Strain modulated magnetocaloric effect in (111) oriented La0.7Sr0.3MnO3-SrRuO3 superlattices.

Magnetocaloric effect in (111)-oriented La0.7Sr0.3MnO3-SrRuO3 (LSMO-SRO) superlattices grown with both the stacking orders by reversing the individual layer thickness on (111)-oriented SrTiO3(STO) substrates using the pulsed laser deposition technique has been studied. Pseudomorphic growth with 0.64% in-plane tensile strain in [11 unit cell (u.c.)SRO/3u.c.LSMO]×15 superlattice is favourable for a larger change in entropy (ΔSM) as compared to relaxed growth with in-plane compressive strain in [11u.c.LSMO/3u.c.SRO]×15 superlattice. The reduction of ΔSM in [11u.c.LSMO/3u.c.SRO]×15 could be due to the orientation-dependent in-phase and out-of-phase tilt of the unit cell between ±1° along the 103pc of the {103}pc, which softens the exchange coupling and leads to the faster alignment of the magnetization near the Curie temperature (TC). Stabilization of the orthorhombic phase of LSMO in the superlattices with both stacking orders is evidenced from the existence of anomaly around the TC of LSMO and SRO in the temperature-dependent phonon frequency shifts. Reduction in symmetry of LSMO from the rhombohedral to orthorhombic structure modulates the Mn-O-Mn bond length and angles, which induces the spin reorientations and hence, modifies the electronic and magnetic properties in these LSMO-SRO superlattices. The ΔSM of these superlattices suggest that the strain, magnitude of the magnetic field, volume and magnetization of the ferromagnet can control the magnetocaloric effect. These results will be useful for designing the magnetic entropy based devices to improve renewable energy systems.

Metastable monoclinic [110] layered perovskite Dy2Ti2O7 thin films for ferroelectric applications.

Using the Combinatorial Substrate Epitaxy (CSE) approach, we report the stabilization of Dy2Ti2O7 epitaxial monoclinic, layered-perovskite phase Dy2Ti2O7 thin films. To achieve this, the films are deposited on high density, polished La2Ti2O7 polycrystalline ceramic substrates, which are stable as monoclinic layered-perovskites, and were prepared by conventional sintering. Microstructural analysis using electron backscatter diffraction (EBSD), electron diffraction (ED), and high-resolution transmission electron microscopy (HRTEM) support this observation. Further, they reveal that the cubic pyrochlore phase is observed far from the interface as films are grown thicker (100 nm), confirming the importance of substrate-induced phase and space group selection. This works reinforces the vast potential of CSE to promote the stabilization of metastable phases, thus giving access to new functional oxide materials, across a range of novel material systems including ferroelectrics.

Oxide thin films as bioactive coatings.

Growth and survival of biological cells (eukaryotes and prokaryotes) on artificial environments often depend on their interactions with the specific surface. Various organic materials can be coated on substrates to assist cells' adhesion and other subsequent cellular processes. However, these coatings are expensive, degrade over short time period, and may even interfere with the cells' signaling processes. Therefore, the use of inorganic surfaces in order to control cellular interactions is of scientific importance from fundamental and application perspectives. Among inorganic materials, oxide thin films have received considerable attention. Thin films of oxides have the advantage of tailoring the surfaces for cellular interactions while using a negligible amount of the oxide material. Here, we review the lesser known application of inorganic oxide coatings as biocompatible and implantable platforms for different purposes, such as biofilm inhibition, cell culture and implant enhancements.

Spin-flop and magnetodielectric reversal in Yb substituted GdMnO3.

The evolution of various spin structures in Yb doped GdMnO3 distorted orthorhombic perovskite system was investigated from their magnetic, dielectric and magnetodielectric characteristics. The Gd1-x Yb x MnO3 (0 ⩽ x ⩽ 0.15) revealed an enhanced magnetodielectric coupling when their magnetic structure is guided from ab to the bc-cycloidal spin structure upon Yb doping. The compounds exhibit magnetic field and temperature controlled spin-flop from c to a-axis. Additionally, magnetodielectric reversal is observed for the x = 0.1 sample which depends on both magnetic field and temperature. The resultant correlation between magnetic and electric orderings is discussed in the frame of symmetric and antisymmetric exchange interaction models. These findings provide further insight in understanding the magnetoelectric materials and importantly show a way to tune the magnetic and magnetodielectric properties towards better application potential.

