Anisotropic pinned/biased magnetization in SrRuO3/SrMnO3SrRuO_3/SrMnO_3 superlattices

The exchange coupling at the interfaces of magnetic superlattices consisting of ferromagnetic $SrRuO_3$ and antiferromagnetic $SrMnO_3$ grown on (001) oriented $SrTiO_3$ is studied with in-plane and out-of-plane orientations, with respect to the substrate plane, of the cooling magnetic field. The magnetization of the in-plane, field cooled hysteresis loop is lower than the corresponding in-plane zero-field-cooled hysteresis loop. The out-of-plane field cooled hysteresis loop is shifted, from the origin, along the graphical magnetization axis. We attribute this irreversible rotation of the moment to the pinning/biasing of spin in the $SrRuO_3$ layer in the vicinity of interfaces by the antiferromagnetic $SrMnO_3$ layer.Comment: To be published in Eur. Jour. Phys. B for a topical issue on "new trends in magnetic exchange bias phenomena

Effect of strain on the electrical transport and magnetization of the epitaxial Pr0.5Ca0.5MnO3/La0.5Ca0.5MnO3/ Pr0.5Ca0.5MnO3 trilayer structures

Epitaxial trilayer structures consisting of two antifferomagnetic charge/orbital order insulators Pr0.5Ca0.5MnO3 (PCMO) and La0.5Ca0.5MnO3 (LCMO) are grown on (001)-oriented SrTiO3 and LaAlO3 substrates. In this trilayer series, a thin film of LCMO with various thicknesses is sandwiched between the two fixed thicknesses of PCMO. These samples show a Curie temperature with a hysteretic field dependent magnetization at 10 K, although the individual compounds are antiferromagnetic. The zero field electronic transport of all samples on LAO shows thermally activated behavior, while the thermally activated behavior is suppressed and a metal-like transport is appearing for the samples on STO as the LCMO layer thickness increases above 10 unit cell. We have discussed these magnetic and transport properties of the trilayer structures on STO and LAO by the interfacial effect due to the stabilized CO state and the vibration mode of Jahn-Teller distortion.Comment: To be pubslised in Phys. Rev.

Direct observation of pinned/biased moments in magnetic superlattices

We report the \QTR{it}{pinned/biased} \QTR{it}{moment} in the superlattices consisting of ferromagnetic (FM) SrRuO$_3$ and antiferromgnetic (AFM) SrMnO$_3$. This superlattice system shows anisotropy and oriented pinning/biasing in the field-cooled (FC) hysteresis loop. The in-plane cooling-field provides antiferromagnetic orientations while out-of-plane cooling-field provides ferromagnetic orientations to the pinned/biased moments. The spacer layer thickness, strength and orientation of magnetic field, cooling field, and driving current influence the pinning strength. We propose that the magnetic structure is a repetition of $AFM/Pin/FM$($Free$)/$Pin$ unit below a critical field to explain its magnetic and transport properties. The transport behavior is discussed using the spin-dependent conduction.Comment: to be published in Physical Review

Electronic structure and optical band gap of CoFe2O4 thin films

Electronic structure and optical band gap of CoFe2O4 thin films grown on (001) oriented LaAlO3 have been investigated. Surprisingly, these films show additional Raman modes at room temperature as compared to a bulk spinel structure. The splitting of Raman modes is explained by considering the short-range ordering of Co and Fe cations in octahedral site of spinel structure. In addition, an expansion of band-gap is observed with the reduction of film thickness, which is explained by the quantum size effect and misfit dislocation. Such results provide interesting insights for the growth of spinel phases.Comment: 15 pages, 5 figures, and 1 table; Accepted and to be published/appeared in APL soo

Exchange coupling induced antiferromagnetic-ferromagnetic transition in Pr0.5Ca0.5MnO3/La0.5Ca0.5MnO3Pr_{0.5}Ca_{0.5}MnO_3/La_{0.5}Ca_{0.5}MnO_3 superlattices

Superlattices built from two antiferromagnetic (AFM) charge/orbital order compounds, $Pr_{0.5}Ca_{0.5}MnO_3$ and $La_{0.5}Ca_{0.5}MnO_3$, have been studied as the thickness of $La_{0.5}Ca_{0.5}MnO_3$ ($LCMO$) varied. High structural quality thin films were obtained on $LaAlO_3$ substrates using the pulsed laser deposition technique. An antiferromagnetic-to-ferromagnetic transition, in addition to an enhancement of the coercivity, are observed as the $LCMO$ layer thickness increases. The small shift in the origin of the field-cooled hysteresis loop along the field axis indicates the presence of ferromagnetic and antiferromagnetic phases in the superlattices. We attribute these features to the AFM spin fluctuations at the $Pr_{0.5}Ca_{0.5}MnO_3/La_{0.5}Ca_{0.5}MnO_3$ interfaces resulting from the strain effects.Comment: To be published in Phys. Rev.

