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 the lowest temperature, implying that the ground state is non-collinear antiferromagnetic or helimagnetic. Considering the incoherent metallic behavior and the suppression of the electronic specific heat at the high x region, the possible emergence of a helimagnetic state in Sr1−xCaxCoO3 is discussed in terms of the bandwidth 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.

Searching for novel magnetic phases in a simple system is an attractive, yet challenging, task in condensed matter physics. One promising approach to discovering exotic magnetic phases is to focus on the system with competing exchange interactions. While this is typified by antiferromagnetic (AFM) insulators with geometrically frustrated spin lattice,1 frustrated magnetic metals with competing exchange interactions have attracted attention for their novel spintronic phenomena, as highlighted by the emergence of magnetic skyrmion lattice in non-centrosymmetric magnets with the Dzyaloshinskii–Moriya interaction.2,3

As centro-symmetric counterparts of frustrated magnetic metals, perovskite-type oxides with unusually high valence Fe4+, Co4+, and Ni3+ ions have been known to show rich magnetic phases.4–6 This is exemplified by the observation of various topological spin orders involving multiple-Q helimagnetic (HM) states in SrFeO37–10 and a room-temperature ferromagnetic (FM) order in SrCoO3.11–16 Although they exhibit apparently different magnetic behavior, there are important similarities between them; both of them share the cubic perovskite-type structure, the strong p-d hybridization with a negative charge-transfer energy, and the eg1 configuration, provided that Co4+ is in the unique intermediate spin IS state (t2g4eg1), which has been under debate.11–15,17,18 Considering the fact that the negative charge transfer energy and the eg1 configuration have been discussed as important conditions for the emergence of the spin spiral in SrFeO3,6 it is tempting to expect the emergence of the novel HM phase in the Co4+-containing perovskites as well.

So far, the Fe4+-containing perovskites AFeO3 have been systematically synthesized to study the A-site dependent variation of the magnetic ground state.5,19,22 Substitution of the A-site in SrFeO3 results in a significant change in the helimagnetic propagation vector in BaFeO3 and CaFeO3, the latter of which shows a metal–insulator transition, accompanied by charge disproportionation of Fe4+.5,19–21 On the other hand, the Co4+-containing perovskites—ACoO3, Sr1−xBaxCoO3, and CaCoO3—have been successfully synthesized by a high pressure technique.23–25 While Sr1−xBaxCoO3 has been reported to retain the cubic structure and show the change in the ground state from FM to HM at around x = 0.4, there remains a missing link between cubic SrCoO3 with the FM state and orthorhombically distorted CaCoO3 showing AFM behavior.

In this paper, we have systematically synthesized perovskite-type cobaltates Sr1−xCaxCoO3 by a high-pressure technique and investigated the change in the structure and the magnetic states, while revealing the structure–property relationship. As the Ca content x increases, the structure changes from cubic to orthorhombic at around x = 0.6, and the magnetic ground state changes from FM to AFM at around x = 0.8 in the orthorhombic phase. We discuss the origin of variation in the magnetic ground states and the possibility of the helical spin order in the AFM state in terms of the bandwidth narrowing and the competing exchange interactions inherent to the orthorhombic distortion.

Polycrystalline samples of Sr1−xCaxCoO3 were prepared by following the procedure reported in Ref. 23. The starting materials, SrCO3, CaCO3, and Co3O4, were stoichiometrically mixed. The mixture was heated at 1173 K for 12 h in air. The obtained powder was pelletized and sintered again at 1373 K for 24 h in a flow of oxygen (1 atm), followed by quenching into water at room temperature. Through these processes, oxygen-deficient perovskites Sr1−xCaxCoO3-δ were synthesized. To obtain fully oxidized compounds, the quenched pellet was pulverized and packed into a gold capsule, together with an oxidizer, NaClO3. The capsule was annealed at 753 K for 1 h at a high pressure of 8 GPa using a cubic-anvil-type high-pressure apparatus. Note that the oxidation of the compound with x = 0.2 was completed by the dip in an aqueous solution of sodium hypochlorite for 12 h, which turned out to yield a more oxidized sample as compared to the samples prepared by the high-pressure process. The synchrotron powder x-ray diffraction (XRD) with a wavelength of 0.689 75 Å was carried out at BL-8A, Photon Factory, KEK, Japan. The diffraction patterns were analyzed by the Rietveld refinement using RIETAN-FP.26 The magnetization M, electrical resistivity ρ, and specific heat C were measured using MPMS and PPMS manufactured by Quantum Design, respectively. The magnetization up to 55 T was measured using the non-destructive 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) method27 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. Non-magnetic (NM) and ferromagnetic (FM) configurations 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 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.

