K. R. Nikolaev

Cuprate/manganite heterostructures

Advances in thin film deposition techniques have made it possible to produce heterostructures of high-temperature superconducting compounds and manganite perovskites. The latter exhibit the phenomenon of colossal magnetoresistance (CMR). The half-metallic character of the CMR compounds results in their carriers being spin polarized. Experiments with these heterostructures have demonstrated that the injection of spin-polarized carriers into high-temperature superconductors results in a reduction of parameters such as the critical current and critical temperature. The experimental situation is reviewed, and open questions are identified.

Oscillatory Exchange Coupling and Positive Magnetoresistance in Epitaxial Oxide Heterostructures

Oscillation in the exchange coupling between ferromagnetic La(2/3)Ba(1/3)MnO3 layers with paramagnetic LaNiO3 spacer layer thickness has been observed in epitaxial heterostructures of the two oxides. This behavior is explained within the RKKY model employing an ab initio calculated band structure of LaNiO3, taking into account strong electron scattering in the spacer. Antiferromagnetically coupled superlattices exhibit a positive current-in-plane magnetoresistance.

Differential conductance of the ferromagnet/superconductor interface of DyBa2Cu3O7/La2/3Ba1/3MnO3 heterostructures

The voltage bias, temperature, and magnetic field dependence of the differential conductance of the interface between a half-metallic ferromagnet and a high-Tc superconductor have been studied in DyBa2Cu3O7/La2/3Ba1/3MnO3 heterostructures. Below the superconducting transition the differential conductance exhibits a dip at zero bias whose amplitude is a decreasing function of temperature and/or magnetic field. These results are interpreted qualitatively using a picture based on the suppression of Andreev reflection as a consequence of the high spin polarization of the carriers in the half-metallic ferromagnet.

Indications of antiferromagnetic interlayer coupling in La2/3Ba1/3MnO3/LaNiO3 multilayers

Multilayers consisting of the perovskite metallic oxides, ferromagnetic La2/3Ba1/3MnO3 (LBMO) and paramagnetic LaNiO3 (LNO) have been grown by ozone-assisted molecular beam epitaxy. Structural characterization using in situ reflection high-energy electron diffraction and high-resolution x-ray diffraction reveal a very high degree of crystalline order. Magnetization and hysteresis measurements show evidence of antiferromagnetic coupling between LBMO layers when the LNO spacer is 15 A or less in thickness.

Exchange-biased La2/3Ca1/3(Sr1/3)MnO3 ultrathin films

Exchange-induced unidirectional anisotropy in trilayers consisting of a thin conductive ferromagnetic layer of La2/3Ca1/3(Sr1/3)MnO3 sandwiched between two antiferromagnetic layers of La1/3Ca2/3MnO3 has been demonstrated through studies of magnetization, magnetoresistance, and in-plane anisotropy of magnetoresistance. The structures were grown by ozone-assisted molecular beam epitaxy and were characterized by a number of techniques. The possibility for the development of exchange-biased magnetic tunnel junctions using these nonclassic double-exchange ferromagnets is discussed.

Quantifying interlayer exchange coupling via layer-resolved hysteresis loops in antiferromagnetically coupled manganite/nickelate superlattices

In superlattices made of a half metallic ferromagnet La2/3Ba1/3MnO3(LBMO) and a metallic paramagnet LaNiO3(LNO), the field dependence of the LBMO magnetization was studied using depth- and element-sensitive x-ray resonant magnetic scattering measurements. The superlattices have ten bilayers of LBMO and LNO, and the LBMO layers were antiferromagnetically coupled across LNO spacer layers. From the x-ray measurements, the magnetic hysteresis loop of each LBMO layer was obtained, and subsequently the obtained layer-resolved LBMO hysteresis loops were utilized to determine the interlayer exchange coupling.

Exchange field induced magnetoresistance in colossal magnetoresistance manganites.

The effect of an exchange field on the electrical transport in thin films of metallic ferromagnetic manganites has been investigated. The exchange field was induced both by direct exchange coupling in a ferromagnet/antiferromagnet multilayer and by indirect exchange interaction in a ferromagnet/paramagnet metallic superlattice. The electrical resistance of the metallic manganite layers was found to be determined by the magnitude of the vector sum of the effective exchange field and the external magnetic field.

Spin Injection and Transport in Magnetic-Superconducting Oxide Heterostructures

Advances in thin film deposition techniques, in particular molecular beam epitaxy, have made it possible to produce bilayer heterostructures of high-temperature superconductors and manganite perovskites. The latter exhibit the phenomenon of colossal magnetoresistance (CMR). The half-metallic character of CMR compounds results in their carriers being spin polarized. The geometry of the structures that have been fabricated is such that it is possible to measure the interface conductance–voltage characteristic G(V) as well as the current–voltage characteristic of the superconducting half of the bilayer. Injection of carriers suppresses superconductivity in the latter, with a current gain of order unity. The data exhibit qualitative features of equilibrium theories of spin-polarized transport across an interface between a ferromagnet and an anisotropic superconductor, although a detailed understanding requires generalization of the theory to include out of equilibrium effects.

Temperature dependence of interlayer exchange coupling in manganite-based superlattices

We present a study of the temperature variation of the interlayer coupling strength of antiferromagnetically coupled La2/3Ba1/3MnO3/LaNiO3 heterostructures. No drastic decrease in the coupling strength was observed up to the Curie point of the manganite. This behavior suggests that the spin polarization of the carriers persists to high temperatures.

Spin injection and the interfacial conductance of ferromagnet–superconductor oxide heterostructures

Abstract Bilayers of epitaxially compatible superconducting DyBa2Cu3O7−x (DBCO) and ferromagnetic La2/3Ba1/3MnO3 (LBMO) have been grown using molecular beam epitaxy. In contrast with other investigations there was no buffer layer separating the ferromagnetic and superconducting layers. Structures were patterned into mesas for vertical transport measurements. Electrical leads permitted simultaneous study of the interface conductance and the effect on superconductivity of carrier injection. Conductance measurements exhibited features consistent with several aspects of calculations of spin-polarized transport between ferromagnetic and superconducting layers.