined from these displacements of the FC loops of the magnetoresistance loops increase when temperature decreases and its magnitude also depends on the ferromagnetic layer thickness

scriptstyle_2/3

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.

Ca

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=...

scriptstyle_1/3

In this work, we report a study of the temperature dependence of magnetotransport properties for a series of [AF-LCMO (7.6nm)/F-LCMO (tF)]N superlattices. Superlattices of F-LCMO and AF-LCMO layers were grown on (001)-oriented SrTiO3 substrates via a high-pressure dc sputtering process. The modulation period have been experimentally derived from X-ray diffraction measurements, confirming that for all superlattices actual layer thicknesses were within 10% of the nominal values. A high resolution transmission electron microscopy micrograph of a superlattice showing sharp interfaces with interfacial roughness value around 0.38 nm. Atomic force microscopy (AFM) images of the 190 nm-total thickness multilayer show surface roughness below 10 nm. Several properties associated with CMR and exchange bias effects have been studied in F-LCMO/AF-LCMO multilayers.

MnO

Metal oxides show fascinating physical properties such as ferromagnetism, antiferromagnetism, high temperature superconductivity, ferroelectricity, or even multiferroicity. For many possible electronic applications as well as fundamental studies, it is essential to fabricate epitaxial layered films and multilayers of these materials having complex lattice structures with sharp interfaces, preserving epitaxiallity through the whole structure. We have grown these kind of oxide heterostructures on single crystal (001) oriented SrTiO3 substrates by using an in-situ DC sputtering technique at high oxygen pressures. Specifically, we report the study of magnetic and transport properties in ferromagnetic/antiferromagnetic, F/AF, heterostructures based on the Ca-doped lanthanum manganite system. We artificially grew La2/3Ca1/3MnO3/La1/3Ca2/3MnO3 heterostructures, maintaining constant the total thickness of the sample and systematically varying the thicknesses of the antiferromagnetic layer (tAF) and ferromagnetic layer (tF). Magnetization measurements indicate a dependence of Curie temperature, exchange bias field, and magnetoresistance behavior with the tAF/tF ratio.