Figure 1(a) shows magnified powder XRD patterns of Sr1−xCaxCoO3, with selected compositions of x = 0.2, 0.4, 0.6, 0.7, 0.8, and 1.0. The diffraction patterns for x = 0.2 and 0.4 and x = 0.8 and 1.0 were indexed with a cubic unit cell (S.G.: Pm-3m) and a GdFeO3-type orthorhombic unit cell (S.G.: Pbnm), respectively. The XRD patterns with Rietveld refinement for the cubic phase with x = 0.4 and the orthorhombic phase with x = 0.8 can be found in Fig. S1 in the supplementary material. The lattice parameters are summarized in 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 on a microscopic scale. In Figs. 1(c) and 1(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 Ca2+ compared to Sr2+. Using the Co–O bond lengths, we estimated the bond-valence sums for Co ions,29 which are in the range +3.86–4.08 (see Table I). Thermo-gravimetric (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 the supplementary material).

FIG. 1.

(a) Synchrotron powder XRD patterns for Sr1−xCaxCoO3. (b) Crystal structures of the cubic-perovskite SrCoO3 and orthorhombic-perovskite CaCoO3 projected along the c axis. The blue, green, and yellow spheres correspond to Co, Sr, and Ca ions, respectively. (c) Unit cell volume V divided by the number of atoms per unit cell Z, and (d) average Co–O–Co bond angle as a function of x for Sr1−xCaxCoO3. Black dots in (c) and (d) are the data of the single crystalline sample.11 The inset in Fig. 1(d) shows the schematic illustration of the nearest-neighbor (NN) interaction J1 and next-nearest-neighbor (NNN) interaction J2 for the orthorhombic structures. (e) Magnetic phase diagram of Sr1−xCaxCoO3 with abbreviations: FM for ferromagnetic and AFM for anti-ferromagnetic. Tc of the single crystal at x = 0 is taken from Ref. 11.

FIG. 1.

(a) Synchrotron powder XRD patterns for Sr1−xCaxCoO3. (b) Crystal structures of the cubic-perovskite SrCoO3 and orthorhombic-perovskite CaCoO3 projected along the c axis. The blue, green, and yellow spheres correspond to Co, Sr, and Ca ions, respectively. (c) Unit cell volume V divided by the number of atoms per unit cell Z, and (d) average Co–O–Co bond angle as a function of x for Sr1−xCaxCoO3. Black dots in (c) and (d) are the data of the single crystalline sample.11 The inset in Fig. 1(d) shows the schematic illustration of the nearest-neighbor (NN) interaction J1 and next-nearest-neighbor (NNN) interaction J2 for the orthorhombic structures. (e) Magnetic phase diagram of Sr1−xCaxCoO3 with abbreviations: FM for ferromagnetic and AFM for anti-ferromagnetic. Tc of the single crystal at x = 0 is taken from Ref. 11.

Close modal
TABLE I.

Refined structural parameters and the bond-valence sum (BVS) of Co4+ for Sr1−xCaxCoO3. BVS is calculated by iexp[(r0ri)/0.37], where r0 = 1.75.