Current-induced metallic behavior in Pr0.5_{0.5}Ca0.5_{0.5}MnO3_3 thin films: competition between Joule heating and nonlinear conduction mechanism

Thin films of Pr0.5Ca0.5MnO3 manganites exhibiting charge/orbital-ordered properties with colossal magnetoresistance have been synthesized by the pulsed laser deposition technique on both (100)-SrTiO3 and (100)-LaAlO3 substrates. The effects of current-induced metallic-behavior of the films are investigated as a function of the temperature and the magnetic field. Calculations based on a heat transfer model across the substrate, and our resistivity measurements reveal effects of Joule heating on charge transport over certain ranges of temperatures and magnetic fields. Our results also indicate that a nonlinear conduction, which cannot be explained by homogeneous Joule heating of the film, is observed when the material is less resistive (10-2 W.cm). The origin of this behavior is explained with a model based on local thermal instabilities associated with phase-separation mechanism and a change in the long range charge-ordered state.Comment: To be published in Phys. Rev.

Raman spectra and Magnetization of all-ferromagnetic superlattices grown on (110) oriented SrTiO3

Superlattices consist of two ferromagnets La0.7Sr0.3MnO3(LSMO)and SrRuO3(SRO) were grown in (110)-orientation on SrTiO3(STO) substrates. The x-ray diffraction and Raman spectra of these superlattices show the presence of in-plane compressive strain and orthorhombic structure of less than 4 u.c. thick LSMO spacer,respectively. Magnetic measurements reveal several features including reduced magnetization, enhanced coercivity, antiferromagnetic coupling, and switching from antiferromagnetic to ferromagnetic coupling with magnetic field orientations. These magnetic properties are explained by the observed orthorhombic structure of spacer LSMO in Raman scattering which occurs due to the modification in the stereochemistry of Mn at the interfaces of SRO and LSMO

Effect of electric/magnetic field on pinned/biased moments at the interfaces of magnetic superlattices

We have observed the pinned/biased moments in the superlattices consisting of ferromagnetic (FM) SrRuO3 (SRO) and antiferromagnetic (AFM) SrMnO3 (SMO)bilayer. The alternate stacking of SRO and SMO leading to a low field positive magnetoresistance with enhanced hysteretic field dependent magnetoresistance under the application of the out-of-plane magnetic field. We attribute these effects to the observed biased/pinned magnetic moments in the SRO layer in the vicinity of the interfaces. In addition, the biased/pinned moments can be oriented under the application of either the out-of-plane magnetic field or a combination of out-of-plane magnetic field and in-plane electric field. These results will bring new insights in the understanding of the coupling at the AFM/FM interface which can be useful for creating new exotic phenomena at the interfaces of the multilayer.Comment: To be published in Journal of Applied Physic

Magnetotransport properties of ferromagnetic Pr0.85Ca0.15MnO3/ferroelectric Ba0.6Sr0.4TiO3 superlattice films

Artificial superlattices designed with ferromagnetic Pr0.85Ca0.15MnO3 insulating layer and ferroelectric Ba0.6Sr0.4TiO3 layer were grown on (100) SrTiO3 substrates. The magnetotransport properties were measured with the current perpendicular to the plane geometry. An increase in magnetoresistance (MR), with no significant low field effect, was observed as the number of ferroelectric Ba0.6Sr0.4TiO3 layer thickness increases even up to 9 unit cells. For example, the superlattice [(Pr0.85Ca0.15MnO3)10(Ba0.6Sr0.4TiO3)9]25 shows 35 % MR at 100 K, though the Pr0.85Ca0.15MnO3 film was a robust insulator with negligible MR even at high applied magnetic field. This observed large MR cannot be explained by simple interfacial ferromagnetism or by the tunneling magnetoresistance. One possible explanation could be the effect due to the ferroelectric spacer layer and the associated magnetoelectric coupling.Comment: To be published in Appl. Phys. Let

EFFECTIVENESS OF EARLY WARNING MODELS: A CRITICAL REVIEW AND NEW AGENDA FOR FUTURE DIRECTION

This paper suggests a new agenda for constructing early warning models (EWMs) to enhance their effectiveness in predicting financial crises. The central argument of the new agenda aims to eradicate the weaknesses of existing EWMs, since their failure to predict the global financial crisis of 2007–2008 demonstrates the need to improve their efficiency. We document the history of EWMs and propose a new agenda as follows: 1) the accurate measurement of a financial crisis, 2) implementation of a fourthgeneration crisis model to capture the dynamic nature of the financial crisis, and 3) the inclusion of interconnectedness/contagion variables as explanatory variables for the financial crisis.This paper suggests a new agenda for constructing early warning models (EWMs) to enhance their effectiveness in predicting financial crises. The central argument of the new agenda aims to eradicate the weaknesses of existing EWMs, since their failure to predict the global financial crisis of 2007–2008 demonstrates the need to improve their efficiency. We document the history of EWMs and propose a new agenda as follows: 1) the accurate measurement of a financial crisis, 2) implementation of a fourthgeneration crisis model to capture the dynamic nature of the financial crisis, and 3) the inclusion of interconnectedness/contagion variables as explanatory variables for the financial crisis