Room-temperature valence transition in a strain-tuned perovskite oxide

Abstract

Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr1-xCaxCoO3). Here, we show that in thin films of the complex perovskite (Pr1-yYy)1-xCaxCoO3-δ, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. The technological implications of this result are accompanied by fundamental prospects, as complete strain control of the electronic ground state is demonstrated, from ferromagnetic metal under tension to nonmagnetic insulator under compression, thereby exposing a potential novel quantum critical point.

Large Perpendicular Magnetic Anisotropy Induced by an Intersite Charge Transfer in Strained EuVO2H Films

“…The ICT discovered in this study could lead to new developments in materials design. Because in many perovskites (e.g., Bi 1– x La x NiO 3 , , (Pr,Y) 1– x Ca x CoO 3 , , PbCoO 3 , and LaCu 3 Fe 4 O 12 , ) and ilmenites (e.g., FeTiO 3 and AgRuO 3 ), general guidelines and predictions are not established due to their 3D and thus complex electronic structures. In contrast, in EuVO 2 H, electron injection occurs from the EuH layer into the VO 2 layer.…”

“…Despite the difference in valence of the starting oxides (Eu III V III O 3 and Sr II V VI O 3 ), the bulk EuVO 2 H has almost identical lattice parameters to SrVO 2 H, indicating the Eu 2+ /V 3+ state, and exhibits a ferromagnetic transition by Eu 2+ ( J = 7/2) at a relatively high Curie temperature, T C , of 10 K, in addition to the 2D Mott insulating state by V 3+ ( S = 1) like SrVO 2 H. Interestingly, the interplay between the two layers, namely, an intersite charge transfer (ICT) from Eu 2+ to V 3+ , occurs by applying the external pressure to the bulk EuVO 2 H or compressive biaxial strain from a SrTiO 3 (STO) substrate to the epitaxial thin films. The ICT phenomenon has been reported in perovskite oxides and related compounds such as Bi 1– x La x NiO 3 , − but has never been realized in mixed-anion compounds. The thin films with ICT exhibit a significant increase in T C , which depends on the film thickness (or the degree of relaxation) and rises up to 40 K. Despite the absence of the orbital degree of freedom in Eu 2+ ( L = 0, J = S = 7/2), EuVO 2 H thin films with less relaxation (<100 nm) exhibit a large magnetic anisotropy, with the easy axis perpendicular to the film.…”

Intersite Charge Transfer through Eu 4f Band Engineering in Eu1–xSrxVO2H Oxyhydride Films

“…This implies the possibility of independent control over the d and f bands, an achievement unexplored in previous oxide research. For example, (Pr,Ca)­CoO 3 undergoes charge transfer between the Pr 4f and Co 3d bands under external stimuli such as strain and elemental substitution of Y ions, yet the substitution of Ca ions affects both 3d and 4f bands. , This primarily stems from the fact that octahedral rotations upon Ca substitution influence both bond distances and angles for the 3d and 4f elements. In contrast, the anion-ordered layered structure of EuVO 2 H allows independent manipulation of the 3d and 4f bands, a concept realized by the proper arrangement of multiple anions.…”

Much more to explore with an oxidation state of nearly four: Pr valence instability in intermetallic m -Pr 2 Co 3 Ge 5

“…Valence instabilities in lanthanide compounds can have substantial impacts on crystal structure and physical properties (13)(14)(15)(16). Targeting systems with structural phase transformations can provide an exciting path forward for strongly correlated materials, specifically in identifying valence-unstable states in dominantly trivalent lanthanides like Pr with the opportunity to access technologically relevant quantum states (17)(18)(19)(20). Still, Pr compounds are typically trivalent, with only a small number of dominantly tetravalent Pr-containing oxides and fluorides reported (21).…”