State Of La0 Research Articles

Charge-exchange-driven interfacial antiferromagnetic ground state in La0.8Sr0.2MnO3 ultrathin films

The evolution of the magnetic ground state of ultrathin 0–10 unit cells (uc) thick La0.8Sr0.2MnO3 films interfaced to an antiferromagnetic La0.45Sr0.55MnO3/SrTiO3(001) buffer layer was investigated with x-ray photoemission electron microscopy. For 0–3 uc La0.8Sr0.2MnO3, we observe antiferromagnetic domains but no ferromagnetic contrast, showing that nominally ferromagnetic La0.8Sr0.2MnO3 adopts the antiferromagnetic ground state of the buffer layer. For larger thicknesses, ferromagnetic domains emerge, confirming that the additional layers revert to the ferromagnetic ground state. We also observe a drastic increase in the complexity of the domain configuration between 3 and 5 uc, which we attribute to competing magnetic and electronic ground states in the system. We attribute the interfacial modified magnetic ground state to charge sharing at the interface due to the chemical potential mismatch, which leads to hole doping at the La0.8Sr0.2MnO3 interface. The present work sheds light on the impact of charge sharing at the interface of complex oxide materials, in particular on the magnetic and electronic states, and presents a strategy for modulating the electronic ground state properties at metallic interfaces.

The Role of Phosphate Functionalization on the Oxygen Evolution Reaction Activity of Cobalt‐Based Oxides at Different pH Values

Cobalt‐based oxides have attracted attention as active electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. However, highly OER active catalysts at near‐neutral pHs are also desired for practical applications. Herein, a dry phosphate functionalization process is presented to enhance the OER activity of different cobalt‐based catalysts at near‐neutral pHs. Electrochemical evaluations show that the P‐functionalization can effectively improve the OER activity at near‐neutral pHs for La0.2Sr0.8CoO3–δ , La0.2Sr0.8Co0.8Fe0.2O3–δ , and CoO x catalysts, but not for La0.5Sr1.5CoO4–δ . Bulk and surface sensitive X‐ray absorption spectroscopy and X‐ray photoelectron spectroscopy unveil the influence of P incorporated in the order of ppm on the electronic state, local structure, and surface composition of the investigated catalysts. The P‐functionalization reduces the Co oxidation state in La0.2Sr0.8CoO3–δ and La0.5Sr1.5CoO4–δ , but the latter also presents significant Sr‐based segregations on the surface‐inhibiting OER activity at near‐neutral pHs. Differently, La0.2Sr0.8CoO3–δ , and to a lesser extent La0.2Sr0.8Co0.8Fe0.2O3–δ and CoO x , shows improved OER activity at neutral pH after the P‐functionalization. The findings disclose that P‐functionalization successfully enhances OER activity at near‐neutral pHs and that both phosphate ion assistance in the OER mechanism and catalyst Co oxidation state can play a role in the enhanced OER activity.

Asymmetric ground states in La0.67Sr0.33MnO3/BaTiO3 heterostructures induced by flexoelectric bending

Misfit strain delivered from single-crystal substrates typically modifies the ground states of transition metal oxides, generating increasing interest in designing modern transducers and sensors. Here, we demonstrate that magnetotransport properties of La0.67Sr0.33MnO3 (LSMO) films were continuously tuned by uniaxial strain produced by a home-designed bending jig. The electrical conductivity and Curie temperature of LSMO films are enhanced by bending stresses. The resistivity of u-shape bended LSMO decays three times faster than that of n-shape bended LSMO as a response to the same magnitude of strain. The asymmetric magnetic states in uniaxially strained LSMO are attributed to the dual actions of Jahn–Teller distortion and strain gradient mediated flexoelectric fields in an adjacent ferroelectric layer. These findings of multi-field regulation in a single material provide a feasible means for developing flexible electronic and spintronic devices.