Effect of La0.7Sr0.3MnO3 crystal structures on magnetization of (1 1 1) oriented La0.7Sr0.3MnO3-SrRuO3 superlattices.

A series of superlattices consisting of 15 bilayers of ferromagnetic La0.7Sr0.3MnO3 (LSMO) and SrRuO3 (SRO) were grown with either stacking order on (1 1 1) oriented SrTiO3 (STO) substrates using the pulsed laser deposition technique. The Raman spectra of these superlattices show the existence of rhombohedral and orthorhombic crystal structures of LSMO in (111)STO/[11-unit cell (u.c.) LSMO/n-u.c. SRO]X15 superlattices with n = 2 and 3. Interestingly, the Raman spectra of (1 1 1)STO/[11-u.c. SRO/n-u.c. LSMO]X15 superlattices with n = 2 and 3 show only the orthorhombic structure of LSMO. The (1 1 1)STO/[11-u.c. LSMO/n-u.c. SRO]X15 superlattices exhibit enhanced magnetization with weak antiferromagnetic coupling whereas reduced magnetization with strong antiferromagnetic coupling is observed in (1 1 1)STO/[11-u.c. SRO/n-u.c. LSMO]X15 superlattices. The observed magnetic properties of these superlattices can be explained by the interfacial structural coupling, as evident from their Raman spectra which suggest a modification in the stereochemistry of Mn at the interfaces.

Kondo effect goes anisotropic in vanadate oxide superlattices.

We study the transport properties in SrVO3/LaVO3 (SVO/LVO) superlattices deposited on SrTiO3 (STO) substrates. We show that the electronic conduction occurs in the metallic LVO layers with a galvanomagnetism typical of a 2D Fermi surface. In addition, a Kondo-like component appears in both the thermal variation of resistivity and the magnetoresistance. Surprisingly, in this system where the STO interface does not contribute to the measured conduction, the Kondo correction is strongly anisotropic. We show that the growth temperature allows a direct control of this contribution. Finally, the key role of vanadium mixed valency stabilized by oxygen vacancies is enlightened.

Structural analysis of strained LaVO3 thin films.

While structure refinement is routinely achieved for simple bulk materials, the accurate structural determination still poses challenges for thin films due on the one hand to the small amount of material deposited on the thicker substrate and, on the other hand, to the intricate epitaxial relationships that substantially complicate standard x-ray diffraction analysis. Using both electron and x-ray diffraction, we analyze the crystal structure of epitaxial LaVO3 thin films grown on (1 0 0)-oriented SrTiO3. Transmission electron microscopy study reveals that the thin films are epitaxially grown on SrTiO3 and points to the presence of 90° oriented domains. The mapping of the reciprocal space obtained by high resolution x-ray diffraction permits refinement of the lattice parameters. We finally deduce that strain accommodation imposes a monoclinic structure onto the LaVO3 film. The reciprocal space maps are numerically processed and the extracted data computed to refine the atomic positions, which are compared to those obtained using precession electron diffraction tomography.

Two components for one resistivity in LaVO3/SrTiO3 heterostructure.

A series of 100 nm LaVO3 thin films have been synthesized on (0 0 1)-oriented SrTiO3 substrates using the pulsed laser deposition technique, and the effects of growth temperature are analyzed. Transport properties reveal a large electronic mobility and a non-linear Hall effect at low temperature. In addition, a cross-over from a semiconducting state at high-temperature to a metallic state at low-temperature is observed, with a clear enhancement of the metallic character as the growth temperature increases. Optical absorption measurements combined with the two-bands analysis of the Hall effect show that the metallicity is induced by the diffusion of oxygen vacancies in the SrTiO3 substrate. These results allow us to understand that the film/substrate heterostructure behaves as an original semiconducting-metallic parallel resistor, and electronic transport properties are consistently explained.

Structure and magnetism of epitaxial PrVO3 films.

The interplay between charge, spin, orbital and lattice degrees of freedom in transition metal oxides has motivated extensive research aiming to understand the coupling phenomena in these multifunctional materials. Among them, rare earth vanadates are Mott insulators characterized by spin and orbital orderings strongly influenced by lattice distortions. Using epitaxial strain as a means to tailor the unit cell deformation, we report here on the first thin films of PrVO3 grown on (001)-oriented SrTiO3 substrate by pulsed laser deposition. An extensive structural characterization of the PrVO3 films, combining x-ray diffraction and high-resolution transmission electron microscopy studies, reveals the presence of oriented domains and a unit cell deformation tailored by the growth conditions. We have also investigated the physical properties of the PrVO3 films. We show that, while PrVO3 exhibits an insulating character, magnetic measurements indicate low-temperature hard-ferromagnetic behavior below 80 K. We discuss these properties in view of the thin-film structure.

