Transport properties, thermodynamic properties, and electronic structure of SrRuO3

Electron localization and magnetism in SrRuO3 with non-magnetic cation substitution

The destruction of the ferromagnetism of alloyed SrRuO(3) can be caused by electron localization at the substitution sites. Among all the non-magnetic cations that enter the B site, Zr(4+) is the least disruptive to conductivity and ferromagnetism. This is because Zr(4+) does not cause any charge disorder, and its empty d electron states which are poorly matched in energy with the Ru t(2g)(4) states cause the least resonance scattering of Ru's d electrons. Conducting Sr(Ru, Zr)O(3) may be used as an electrode for perovskite-based thin film devices, while its insulating counterpart provides unprecedented magnetoresistance, seldom seen in other non-manganite and non-cobaltite perovskites.

Electron backscattering diffraction and X-ray diffraction studies of interface relationships in Sr3Ru2O7/Sr2RuO4 eutectic crystals

Sr3Ru2O7/Sr2RuO4 eutectic system is investigated by electron backscattering diffraction (EBSD) and X-ray diffraction (XRD). The eutectic growth enables the solidification of the two phases in an ordered lamellar pattern extending along the growth direction, namely the b-axis direction. The eutectic material thus provides in the a-c plane two distinct interfaces having different microstructures with respect to the growth direction. Our analysis shows that, across the inplane c-axis direction (characterized by a poor lattice matching), the b-axis orientation is not constant at the individual interfaces, showing an orientation spread of about 5°. However, across the in-plane a-axis direction (characterized by a good lattice matching), the b-axis orientation does not change within a few tenths of degree (about 0.25°). Such information at nanoscale is also verified on a macroscopic level by standard XRD investigation.

First-principles investigation of the structural, magnetic and electronic properties of perovskite SrRu(1-x)Mn(x)O(3)

We have investigated the structural, magnetic and electronic properties of single-crystal SrRu(1-x)Mn(x)O(3), using first-principles density functional theory within the generalized gradient approximation (GGA)+U schemes. The entire series of SrRu(1-x)Mn(x)O(3) (x = 0, 0.25, 0.5 and 1) is stabilized in the single-crystal perovskite structure which is in agreement with experimental findings. Our spin-polarized calculations give a metallic ground state for the x<0.5 regime and an insulator ground state for the x≥0.5 regime. The magnetic structure for x = 0 is found to be the ferromagnetic state while the magnetic structures for 0<x<0.5 are the ferrimagnetic state where any Mn ions are coupled antiparallel to the Ru at the near sites. The magnetic structures for x≥0.5 are found to be the antiferromagnetic states. The substitution of itinerant Ru ions by localized Mn ions enhances the p-d coupling between O and the transition metal. It also strongly drives the system from the ferromagnetic metal to the antiferromagnetic insulator.

Itinerant electron magnetism in CaRu(1-x)Mn(x)O(3) (0 ≤ x ≤ 0.5)

The effect of Mn substitution in paramagnetic metal CaRuO(3) was studied by magnetization and neutron diffraction measurements. Development of ferromagnetic order is observed for x≥0.2 in CaRu(1-x)Mn(x)O(3). For the sample with x = 0.4, the Curie temperature of ∼160 K is obtained from the Arrott plot and the ratio of effective moment and saturation moment P(eff)/M(0) is estimated to be ∼4.8. We further found that the magnetization is significantly suppressed with decreasing temperature T below ∼90 K. In the neutron diffraction experiment at T = 15 K, we observed the evolution of a magnetic Bragg peak originating from the G-type antiferromagnetic order as well as the ferromagnetic one. This strongly suggests that both ferromagnetic and antiferromagnetic states are coexistent with each other at low temperatures. In the M(T)(0)(2) against T(2) plot (here, M(T)(0) is a spontaneous magnetization estimated from the Arrott plot), M(T)(0)(2) linearly increases with decreasing T(2) in the ferromagnetic region between ∼90 and 160 K. The ferromagnetic properties of the CaRu(1-x)Mn(x)O(3) system (x≤0.5) are well explained in terms of spin fluctuation theory based on the itinerant electron model rather than the localized spin model.

