Positive exchange bias in ferromagnetic La0.67Sr0.33MnO3 / SrRuO3 bilayers

Epitaxial La0.67Sr0.33MnO3 (LSMO)/ SrRuO3 (SRO) ferromagnetic bilayers have been grown on (001) SrTiO3 (STO) substrates by pulsed laser deposition with atomic layer control. We observe a shift in the magnetic hysteresis loop of the LSMO layer in the same direction as the applied biasing field (positive exchange bias). The effect is not present above the Curie temperature of the SRO layer (), and its magnitude increases rapidly as the temperature is lowered below . The direction of the shift is consistent with an antiferromagnetic exchange coupling between the ferromagnetic LSMO layer and the ferromagnetic SRO layer. We propose that atomic layer charge transfer modifies the electronic state at the interface, resulting in the observed antiferromagnetic interfacial exchange coupling.Comment: accepted to Applied Physics Letter

Electronic inhomogeneity at magnetic domain walls in strongly-correlated systems

We show that nano-scale variations of the order parameter in strongly-correlated systems can induce local spatial regions such as domain walls that exhibit electronic properties representative of a different, but nearby, part of the phase diagram. This is done by means of a Landau-Ginzburg analysis of a metallic ferromagnetic system near an antiferromagnetic phase boundary. The strong spin gradients at a wall between domains of different spin orientation drive the formation of a new type of domain wall, where the central core is an insulating antiferromagnet, and connects two metallic ferromagnetic domains. We calculate the charge transport properties of this wall, and find that its resistance is large enough to account for recent experimental results in colossal magnetoresistance materials. The technological implications of this finding for switchable magnetic media are discussed.Comment: Version submitted to Physical Review Letters, except for minor revisions to reference

Localization of Two-dimensional Electron Gas in LaAlO3/SrTiO3 Heterostructures

We report strong localization of 2D electron gas in LaAlO3 / SrTiO3 epitaxial thin-film heterostructures grown on (LaAlO3)0.3-(Sr2AlTaO3)0.7 substrates by using pulsed laser deposition with in-situ reflection high-energy electron diffraction. Using longitudinal and transverse magnetotransport measurements, we have determined that disorder at the interface influences the conduction behavior, and that increasing the carrier concentration by growing at lower oxygen partial pressure changes the conduction from strongly localized at low carrier concentration to metallic at higher carrier concentration, with indications of weak localization. We interpret this behavior in terms of a changing occupation of Ti 3d bands near the interface, each with a different spatial extent and susceptibility to localization by disorder, and differences in carrier confinement due to misfit strain and point defects.Comment: 12 pages, 4 figure

Conductance asymmetry in point-contacts on epitaxial thin films of Ba(Fe0.92_{0.92}Co0.08_{0.08})2_2As2_2

Point-contact spectroscopy is a powerful tool for probing superconductors. One of the most common observations in the point-contact spectra on the recently discovered ferropnictide superconductors is a large conductance asymmetry with respect to voltage across the point-contact. In this paper we show that the antisymmetric part of the point-contact spectrum between a silver tip and an epitaxial thin film of Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ shows certain unique features. These features have an interesting evolution with increasing temperature up to a temperature that is 30% larger than the critical temperature $T_c$ of the superconductor. We argue that this evolution can be associated with the rich normal state properties of these materials.Comment: 4 pages, 2 figure

Structure of the superconducting state in a fully frustrated wire network with dice lattice geometry

The superconducting state in a fully frustrated wire network with the dice lattice geometry is investigated in the vicinity of the transition temperature. Using Abrikosov's variational procedure, we write the Ginzburg-Landau free energy functional projected on its unstable supspace as an effective model on the triangular lattice of sixfold coordinated sites. For this latter model, we obtain a large class of degenerate equilibrium configurations in one to one correspondence with those previously constructed for the pure XY model on the maximally frustrated dice lattice. The entropy of these states is proportional to the linear size of the system. Finally we show that magnetic interactions between currents provide a degeneracy lifting mechanism.Comment: The final version (as published in Phys. Rev. B). Substantial corrections have been made to Sec.

