Giant caloric effects in charge-spin-lattice coupled transition-metal oxides

The caloric effects of solids can provide us with highly efficient and environmentally friendly energy systems. Exploring novel caloric materials is challenging but critically important in developing future technologies. Typical solid caloric effects are magnetocaloric, electrocaloric, and barocaloric effects induced respectively by magnetic fields, electric fields, and pressure, and materials showing large caloric responses through the effects attract lots of recent attention. In this perspective article, novel transition-metal oxides that show giant caloric effects are highlighted. The compounds are NdCu₃Fe₄O₁₂ and BiCu₃Cr₄O₁₂ containing unusually high valence states of transition-metal ions and show charge transitions to relieve the electronic instabilities. The charge, spin, and lattice degrees of freedom in the compounds are strongly correlated and the primarily induced charge transitions cause unusual first-order magnetic phase transitions that provide significant latent heat. Importantly, the large latent heat can be utilized through the caloric effects in multiple ways. The details of the giant caloric effects in the charge–spin–lattice coupled transition-metal oxides are summarized and the mechanism of the effects is discussed

Influence of deposition rate on magnetic properties of inverse-spinel NiCo2O4epitaxial thin films grown by pulsed laser deposition

We investigated the influence of the deposition rate on structural and magnetic properties of inverse-spinel ferrimagnet NiCo₂O₄ epitaxial films grown by pulsed laser deposition. While films' lattice constants are insensitive to the deposition rate, saturation magnetization, and perpendicular magnetic anisotropy for the film grown with a high deposition rate are reduced. These results imply that growing NiCo₂O₄ films with a high deposition rate leads to occupations of the tetrahedral site by Ni, although Ni ideally occupies only the octahedral site. Controlling the deposition rate and modulating the cation distribution is the key for tuning the magnetic properties in NiCo₂O₄ films

Thermal properties and phase transition behaviors of possible caloric materials Bi₀.₉₅Ln₀.₀₅NiO₃

Thermal properties and phase transition behaviors of possible caloric materials Bi₀.₉₅Ln₀.₀₅NiO₃ (Ln = La, Nd, Sm, Eu, Gd, Dy), which show intersite charge transfer between Bi and Ni ions, were investigated. Although a few of the compounds showed large latent heats at the intersite-charge-transfer transition temperatures, the values are not comparable to that observed in the giant caloric effect compound NdCu₃Fe₄O₁₂. In the present Bi₀.₉₅Ln₀.₀₅NiO₃, contrary to our expectation, the magnetic transitions of Ni²⁺ spins are not induced by the intersite-charge-transfer transitions and the magnetic entropy changes do not contribute to the latent heat produced by the intersite-charge-transfer transitions. To obtain giant caloric effects, materials for which the “intrinsic” magnetic transition temperatures are much higher than the charge-transfer-transition temperatures may be needed

Crystal Structures at Atomic Resolution of the Perovskite Related GdBaMnFeO5 and Its Oxidized GdBaMnFeO6

Perovskite-related GdBaMnFeO5 and the corresponding oxidized phase GdBaMnFeO6, with long-range layered-type ordering of the Ba and Gd atoms have been synthesized. Oxidation retains the cation ordering but drives a modulation of the crystal structure associated with the incorporation of the oxygen atoms between the Gd layers. Oxidation of GdBaMnFeO5 increases the oxidation state of Mn from 2+ to 4+, while the oxidation state of Fe remains 3+. Determination of the crystal structure of both GdBaMnFeO5 and GdBaMnFeO6 is carried out at atomic resolution by means of a combination of advanced transmission electron microscopy techniques. Crystal structure refinements from synchrotron X-ray diffraction data support the structural models proposed from the TEM data. The oxidation states of the Mn and Fe atoms are evaluated by means of EELS and Mö ssbauer spectroscopy, which also reveals the different magnetic behavior of these oxides

Stabilization of Ferroelectric Hf0.5Zr0.5O2 Epitaxial Films via Monolayer Reconstruction Driven by Interfacial Redox Reaction

The binary fluorite oxide Hf0.5Zr0.5O2 tends to grab a significant amount of notice due to the distinct and superior ferroelectricity found in its metastable phase. Stabilizing the metastable ferroelectric phase and delineating the underlying growth mechanism, however, are still challenging. Recent discoveries of metastable ferroelectric Hf0.5Zr0.5O2 epitaxially grown on structurally dissimilar perovskite oxides have triggered intensive investigations on the ferroelectricity in materials that are nonpolar in bulk form. Nonetheless, the growth mechanism for the unique fluorite/perovskite heterostructures has yet to be fully explored. Here we show that the metastable ferroelectric Hf0.5Zr0.5O2 films can be stabilized even on a one-unit-cell-thick perovskite La0.67Sr0.33MnO3 buffer layer. In collaboration with scanning transmittance electron microscopy (STEM) based characterizations, we show that monolayer reconstruction driven by interfacial redox reactions plays a vital role in the formation of a unique heterointerface between the two structurally dissimilar oxides, providing the template monolayer that facilitates the epitaxial growth of the metastable HZO films. Our findings offer significant insights into the stabilization mechanism of the ferroelectric Hf0.5Zr0.5O2, and this mechanism could be extended for exploring functional metastable phases of various metal oxides