Possible helimagnetic order in Co4+-containing perovskites Sr1-xCaxCoO3

We systematically synthesized perovskite-type oxides Sr1-xCaxCoO3 containing unusually high valence Co4+ ions by a high pressure technique, and investigated the effect of systematic lattice change on the magnetic and electronic properties. As the Ca content x exceeds about 0.6, the structure changes from cubic to orthorhombic, which is supported by the first-principles calculations of enthalpy. Upon the orthorhombic distortion, the ground state remains to be apparently ferromagnetic with a slight drop of the Curie temperature. Importantly, the compounds with x larger than 0.8 show antiferromagnetic behavior with positive Weiss temperatures and nonlinear magnetization curves at lowest temperature, implying that the ground state is noncollinear antiferromagnetic or helimagnetic. Considering the incoherent metallic behavior and the suppression of the electronic specific heat at high x region, the possible emergence of a helimagnetic state in Sr1-xCaxCoO3 is discussed in terms of the band-width narrowing and the double-exchange mechanism with the negative charge transfer energy as well as the spin frustration owing to the next-nearest neighbor interaction.

and PPMS manufactured by Quantum Design, respectively. The magnetization up to 55 T was measured using the nondestructive pulsed magnet with a pulse duration of 36 ms at the International MegaGauss Science Laboratory at the Institute for Solid State Physics.
To evaluate the structural stability of SrCoO3 and CaCoO3 under ambient and high pressures, we performed DFT calculations using the plane-wave-basis projector augmented wave (PAW) method 27 with GGA-PBEsol approximations, as implemented in Quantum Espresso package 28 . We performed structural optimization calculations on cubic and orthorhombic perovskite-type structures at 0 and 10 GPa. Nonmagnetic (NM) and ferromagnetic (FM) configuration were assumed for Co ions, and the electron correlation U was set to 5 eV.
The cutoff energies for wavefunctions and charge density were set to 60 Ry and 445 Ry, respectively. The Monkhorst-Pack k-point meshes of 9 × 9 × 9 and 6 × 4 × 6 or more were adopted for the cubic and the orthorhombic structures, respectively.  Table I, and the refined structures for x = 0 and 1.0 are shown in Fig. 1(b). It is noted that the diffraction peaks of x=0.6 and 0.7 tend to be diffused, implying that these samples are located around the first-order structural phase boundary and contain both cubic and orthorhombic phases coexisting in a microscopic scale. In Figs. 1(c) and (d), the unit cell volume divided by the number of atoms per unit cell Z and the Co-O-Co bond angle are presented as a function of x. The unit cell volume of both structures decreases with increasing x, reflecting the smaller ionic radius of Ca 2+ than that of Sr 2+ . Using the Co-O bond lengths, we estimated the bond-valence sums for Co ions 29 , which are in the range of +3.86 ~ 4.08 (see Table I). Thermogravimetric (TG) measurements support this high Co valence; the TG measurements suggest that this system is sufficiently oxidized with little oxygen deficiency, implying that the valence of Co is close to +4 (see Supplementary Information).
To examine the stability of cubic and orthorhombic perovskite structures for SrCoO3 and CaCoO3, respectively, the enthalpies at 0 and 10 GPa were calculated based on ab-initio calculations. Figure 2 shows the enthalpy of the orthorhombic phase relative to that of the cubic phase for SrCoO3 and CaCoO3 with ferromagnetic (FM) and nonmagnetic (NM) states. It is notable that the orthorhombic structure tends to be more stable than the cubic structure not only for CaCoO3 but for SrCoO3. Assuming the FM state at a high pressure of 10 GPa, while the orthorhombic structure remains to be stable for CaCoO3, the cubic structure becomes more stable than the orthorhombic one for SrCoO3, implying the importance of the FM interaction for the phase formation. This result is qualitatively consistent with our experimental results that the cubic structure found for SrCoO3 is replaced by the orthorhombic structure as the Ca content x increases. Further insight into the magnetic ground state of Sr1-xCaxCoO3 is provided by the field dependence of M at 2 K as shown in Fig. 3(c). For x = 0.2 and 0.4, the magnetization shows a characteristic field dependence as ferromagnetic SrCoO3. The saturated moment of ~ 2.0 μB/Co is slightly smaller than that of the single crystal of SrCoO3 (~2.5 μB/Co) 11 , but comparable to that of the polycrystalline sample (~2.1 μB/Co) [30][31][32] . The FM behavior was also observed for x = 0.6-0.7, while the hysteresis loop was enlarged and the saturation moment Sr1-xBaxCoO3 has been also found to be independent of the Ba content 25 , it is presumable that the intermediate spin state in the Co 4+ -containing perovskites is stable over a wide range with respect to the Co-O bond length.