Magnetocaloric effect and improved relative cooling power in (La(0.7)Sr(0.3)MnO(3)/SrRuO(3)) superlattices.

Magnetic properties of a series of (La(0.7)Sr(0.3)MnO(3)/SrRuO(3)) superlattices, where the SrRuO(3) layer thickness is varying, are examined. A room-temperature magnetocaloric effect is obtained owing to the finite size effect which reduces the T(C) of La(0.7)Sr(0.3)MnO(3) layers. While the working temperature ranges are enlarged, - ΔS(M)(max) values remain similar to the values in polycrystalline La(0.7)Sr(0.3)MnO(3). Consequently, the relative cooling powers are significantly improved, the microscopic mechanism of which is related to the effect of the interfaces at La(0.7)Sr(0.3)MnO(3)/SrRuO(3) and higher nanostructural disorder. This study indicates that artificial oxide superlattices/multilayers might provide an alternative pathway in searching for efficient room-temperature magnetic refrigerator for (nano) micro-scale systems.

Tunable magnetic interaction at the atomic scale in oxide heterostructures.

We report on a systematic study of a number of structurally identical but chemically distinct transition metal oxides in order to determine how the material-specific properties such as the composition and the strain affect the properties at the interface of heterostructures. Our study considers a series of structures containing two layers of ferromagnetic SrRuO3, with antiferromagnetic insulating manganites sandwiched in between. The results demonstrate how to control the strength and relative orientation of interfacial ferromagnetism in correlated electron materials by means of valence state variation and substrate-induced strain, respectively.

Polar properties of Eu(0.6)Y(0.4)MnO(3) ceramics and their magnetic field dependence.

Eu(1-x)Y(x)MnO(3), compared against other magnetoelectric systems, exhibits very distinctive features. Its magnetoelectric properties are driven by the magnetic spin of the Mn(3+) ion, but they can be drastically changed by varying the content of Y(3+), which does not carry any magnetic moment. Although the x = 0.40 composition has been studied extensively, some basic areas still remain to be thoroughly understood. Thus, this work is aimed at studying some of its polar properties and their magnetic field dependence as well. The experimental results reported here show that this material is very easily polarizable under external electric fields, and so, whenever the polarization is obtained from time integration of the displacement currents, an induced polarization is superposed on the spontaneous one, eventually masking the occurrence of ferroelectricity. We have found clear evidence for the influence of a magnetic field in the polar properties of Eu(0.6)Y(0.4)MnO(3). The study of electric polarization of Eu(0.6)Y(0.4)MnO(3) under an external magnetic field yields a value with the same order of magnitude of the remanent polarization as was determined from polarization reversal experiments. The comparison of the magnetically induced changes in the polarization obtained for polycrystalline samples and single crystals confirms the threshold magnetic field value for the polarization rotation from the a-direction to the c-direction, and provides evidence of the importance of the granular nature of the samples in the polar response to the magnetic field.

The Almeida-Thouless line in BiFeO(3): is bismuth ferrite a mean field spin glass?

Low-temperature magnetic properties of epitaxial BiFeO(3) (BFO) thin films grown on (111) SrTiO(3) substrates have been studied. Zero-field-cooled and field-cooled magnetization curves show a large discrepancy beginning at a characteristic temperature T(f) that is dependent on the magnetic field strength. T(f)(H) varies according to the well known de Almeida-Thouless line [Formula: see text] suggesting an acentric long-range spin-glass behavior and mean field system.

Electrical transport in the ferromagnetic state of manganites: small-polaron metallic conduction at low temperatures.

We report measurements of the resistivity in the ferromagnetic state of epitaxial thin films of La(1-x)Ca(x)MnO3 and the low-temperature specific heat of a polycrystalline La0.8Ca0.2MnO3. The resistivity below 100 K can be well fitted by rho-rho(0) = Eomega(s)/sinh (2)(hs/ 2k(B)T) with h(omega)(s)/k(B) approximately 80 K and E being a constant. Such behavior is consistent with small-polaron coherent motion which involves a relaxation due to a soft optical phonon mode that is strongly coupled to the carriers. The specific-heat data also suggest the existence of such a phonon mode. The present results thus provide evidence for small-polaron metallic conduction in the ferromagnetic state of manganites.