Tensile-Strain-Dependent Spin States in Epitaxial LaCoO_{3} Thin Films

Manipulating the Spin State of Spinel Octahedral Sites via a π-π Type Orbital Coupling to Boost Water Oxidation

Spin state is often regarded as the crucial valve to release the reactivity of energy-related catalysts, yet it is also challenging to precisely manipulate, especially for the active center ions occupied at the specific geometric sites. Herein, a π-π type orbital coupling of 3d (Co)-2p (O)-4f (Ce) was employed to regulate the spin state of octahedral cobalt sites (CoOh) in the composite of Co3O4/CeO2. More specifically, the equivalent high-spin ratio of CoOh can reach to 54.7 % via tuning the CeO2 content, thereby triggering the average eg filling (1.094) close to the theoretical optimum value. The corresponding catalyst exhibits a superior water oxidation performance with an overpotential of 251 mV at 10 mA cm-2, rivaling most cobalt-based oxides state-of-the-art. The π-π type coupling corroborated by the matched energy levels between Ce t1u/t2u-O and CoOh t2g-O π type bond in the calculated crystal orbital Hamilton population and partial density of states profiles, stimulates a π-donation between O 2p and π-symmetric Ce 4fyz 2 orbital, consequently facilitating the electrons hopping from t2g to eg orbital of CoOh. This work offers an in-depth insight into understanding the 4f and 3d orbital coupling for spin state optimization in composite oxides.

Spin State Disproportionation in Insulating Ferromagnetic LaCoO3 Epitaxial Thin Films

The origin of insulating ferromagnetism in epitaxial LaCoO3 films under tensile strain remains elusive despite extensive research efforts are devoted. Surprisingly, the spin state of its Co ions, the main parameter of its ferromagnetism, is still to be determined. Here, the spin state in epitaxial LaCoO3 thin films is systematically investigated to clarify the mechanism of strain-induced ferromagnetism using element-specific X-ray absorption spectroscopy and dichroism. Combining with the configuration interaction cluster calculations, it is unambiguously demonstrated that Co3+ in LaCoO3 films under compressive strain (on LaAlO3 substrate) is practically a low-spin state, whereas Co3+ in LaCoO3 films under tensile strain (on SrTiO3 substrate) have mixed high-spin and low-spin states with a ratio close to 1:3. From the identification of this spin state ratio, it is inferred that the dark strips observed by high-resolution scanning transmission electron microscopy indicate the position of Co3+ high-spin state, i.e., an observation of a spin state disproportionation in tensile-strained LaCoO3 films. This consequently explains the nature of ferromagnetism in LaCoO3 films. The study highlights the importance of spin state degrees of freedom, along with thin-film strain engineering, in creating new physical properties that do not exist in bulk materials.

Positive temperature coefficient of the thermal conductivity above room temperature in a perovskite cobaltite

The thermal conductivity above room temperature is investigated for LaCoO₃-based materials showing spin-state and insulator-metal crossovers. A positive temperature coefficient (PTC) of the thermal conductivity is observed during the insulator-metal crossover around 500 K. Our analysis indicates that the phononic thermal transport is also enhanced in addition to the electronic contribution as the insulator-metal crossover takes place. The enhancement of the phononic component is ascribed to the reduction of the incoherent local lattice distortion coupled with the spin/orbital state of each Co³⁺ ion, which is induced by the enhanced spin-state fluctuation between low and excited spin-states. Moreover, fine tunability for the PTC of the thermal conductivity is demonstrated via doping hole-type carriers into LaCoO₃. The observed enhancement ratio of the thermal conductivity κ _T (773 K) / κ _T (323 K) = 2.6 in La_0.95Sr_0.05CoO₃ is the largest value among oxide materials which exhibit a PTC of their thermal conductivity above room temperature. The thermal rectification ratio is estimated to reach 61% for a hypothetical thermal diode consisting of La_0.95Sr_0.05CoO₃ and LaGaO₃, the latter of which is a typical band insulator. These results indicate that utilizing spin-state and orbital degrees of freedom in strongly correlated materials is a useful strategy for tuning thermal transport properties, especially for designing thermal diodes.

