Pressure-induced spin-state and insulator-metal transition in Sr 2 CoO 3 F by first principles

1. Pressure-induced spin-state and insulator-metal transition in Sr2CoO3F by first principles Xue-dong Ou and Hua Wu We have studied the electronic structure and magnetic property of the pressurized Sr2CoO3F, using configuration-state-constrained GGA+U calculations. The obtained results show that a moderate pressure(about 6 GP) could readily induce a transition from antiferromagentic (AF) insulator with high spin (HS, S=2) to ferromagnetic (FM) half-metal with intermediate-spin (IS, S=1). Our results account well for the recent experimental data, and it is suggested that a novel FM half-metallic ground state may be explored by Ca-doping. Ⅰ. Introduction Ⅱ. Spin state transition and multiplet effect Strongly correlated cobalt oxides are interesting materials which display spectacular properties such as giant magneto resistance, superconductivity and large thermoelectric power [1-2]. They have an important spin-state issue. Spin state and its transition are often associated with fascinating magnetic and electronic properties. An intermediate-spin state could give rise to a FM half-metallic state that is of great interest. Figure 1b. The parameter D, defined as the distance between the Co ion and the O4 basal square, is related to bandwidth of the x2-y2 pdσ state. It may tune an insulator-metal and/or AFM-FM transition in layered cobalt oxides • Figure 2.Configuration states of Co3+ ion.(a). A competition between crystal field and Hund exchange leads to different spin states. (b) A multiplet effect in the IS state is due to inter-orbital interactions. Figure 1a. The structure of Sr2CoO3F Ⅲ. Pressure-induced spin-state and insulator-metal transitions HS IS (a) (b) Figure 3a. Pressure dependence of the electric resistivity of Sr2CoO3F [3]. Figure 3b.The E-V and P-V curve by GGA+U calculations. Figure 3a shows a decreasing electric resistivity of Sr2CoO3F under pressure, implying the onset of a metallic state [3]. Through configuration-state-constrained GGA+U calculations (Fig. 3b), we find that an HS-IS transition and an associated AF insulator-FM metal transition occur at about 6 GPa (with about 3% volume compression). This agrees with the experiment data [3]. The P-V curve is fitted by Brich-Murnaghan equation of state as follows: Figure 4. Total density of states (DOS) and orbitally resolved Co-3d DOS. (a) High spin (HS) insulator with G-type AF order is the ground state at ambient pressure, which agrees well with the experimental results [3]. (b) Under the pressure of about 6 GPa, spin state and I-M transition occurr, and the IS half-metalli state becomes most stable. Ⅳ. Discussion and conclusion In 3d transition-metal oxides, there could be a spin crossover, if the Hund exchange energy Eex and the crystal field energy ECFare comparable. The former favors a high-spin state, while the latter prefers to stabilize a low-spin state. The energy difference between Eex and ECF could be tuned effectively by an external pressure or a chemical substitution. Then interesting magnetic and electronic properties may emerge. Sr2CoO3Fis found to have an interesting transition from HS AF insulator to IS FM half-metal, under a moderate pressure of about 6 GPa. As the D parameter (or the Co-O-Co bond angle) controls the x2-y2 bandwidth effectively, one would need to further reduce it to increase the bandwidth. This plays a vital role in forming the IS FM half-metallic state. We suggest that Ca-doping could shrink the lattice and reduce the D parameter, and as a result, the IS FM half-metallic phase could become the ground state at ambient pressure. Currently we are exploring such a possible half-metallic oxide. References [1] K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi, R. A. Dilanian, and T. Sasaki, Nature (London) 422, 53 (2003). [2] T. Jia, H. Wu, G. Zhang, X. Zhang, Y. Guo, Z. Zeng, H.Q. Lin.Phys. Rev. B 83, 174433 (2011). [3] Y.Tsujimoto, C.I. Sathish, K. P. Hong, K. Oka, M. Azuma, Y. F. Guo, Y. Matsushita, K. Yamaura, and E. Takayama-Muromachi, Inorg. Chem. 51 ,4802 (2012).