Tuning ferroelectrics to antiferroelectrics in multiferroic LaxSr1−xFe12O19 ceramics

The combination of antiferroelectricity (AFE) and ferromagnetism (FM) in one structure would allow the development of new type of multiferroic candidates, which be applicable not only in magnetoelectric memories but also in novel energy storage devices. Here we propose a novel type of multiferroic candidate LaxSr1-xFe12O19, whose room temperature state could betuned from ferroelectrics (FE) to antiferroelectrics by changing x from 0 to 0.5. The emphasis of this paper will be focused on the La0.5Sr0.5Fe12O19 system, in which full AFE and FM coexist. The pure antiferroelectric behavior in La0.5Sr0.5Fe12O19 ceramics is demonstrated by double polarization-electric field (P-E) hysteresis loops, which are fully separated by a linear antiferroelectric AFE component with zero net polarization. The material of La0.2Sr0.7Fe12O19 with the intermediate composition exhibits a hybrid ferroelectric/antiferroelectric state. The recoverable energy density of the antiferroelectric La0.5Sr0.5Fe12O19 phase reaches 14.3 J/cm3. This material demonstrates strong magnetoelectric coupling and giant magnetoresistance (GMR) effect. A 1.1T magnetic field generates electronic polarization up to 0.95uC/cm2, reduce the resistance by 117%, enhances dielectric constants by 540% and right shifts the maximum dielectric loss peak by 208 kHz. The combined functional responses provide an opportunity to develop novel multifunctional electric and energy storage devices.

Magnetic Anisotropy Induced by Orbital Occupation States in La0.67Sr0.33MnO3 FilmsSupported by the National Key Research Program of China (Grant Nos. 2017YFA0206200, 2016YFA0300701, and 2018YFA0208402), the National Natural Science Foundation of China (Grant Nos. 11934017, 11874413, 11574376, and 51972333), the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No. XDB33030200), and the Youth Innovation Promotion Association of the Chinese

Interface engineering is an effective and feasible method to regulate the magnetic anisotropy of films by altering interfacial states between films. Using the technique of pulsed laser deposition, we prepared La0.67Sr0.33MnO3 (LSMO) and La0.67Sr0.33MnO3/SrCoO2.5 (LSMO/SCO) films on (110)-oriented La0.3Sr0.7Al0.65Ta0.35O3 substrates. By covering the SCO film above the LSMO film, we transformed the easy magnetization axis of LSMO from the [001] axis to the [] axis in the film plane. Based on statistical analyses, we find that the corresponding Mn–Mn ionic distances are different in the two types of LSMO films, causing different distortions of Mn–O octahedron in LSMO. In addition, it also induces diverse electronic occupation states in Mn3+ ions. The e g electron of Mn3+ occupies 3z 2 – r 2 and x 2 – y 2 orbitals in the LSMO and LSMO/SCO, respectively. We conclude that the electronic spin reorientation leads to the transformation of the easy magnetization axis in the LSMO films.

Advances in strain engineering on oxide heteroepitaxy

Advances in strain engineering on oxide heteroepitaxy

First-Principles study of the structural, electronic, magnetic, and ferroelectric properties of hypothetical La0.75Bi0.25Cr1-xFexO3 solid solutions

Calculations based on the density functional theory were conducted to identify the presence of multiferroic properties in a hypothetical La0.75Bi0.25Cr1-xFexO3 solid solution with x = 0.5 and 0.75. This substance was based on a combination of the ferroelectric state in La0.75Bi0.25CrO3. The magnetic properties were induced by substituting Cr3+ with Fe3+ occupying the B site. The ferromagnetic order was more stable in the La0.75Bi0.25Cr0.5Fe0.5O3 solid solution with a total magnetization MT = 15.89 μB/cell in the ground state. Conversely, a ferrimagnetic order prevailed in the La0.75Bi0.25Cr0.25Fe0.75O3 solid solution. The formation energy demonstrated that the substances were energetically stable with a high potential for synthesis. The band gap values of the substances were in a range of 2.11–3.24 eV, which was indicative of their potential application as magnetic semiconductors. The La0.75Bi0.25Cr0.5Fe0.5O3 and La0.75Bi0.25Cr0.25Fe0.75O3 solid solutions with orthorhombic structures exhibited ferroelectric properties with spontaneous polarization values of 11.43 μC/cm2 (along the [101] direction) and 9.02 μC/cm2 (along the [1¯ 01] direction), respectively. The coexistence of the ferroelectric and ferromagnetic (ferrimagnetic) orders in the La0.75Bi0.25Cr0.5Fe0.5O3 (La0.75Bi0.25Cr0.25Fe0.75O3) solid solution promoted the potential for coupling between the magnetization and polarization, giving these materials significant potential for use in magnetoelectric devices.