Here we note that SrFeO3 shows a similar field-induced transition from helimagnetic to conical spin state at 4 K and a positive Weiss temperature θ. Figure 4(c) shows the x dependence of the Weiss temperature θ for Sr1-xCaxCoO3, which is evaluated by the Curie-Weiss law. Upon the increment of the Ca content x, θ decreases monotonically while maintaining a positive value indicating the predominance of the FM interaction in all x regions. In fact, CaCoO3 of a cubic variant is reported to show FM behavior and the stability of FM state at high pressures is also predicted in the first-principles calculations 33 . We, hence, presume that CaCoO3 adopts HM spin structure rather than G-type spin structure in the AFM phase as in the case of SrFeO3 showing the pressure-induced-transition from a HM to FM state 34 . . For CaCoO3, Δd is evaluated to be ~ 3 × 10 -5 which is substantially smaller than that of the JT system LaMnO3 (~5 × 10 -3 ), 39 but comparable to that of the non-JT system RCoO3 (~5 × 10 -5 , R=Pr...Lu). 40 The orthorhombic structural distortion influences the electronic properties as characterized by the x dependence of ρ and C. Figure 4(a) shows the temperature dependence of ρ, which is seemingly nonmetallic with a small magnitude less than 3 mΩcm at room temperature. The ratio of the electrical resistivity at 2 K to that at room temperature ρ2K/ρ300K is enhanced especially for x > 0.7 as shown in Fig. 4(d), implying that the orthorhombic distortion changes the system to the incoherent metallic state, which is called a "bad metal", ubiquitously observed in the vicinity of the Mott insulator. The substantial orthorhombic distortion also affects the specific heat C. The T 2 dependence of C/T is shown in Fig. 4(b). The Debye temperature θD and the electronic specific heat coefficient γ were evaluated with the equation C/T=γ + (12/5)π 4 NRθD -3 T 2 (R=8.31 J/mol K and N=5). θD changes discontinuously at the critical composition (x = 0.6), reflecting the structure transition from the cubic to orthorhombic phase, whereas γ shows a gradual change with a kink at x = 0.6 (see Fig. 4(e)).
The observation of large γ of 40~50 mJmol -1 K -2 in all compositions suggests that the effective mass of the conduction electron is enhanced by the electron correlation, as reported for the perovskite-type cobalt oxides (Ca,Y)Cu3Co4O12. 41 As the Ca content increases in the orthorhombic phase, γ gradually decreases presumably owing to the pseudo-gap formation causing the suppression of the density of states at the Fermi level, which is accompanied by the orthorhombic distortion. A recent band calculation with GGA+U for the orthorhombically distorted CaCoO3 shows that only the majority eg bands are responsible for metallicity, while the minority t2g band opens a band gap 42 . These theoretical conjectures are compatible with the variation of the electronic properties of Sr1-xCaxCoO3, i.e., the incoherent metallic behavior with the low resistivity value (< 10 mΩcm) and suppression of γ can be explained by the gap formation of the t2g bands while maintaining the itinerant eg bands. This is in contrast to the insulating ground state of CaFeO3 exhibiting the charge disproportionation of Fe 4+ ions with eg bands near the Fermi level.
Finally, let us discuss the lattice dependent variation of the magnetic ground states of Sr1-xCaxCoO3. We can propose two possible mechanisms of the AFM(HM) ground state near x = 1. One is the double exchange mechanism for transition-metal oxides with the eg 1 configuration and a negative p-d charge-transfer energy Δ. 6,43 Assuming that this model, in which the pd hybridization plays an important role on the phase stability, holds for Sr1-xCaxCoO3 with the eg 1 configuration and negative Δ, the orthorhombic distortion shown in Fig.   1(d) presumably stabilizes the HM state rather than the FM state through the reduction of the pd hybridization.
The other is the frustration between the FM double exchange interaction J1 and the AFM superexchange interaction J2, the latter being comparable to the former when the orthorhombic distortion is significant as shown in Fig. 1(d). 43