Tailoring a two-dimensional electron gas at the LaAlO3/SrTiO3 (001) interface by epitaxial strain

Recently a metallic state was discovered at the interface between insulating oxides, most notably LaAlO3 and SrTiO3. Properties of this two-dimensional electron gas (2DEG) have attracted significant interest due to its potential applications in nanoelectronics. Control over this carrier density and mobility of the 2DEG is essential for applications of these novel systems, and may be achieved by epitaxial strain. However, despite the rich nature of strain effects on oxide materials properties, such as ferroelectricity, magnetism, and superconductivity, the relationship between the strain and electrical properties of the 2DEG at the LaAlO3/SrTiO3 heterointerface remains largely unexplored. Here, we use different lattice constant single crystal substrates to produce LaAlO3/SrTiO3 interfaces with controlled levels of biaxial epitaxial strain. We have found that tensile strained SrTiO3 destroys the conducting 2DEG, while compressively strained SrTiO3 retains the 2DEG, but with a carrier concentration reduced in comparison to the unstrained LaAlO3/SrTiO3 interface. We have also found that the critical LaAlO3 overlayer thickness for 2DEG formation increases with SrTiO3 compressive strain. Our first-principles calculations suggest that a strain-induced electric polarization in the SrTiO3 layer is responsible for this behavior. It is directed away from the interface and hence creates a negative polarization charge opposing that of the polar LaAlO3 layer. This both increases the critical thickness of the LaAlO3 layer, and reduces carrier concentration above the critical thickness, in agreement with our experimental results. Our findings suggest that epitaxial strain can be used to tailor 2DEGs properties of the LaAlO3/SrTiO3 heterointerface

Superconducting Phase with Fractional Vortices in the Frustrated Kagome Wire Network at f=1/2

In classical XY kagome antiferromagnets, there can be a novel low temperature phase where $\psi^3=e^{i3\theta}$ has quasi-long-range order but $\psi$ is disordered, as well as more conventional antiferromagnetic phases where $\psi$ is ordered in various possible patterns ($\theta$ is the angle of orientation of the spin). To investigate when these phases exist in a physical system, we study superconducting kagome wire networks in a transverse magnetic field when the magnetic flux through an elementary triangle is a half of a flux quantum. Within Ginzburg-Landau theory, we calculate the helicity moduli of each phase to estimate the Kosterlitz-Thouless (KT) transition temperatures. Then at the KT temperatures, we estimate the barriers to move vortices and effects that lift the large degeneracy in the possible $\psi$ patterns. The effects we have considered are inductive couplings, non-zero wire width, and the order-by-disorder effect due to thermal fluctuations. The first two effects prefer $q=0$ patterns while the last one selects a $\sqrt{3}\times\sqrt{3}$ pattern of supercurrents. Using the parameters of recent experiments, we conclude that at the KT temperature, the non-zero wire width effect dominates, which stabilizes a conventional superconducting phase with a $q=0$ current pattern. However, by adjusting the experimental parameters, for example by bending the wires a little, it appears that the novel $\psi^3$ superconducting phase can instead be stabilized. The barriers to vortex motion are low enough that the system can equilibrate into this phase.Comment: 30 pages including figure

Oxide two-dimensional electron gas with high mobility at room-temperature

The prospect of 2‐dimensional electron gases (2DEGs) possessing high mobility at room temperature in wide‐bandgap perovskite stannates is enticing for oxide electronics, particularly to realize transparent and high‐electron mobility transistors. Nonetheless only a small number of studies to date report 2DEGs in BaSnO(3)‐based heterostructures. Here, 2DEG formation at the LaScO(3)/BaSnO(3) (LSO/BSO) interface with a room‐temperature mobility of 60 cm(2) V(−1) s(−1) at a carrier concentration of 1.7 × 10(13) cm(–2) is reported. This is an order of magnitude higher mobility at room temperature than achieved in SrTiO(3)‐based 2DEGs. This is achieved by combining a thick BSO buffer layer with an ex situ high‐temperature treatment, which not only reduces the dislocation density but also produces a SnO(2)‐terminated atomically flat surface, followed by the growth of an overlying BSO/LSO interface. Using weak beam dark‐field transmission electron microscopy imaging and in‐line electron holography technique, a reduction of the threading dislocation density is revealed, and direct evidence for the spatial confinement of a 2DEG at the BSO/LSO interface is provided. This work opens a new pathway to explore the exciting physics of stannate‐based 2DEGs at application‐relevant temperatures for oxide nanoelectronics