Iron oxidation state in La0.7Sr1.3Fe0.7Ti0.3O4 and La0.5Sr1.5Fe0.5Ti0.5O4 layered perovskites: Magnetic properties

La0.5Sr1.5Fe0.5Ti0.5O4 and La0.7Sr1.3Fe0.7Ti0.3O4 solid solutions with the layered perovskite structure were synthesized using a solid state method. Structural properties of obtained samples were characterized using X-ray diffraction and X-ray fluorescence analyses. Magnetic properties were investigated using magnetometry, electron spin resonance (ESR) and Mössbauer spectroscopy methods. Based on magnetization and ESR measurements it was suggested the presence of Fe4+ ions in addition to trivalent iron ions that was exactly confirmed by Mössbauer spectroscopy investigations. Based on all experimental results one can suggest the presence of the electronic phase separation in the investigated samples – the simultaneous existence of the paramagnetic phase and magnetically correlated regions, which form due to the mixed-valence iron ions. So the paramagnetic phase with strong antiferromagnetic correlation exists in both samples, while the second phase is ferromagnetically and ferrimagnetically correlated regions in La0.5Sr1.5Fe0.5Ti0.5O4 and La0.7Sr1.3Fe0.7Ti0.3O4, respectively.

Superparamagnetic state in La0.7Sr0.3MnO3 thin films obtained by rf-sputtering

A novel superparamagnetic state has been observed in high quality La0.7Sr0.3MnO3 (LSMO) thin films directly grown by rf-sputtering on SiOx/Si(100) substrates. The films are nanostructured without grain boundaries, constituted by locally epitaxial nanoregions grown layer-by-layer with out-of-plane (012) preferential orientation, induced by the constrain of the native silicon oxide. Low magnetic field ZFC-FC magnetization curves show a cross-over from superparamagnetic to ferromagnetic state dependent of the thickness. The thicker film (140 nm) exhibits typical ferromagnetic order. The thinner films (40 and 60 nm) exhibit superparamagnetic behavior attributed to interacting ferromagnetic monodomain nanoregions with critical size, random in-plane oriented, where the inter-monodomain boundaries with surface spin-glass structure regulate the blocking of magnetization depending on the magnetic field intensity. M(H) hysteresis loops showed noticeable coercive fields in all samples, larger than those reported for LSMO. Such properties of half-metal LSMO film foresee potential integration in new Si-technology nanodevices in Spintronics.

Effect of oxygen vacancies on electrical conductivity of La0.5Sr0.5FeO3−δ from first-principles calculations

Metal to semiconductor transition by hole compensation of excess electrons from VO and localized VO state in La0.5Sr0.5FeO3−δ under low PO2.

Interface Engineered Room-Temperature Ferromagnetic Insulating State in Ultrathin Manganite Films.

Ultrathin epitaxial films of ferromagnetic insulators (FMIs) with Curie temperatures near room temperature are critically needed for use in dissipationless quantum computation and spintronic devices. However, such materials are extremely rare. Here, a room‐temperature FMI is achieved in ultrathin La0.9Ba0.1MnO3 films grown on SrTiO3 substrates via an interface proximity effect. Detailed scanning transmission electron microscopy images clearly demonstrate that MnO6 octahedral rotations in La0.9Ba0.1MnO3 close to the interface are strongly suppressed. As determined from in situ X‐ray photoemission spectroscopy, O K‐edge X‐ray absorption spectroscopy, and density functional theory, the realization of the FMI state arises from a reduction of Mn eg bandwidth caused by the quenched MnO6 octahedral rotations. The emerging FMI state in La0.9Ba0.1MnO3 together with necessary coherent interface achieved with the perovskite substrate gives very high potential for future high performance electronic devices.

