Strongly Coupled Magnetic and Electronic Transitions in Multivalent Strontium Cobaltites

J H Lee,H Jeen,M S Yeom,Woo Seok Choi,J Nam,H N Lee,H.-J Lee,J H Seo

doi:10.1038/s41598-017-16246-z

Abstract

The topotactic phase transition in SrCoOx (x = 2.5–3.0) makes it possible to reversibly transit between the two distinct phases, i.e. the brownmillerite SrCoO2.5 that is a room-temperature antiferromagnetic insulator (AFM-I) and the perovskite SrCoO3 that is a ferromagnetic metal (FM-M), owing to their multiple valence states. For the intermediate x values, the two distinct phases are expected to strongly compete with each other. With oxidation of SrCoO2.5, however, it has been conjectured that the magnetic transition is decoupled to the electronic phase transition, i.e., the AFM-to-FM transition occurs before the insulator-to-metal transition (IMT), which is still controversial. Here, we bridge the gap between the two-phase transitions by density-functional theory calculations combined with optical spectroscopy. We confirm that the IMT actually occurs concomitantly with the FM transition near the oxygen content x = 2.75. Strong charge-spin coupling drives the concurrent IMT and AFM-to-FM transition, which fosters the near room-T magnetic transition characteristic. Ultimately, our study demonstrates that SrCoOx is an intriguingly rare candidate for inducing coupled magnetic and electronic transition via fast and reversible redox reactions.

Highlights

Oxygen stoichiometry in the transition metal oxides (TMOs) plays an essential role in determining the physical properties, including optoelectronic and magnetic properties[8,9,10]
We present a combined study of density-functional theory (DFT) calculations and optical spectroscopy to reveal the coupling between itinerancy of charge carriers and magnetic transition mediated by the oxygen concentration in SCOx
A rather rapid change is found in magnetic energy and moment at around x = 2.75, which may be affected by the insulator-to-metal transition (IMT)

Summary

Result

IMT with oxidation revealed by optical conductivity. Figure 1 shows oxygen content (x) dependent optical conductivity spectra (σ1(ω)) computed using DFT calculations (Fig. 1(a)) and recorded by spectroscopic ellipsometry (Fig. 1(b)). Note that the oxidization state was confirmed by comparing x-ray absorption spectroscopy data with known spectra from bulk This comparison clearly shows that electronic property, including insulating/metal character, is strongly subject to magnetic ordering; and metallic characters should be induced by the emergence of FM ordering at x ~2.75. Our combined optical study with DFT calculations shows a coupled magnetic IMT at x = 2.75, the dc transport measurement shows the IMT at around x = 2.9 This offset of the IMT could be associated with extrinsic effects, such as phase separation by forming puddles of conducting charges, domain boundaries, etc., which can reduce electrical conductivity yielding poor electronic conduction in dc-measurements.

Conclusions

Methods