Orbital orderings and optical conductivity of SrRuO(3) and CaRuO(3): first-principles studies

The Jahn-Teller (JT) distortion induced orbital order and optical conductivity in SrRuO(3) and CaRuO(3) are investigated by first-principles calculations. The total energy and optical conductivity of all the spin ordering states of SrRuO(3) and CaRuO(3) are calculated in the LDA+U scheme with U(eff) = 2.5 eV. The down-spin t(2g) of Ru show antiferromagnetic-like orbital order in the a-b plane. We observe a d-d transition peak at an energy of about 1.0 eV in the calculated optical conductivity and analyze the reason for it not being observed in previous experiments.

High-pressure synthesis of the cubic perovskite BaRuO3 and evolution of ferromagnetism in ARuO3 (A = Ca, Sr, Ba) ruthenates

The cubic perovskite BaRuO(3) has been synthesized under 18 GPa at 1,000 degrees C. Rietveld refinement indicates that the new compound has a stretched Ru-O bond. The cubic perovskite BaRuO(3) remains metallic to 4 K and exhibits a ferromagnetic transition at T(c) = 60 K, which is significantly lower than the T(c) approximately = 160 K for SrRuO(3). The availability of cubic perovskite BaRuO(3) not only makes it possible to map out the evolution of magnetism in the whole series of ARuO(3) (A = Ca, Sr, Ba) as a function of the ionic size of the A-site r(A,) but also completes the polytypes of BaRuO(3). Extension of the plot of T(c) versus r(A) in perovskites ARuO(3) (A = Ca, Sr, Ba) shows that T(c) does not increase as the cubic structure is approached, but has a maximum for orthorhombic SrRuO(3). Suppressing T(c) by Ca and Ba doping in SrRuO(3) is distinguished by sharply different magnetic susceptibilities chi(T) of the paramagnetic phase. This distinction has been interpreted in the context of a Griffiths' phase on the (Ca Sr)RuO(3) side and bandwidth broadening on the (Sr,Ba)RuO(3) side.

Carrier-mediated magnetoelectricity in complex oxide heterostructures

Increasing demands for high-density, stable nanoscale memory elements, as well as fundamental discoveries in the field of spintronics, have led to renewed interest in exploring the coupling between magnetism and electric fields. Although conventional magnetoelectric routes often result in weak responses, there is considerable current research activity focused on identifying new mechanisms for magnetoelectric coupling. Here we demonstrate a linear magnetoelectric effect that arises from a carrier-mediated mechanism, and is a universal feature of the interface between a dielectric and a spin-polarized metal. Using first-principles density functional calculations, we illustrate this effect at the SrRuO3/SrTiO3 interface and describe its origin. To formally quantify the magnetic response of such an interface to an applied electric field, we introduce and define the concept of spin capacitance. In addition to its magnetoelectric and spin capacitive behaviour, the interface displays a spatial coexistence of magnetism and dielectric polarization, suggesting a route to a new type of interfacial multiferroic.

Local tunneling spectroscopy across a metamagnetic critical point in the bilayer ruthenate Sr3Ru2O7

The local spectroscopic signatures of metamagnetic criticality in Sr(3)Ru(2)O(7) were explored using scanning tunneling microscopy (STM). Singular features in the tunneling spectrum were found close to the Fermi level, as would be expected in a Stoner picture of itinerant electron metamagnetism. These features showed a pronounced magnetic field dependence across the metamagnetic critical point, which cannot be understood in terms of a naive Stoner theory. In addition, a pseudogap structure was observed over several tens of meV, accompanied by a c(2 x 2) superstructure in STM images. This result represents a new electronic ordering at the surface in the absence of any measurable surface reconstruction.