Creating a Ferromagnetic Ground State with Tc Above Room Temperature in a Paramagnetic Alloy through Non-Equilibrium Nanostructuring

Materials with strong magnetostructural coupling have complex energy landscapes featuring multiple local ground states, thus making it possible to switch among distinct magnetic-electronic properties. However, these energy minima are rarely accessible by a mere application of an external stimuli to the system in equilibrium state. A ferromagnetic ground state, with T_c above room temperature, can be created in an initially paramagnetic alloy by nonequilibrium nanostructuring. By a dealloying process, bulk chemically disordered FeRh alloys are transformed into a nanoporous structure with the topology of a few nanometer-sized ligaments and nodes. Magnetometry and Mössbauer spectroscopy reveal the coexistence of two magnetic ground states, a conventional low-temperature spin-glass and a hitherto-unknown robust ferromagnetic phase. The emergence of the ferromagnetic phase is validated by density functional theory calculations showing that local tetragonal distortion induced by surface stress favors ferromagnetic ordering. The study provides a means for reaching conventionally inaccessible magnetic states, resulting in a complete on/off ferromagnetic-paramagnetic switching over a broad temperature range.

Two-dimensional magnetic interplay in the tensile-strained LaCoO3 thin films

High-quality epitaxial LaCoO₃ (LCO) thin films have been deposited on SrTiO₃ (STO) substrates with pulsed laser deposition (PLD). We find that the LCO films undergo a typical ferromagnetic-paramagnetic (FM-PM) phase transition at ∼80 K. To understand the nature of magnetic phase transition, various methods, including the modified Arrott plot and critical isotherm analysis, were used to determine the critical exponents, which are β = 0.754(1) with a Curie temperature T_C = 79.8(8) K and γ = 1.52(2) with T_C = 79.9(2) K. The reliability of these critical exponents was confirmed using the Widom scaling relation and the scaling hypothesis. Further analysis revealed that the spin coupling within the LCO films exhibits two-dimensional (2D) long-range magnetic interaction and the magnetic exchange distance decays as J(r) ∼r^-(3.46). Our critical behavior analysis may shed new light on the further understanding of the origin of FM and the relatively fixed T_C in LCO thin films.

Two Spin-State Crystallizations in LaCoO_{3}

We report a magnetostriction study of a perovskite LaCoO_{3} above 100 T using our state-of-the-art strain gauge to investigate an interplay between electron correlation and spin crossover. There has been a controversy regarding whether two novel phases in LaCoO_{3} at high magnetic fields result from crystallizations or Bose-Einstein condensation during spin crossover as manifestations of localization and delocalization in spin states, respectively. We show that both phases are crystallizations rather than condensations, and the two crystallizations are different, based on the observations that the two phases exhibit as magnetostriction plateaux with distinct heights. The crystallizations of spin states have emerged manifesting the localizations and interactions in spin crossover with large and cooperative lattice changes.

Vertical Strain-Driven Antiferromagnetic to Ferromagnetic Phase Transition in EuTiO3 Nanocomposite Thin Films

Three-dimensional (3D) strain induced in self-assembled vertically aligned nanocomposite (VAN) epitaxial films provides an unrivaled method to induce very large strains in thin films. Here, by growing VAN films of EuTiO₃ (ETO)-Eu₂O₃ (EO) with different EO fractions, the vertical strain was systematically increased in ETO, up to 3.15%, and the Eu-Ti-Eu bond angle along ⟨111⟩ decreased by up to 1°, leading to a weakening of the antiferromagnetic interactions and switching from antiferromagnetic to ferromagnetic behavior. Our work has shown for the first time that Eu-Ti-Eu superexchange interactions play a key role in determining the magnetic ground state of ETO. More broadly, our work serves as an exemplar to show that multifunctionalities in strong spin-lattice coupling perovskite oxides can be uniquely tuned at the atomic scale using simple VAN structures.

Nanoscale ferroelastic twins formed in strained LaCoO3 films

The coexistence and coupling of ferroelasticity and magnetic ordering in a single material offers a great opportunity to realize novel devices with multiple tuning knobs. Complex oxides are a particularly promising class of materials to find multiferroic interactions due to their rich phase diagrams, and are sensitive to external perturbations. Still, there are very few examples of these systems. Here, we report the observation of twin domains in ferroelastic LaCoO₃ epitaxial films and their geometric control of structural symmetry intimately linked to the material's electronic and magnetic states. A unidirectional structural modulation is achieved by selective choice of substrates having twofold rotational symmetry. This modulation perturbs the crystal field-splitting energy, leading to unexpected in-plane anisotropy of orbital configuration and magnetization. These findings demonstrate the use of structural modulation to control multiferroic interactions and may enable a great potential for stimulation of exotic phenomena through artificial domain engineering.