Generalized phonon density of states of La2−xSrxCuO4cuprate superconductor

Many body quantum dynamics of phonons is steadily developed by considering the various effects of anharmonicities, defects (consider as doping or impurity concentration) and electron–phonon interactions in model Hamiltonian (instead of BCS Hamiltonian) for a high-temperature superconductor (HTS). This enables to obtain the expressions for the renormalized phonon spectrum, the renormalized phonon density of states (RPDOS). The RPDOS can be resolved into diagonal and nondiagonal parts where the nondiagonal component is found highly impurity-dependent. Considering the suitable Born–Mayer–Huggins (BMH) interaction potential, the renormalized phonon spectrum, RPDOS and generalized phonon density of states (GPDOS) of the La[Formula: see text]Sr[Formula: see text]CuO4layered superconductor have been numerically analyzed and it was found that these quantities depend on doping concentration, anharmonicities, and temperature. The results are compared with the inelastic neutron scattering experimental data of GPDOS for La[Formula: see text]Sr[Formula: see text]CuO4and are found in good agreement. The ratio of deviation in GPDOS to GPDOS at critical temperature ([Formula: see text] K) shows the implicit difference at [Formula: see text]. The impact of defects, anharmonicities, and electron–phonon interactions in the cuprate superconductors virtually modify the scenario of GPDOS and affirm a large number of exotic peaks in the spectrum.

Noise Features of Metastable Resistivity States in La0.86Ca0.14MnO3 Manganite Single Crystal

We summarize the long-term electron transport and electronic noise investigations of low-doped La[Formula: see text]Ca[Formula: see text]MnO3 manganite single crystal being in its several, consecutive metastable resistivity states: the pristine high-resistivity one, the low-resistivity, and finally in the high-resistivity metastable state.

Investigation of Mn 3d derived states in La0.2Sr0.8MnO3

We investigate the Mn 3d derived states in La0.2Sr0.8MnO3 to understand the temperature dependent spectral weight transfer observed in the valence band spectra. Towards this, we have used DFT and DFT+U calculations and x-ray and Mn 3p to 3d resonant photoemission spectroscopic techniques. Our results show that the calculation gives better representation of the experimental valence band spectra for on site Coulomb interaction energy U = 5 eV. The room temperature resonant photoemission technique reveal that the valence band feature ∼2 eV is of mainly 3dn character ( is the ligand hole formed by ligand to metal charge transfer) and the feature ∼5 eV is predominantly of O 2p character. As the sample enters the low temperature tetragonal and insulating phase, the Mn 3d character of feature ∼2 eV increases and of feature ∼5 eV, it decreases.

Time-dependent Ginzburg–Landau model for light-induced superconductivity in the cuprate LESCO

Cavalleri and coworkers have discovered evidence of light-induced superconductivity and related phenomena in several different materials. Here, we suggest that some features may be naturally interpreted using a time-dependent Ginzburg–Landau model. In particular, we focus on the lifetime of the transient state in La1.675Eu0.2Sr0.125CuO4 (LESCO1∕8), which is remarkably long below about 25 K, but exhibits different behavior at higher temperature.

Griffiths-like phase and charge-spin glass state in La1.5Sr0.5CoO4

Hole-doped antiferromagnetic La2CoO4 has been known to possess intriguing electronic and magnetic phases due to the complex temperature and magnetic field dependent behavior of Co3+ ions. Here, we report on magnetic and dielectric observations in the half hole-doped La1.5Sr0.5CoO4. A Griffiths-like phase has been observed in the high temperature region below TG (∼244 K). At low temperature (∼55 K), a metamagnetic phase has been observed where antiferromagnetic correlations appear. On further cooling, a static weak antiferromagnetic ordering appears at ∼30 K, and a charge-spin glass state has been realized at ∼14 K.