Orbital ordering and Jahn-Teller distortion in Perovskite ruthenate SrRuO3

Local density approximation plus on-site Coulomb interaction U band structure calculations reveal that SrRuO3 exhibits a half-metallic ground state with an integer spin moment of 2.0 microB/SrRuO3. An associated tilting 4dt2g orbital ordering on a Ru sublattice is observed under the on-site Coulomb interaction U in the presence of lattice distortion. This finding unravels the on-site Coulomb correlation as the driving force of the 4d orbital ordering and Jahn-Teller distortion as well as of the half-metallic ground state.

Sensitivity to disorder of the metallic state in the ruthenates

We report the results of transport measurements on SrRuO3, Sr3Ru2O7, and CaRuO3. In SrRuO3 and Sr3Ru2O7, our findings are consistent with the predictions of Fermi liquid theory, in contrast to previous reports based on samples with much shorter mean free paths. In CaRuO3, however, a T1.5 power law is seen in the resistivity in the high purity samples studied here. Our work gives concrete evidence that even the metallic state of the ruthenates is highly sensitive to disorder.

Origin of the non-fermi liquid behavior of SrRuO3

Motivated by the unusual features observed in the transport properties of the ferromagnetic "bad metal" SrRuO3, we construct a model incorporating essential features of the realistic structure of this nearly cubic material. In particular, we show how the t2g orbital orientation in the perfectly cubic structure determines the peculiar structure of the hybridization matrix, and demonstrate how the local non-Fermi liquid features arise when interactions are switched on. We discuss the effects of the slight deviation from the cubic structure qualitatively. The model provides a consistent explanation of the features observed recently in the optical response of SrRuO3.

Magnetic field-tuned quantum criticality in the metallic ruthenate Sr3Ru2O7

The concept of quantum criticality is proving to be central to attempts to understand the physics of strongly correlated electrons. Here, we argue that observations on the itinerant metamagnet Sr3Ru2O7 represent good evidence for a new class of quantum critical point, arising when the critical end point terminating a line of first-order transitions is depressed toward zero temperature. This is of interest both in its own right and because of the convenience of having a quantum critical point for which the tuning parameter is the magnetic field. The relationship between the resultant critical fluctuations and novel behavior very near the critical field is discussed.

Metamagnetism and critical fluctuations in high quality single crystals of the bilayer ruthenate Sr3Ru2O7

We report the results of low temperature transport, specific heat, and magnetization measurements on high quality single crystals of the bilayer perovskite Sr3Ru2O7, which is a close relative of the unconventional superconductor Sr2RuO4. Metamagnetism is observed, and transport and thermodynamic evidence for associated critical fluctuations is presented. These relatively unusual fluctuations might be pictured as variations in the Fermi surface topography itself.

Negative spin-polarization of SrRuO3

We have made Meservey-Tedrow style spin-polarized tunneling measurements on SrRuO3, using a SrTiO3 barrier. In contrast to all other materials measured with this technique, we found the spin polarization to be negative, P = -9.5%. This makes it possible to test the generalized Julliere model for a negatively spin-polarized electrode, by measuring the magnetoresistance of a La(0.7)Sr(0. 3)MnO(3)/SrTiO(3)/SrRuO(3) tunnel junction. The generalized Julliere theory predicts it should be inverse.

Domain wall resistivity in SrRuO3

SrRuO3 is an itinerant ferromagnet with T(c) approximately 150 K. When SrRuO3 is cooled through T(c) in zero applied magnetic field, a stripe domain structure appears whose orientation is uniquely determined by the large uniaxial magnetocrystalline anisotropy. We find that the ferromagnetic domain walls clearly enhance the resistivity of SrRuO3 and that the enhancement has different temperature dependence for currents parallel and perpendicular to the domain walls. We discuss possible interpretations of our results.