Frequency-Resolved Evaluation of Electrode Oxidation State in La0.6Sr0.4CoO3 Thin Films with Operando XAS

Commercial adoption and implementation of renewable, but intermittent, energy sources relies on a new generation of high-performance energy conversion and storage devices, such as solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOEC). Performance of the oxygen electrode in SOFC is a limiting factor; however, improvements are hindered by inadequate understanding of electrochemical reaction and ionic diffusion processes. AC impedance and DC polarization techniques are typically used to infer relationships between driving energy and reaction rate, but uniquely understanding reaction mechanisms without direct chemical state measurements of the electrode is challenging. In recent years, workers have developed in operando X-ray absorption spectroscopy (XAS) to study oxygen chemical potentials in SOFC cathodes under operating temperatures and Po2. Determining the oxygen chemical potential is based on shifts in the X-ray absorption edge of 3d transition metals due to valence changes to compensate for oxygen vacancies. So far, operando XAS has only been measured under DC polarization, and thus lacked the ability to probe processes of different timescales [1]. In this work, we extend operando XAS to AC impedance measurements, allowing direct frequency-resolved measurement of electrode oxidation state. This approach can potentially be applied to nonlinear harmonic responses, and the first step in the spatially-resolved mapping of the local steady-periodic electrode response. A dense film of La0.6Sr0.4CoO3 (LSC) was deposited on a mirror polished Gd-doped ceria pellet by pulsed laser deposition as the working electrode with porous Pt counter and reference electrodes. Absorption spectra of the Co K-­edge were collected from fluorescent X-rays to determine the detection energy most sensitive to changes in Co valence state. Operando XAS measurements were conducted at 973 K in 10% O2 at the predetermined X-ray energy, typically around 7.718 keV, under electrical perturbations large enough to excite moderate nonlinearity. The frequency resolved oxidation state response of the electrode is obtained by converting X-ray absorption data to oxygen chemical potentials via sets of calibration XAS spectra at various effective Po2. As a result, the data can be treated by the same analysis methods developed for the voltage response in AC impedance measurements.

Path dependent magnetic states and evidence of kinetically arrested states in Nd doped LaFe11.5Al1.5

First order antiferromagnetic to ferromagnetic transition and path dependent magnetic states in La1−xNdxFe11.5Al1.5 for x∼0.1 are studied at low temperatures via powder x-ray diffraction, magnetization, and specific heat measurements. X-ray diffraction measurements suggest that around 8% of high temperature antiferromagnetic phase is converted to ferromagnetic phase at low temperatures in zero field cooling. A systematic study of temperature and magnetic field dependent magnetization measurements show a non-monotonic variation of upper critical field and re-entrant antiferromagnetic-ferromagnetic-antiferromagnetic transition while warming at an applied magnetic field under zero-field-cooled condition. This has been interpreted in the framework of kinetic arrest model for first order magnetic transition. It is also found that the antiferromagnetic phase is in the non-equilibrium state and behaves as a glass-like magnetic state at low temperatures. The specific heat in field-temperature space is studied and found to have a lower electronic contribution for the non-equilibrium antiferromagnetic state, compared to the equilibrium ferromagnetic state in this compound.

Electrical transport and magnetic properties of Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3 bilayer film on (001) SrTiO3

We have grown Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3 bilayer on (001) single crystal SrTiO3 substrate by pulsed laser deposition technique. The bilayer film is well (001) orientation grown with high crystalline quality. When the applied magnetic field increases up to 9 T, only metal-like transport behavior can be observed below room temperature. The Low-field MR (LFMR) as large as ~−73% has been achieved at an applied field of 1 T and the magnetoresistance (MR) can reach a value of ~−641% at an applied field of 9 T. Ferromagnetic (FM) clusters percolation accompanied with the collapse (melting) of charge-ordered (CO) state in La0.5Ca0.5MnO3 may be responsible for the enhancement of MR. Moreover, the disappearance of exchange bias (EB) in the bilayer demonstrates the existence of FM clusters percolation at the PSMO/LCMO interface. Large LFMR provides possibilities in developing manganite-based FM/CO film devices.

Persistent photoinduced modifications in the phase-separated states of La2−2xSr1+2xMn2O7

Persistent photoinduced modifications in the phase-separated states of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi mathvariant="normal">L</mml:mi><mml:msub><mml:mi mathvariant="normal">a</mml:mi><mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo><mml:mn>2</mml:mn><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:mi mathvariant="normal">S</mml:mi><mml:msub><mml:mi mathvariant="normal">r</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:mn>2</mml:mn><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:mi mathvariant="normal">M</mml:mi><mml:msub><mml:mi mathvariant="normal">n</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>7</mml:mn></mml:msub></mml:mrow></mml:math>