x0.20.40.60.81.0
Space group Pm-3m Pm-3m Pm-3m Pbnm Pbnm Pbnm 
A (Å) 3.825 5(1) 3.804 0(1) 3.7857(4) 5.3264(8) 5.3152(8) 5.2664(5) 
B (Å)    5.3109(9) 5.3139(5) 5.2929(6) 
C (Å)    7.548(17.500(1) 7.4354(6) 
V/Z (Å355.984 55.047 54.253 53.383 52.960 51.815 
Co–O(1) (Å) 1.912 76(6) 1.902 01(7) 1.8928(2) 1.78(3) × 2 1.938(16) × 2 1.914(8) × 2 
    2.07(2) × 2 1.870(16) × 2 1.905(8) × 2 
Co–O(2) (Å)    1.888(1) × 2 1,919(3) × 2 1.890(2) × 2 
⟨Co–O–Co⟩(deg) 180 180 180 155.8(9) × 4 161.4(9) × 4 155.7(5) × 4 
    178(4) × 2 155.4(9) × 2 159.1(7) × 4 
Co valence (BVS) 3.86 3.98 4.08 4.08 3.92 3.97 
Rwp (%) 0.818 1.031 1.114 1.114 1.384 1.961 
1.385 2.563 1.293 1.293 2.519 3.3531 
x0.20.40.60.81.0
Space group Pm-3m Pm-3m Pm-3m Pbnm Pbnm Pbnm 
A (Å) 3.825 5(1) 3.804 0(1) 3.7857(4) 5.3264(8) 5.3152(8) 5.2664(5) 
B (Å)    5.3109(9) 5.3139(5) 5.2929(6) 
C (Å)    7.548(17.500(1) 7.4354(6) 
V/Z (Å355.984 55.047 54.253 53.383 52.960 51.815 
Co–O(1) (Å) 1.912 76(6) 1.902 01(7) 1.8928(2) 1.78(3) × 2 1.938(16) × 2 1.914(8) × 2 
    2.07(2) × 2 1.870(16) × 2 1.905(8) × 2 
Co–O(2) (Å)    1.888(1) × 2 1,919(3) × 2 1.890(2) × 2 
⟨Co–O–Co⟩(deg) 180 180 180 155.8(9) × 4 161.4(9) × 4 155.7(5) × 4 
    178(4) × 2 155.4(9) × 2 159.1(7) × 4 
Co valence (BVS) 3.86 3.98 4.08 4.08 3.92 3.97 
Rwp (%) 0.818 1.031 1.114 1.114 1.384 1.961 
1.385 2.563 1.293 1.293 2.519 3.3531 

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 non-magnetic (NM) states. It is notable that the orthorhombic structure tends to be more stable than the cubic structure not only for CaCoO3, but also 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.

FIG. 2.

(a) Enthalpy difference between the orthorhombic (Pbnm) and cubic (Pm-3m) phases of SrCoO3 and CaCoO3 calculated for (a) and (b) nonmagnetic (NM) and (c) and (d) ferromagnetic (FM) phases at (a) and (c) 0 GPa, and (b) and (d) 10 GPa.

FIG. 2.

(a) Enthalpy difference between the orthorhombic (Pbnm) and cubic (Pm-3m) phases of SrCoO3 and CaCoO3 calculated for (a) and (b) nonmagnetic (NM) and (c) and (d) ferromagnetic (FM) phases at (a) and (c) 0 GPa, and (b) and (d) 10 GPa.

Close modal

Figure 3(a) shows the temperature dependence of magnetization M divided by a magnetic field H of 0.1 T, M/H, measured in field cooling (FC) runs. As x increases from 0 to 0.7, the FM transition temperature Tc decreases systematically, which is accompanied by the emergence of a stepwise transition at x = 0.6 and 0.67. The observation of the successive FM transitions reflects the coexistence of cubic and orthorhombic phases with different Tc, consistent with the XRD experiments. For x = 0.8, with orthorhombic structure, the FM transition was observed at 120 K, followed by the decrease of M/H below 90 K. Eventually at x = 1.0, the FM transition disappeared, and, instead, an antiferromagnetic (AFM) transition was found at 95 K (=TN), as shown in Fig. 3 (b).

FIG. 3.

(a) Temperature dependence of the magnetization divided by the applied magnetic field M/H for Sr1−xCaxCoO3. (b) M/H for x = 1.0 as a function of temperature. (c) Magnetic field dependence of the magnetization for Sr1−xCaxCoO3 at 2 K. (d) Magnetic field dependence of the magnetization for x = 1.0 up to 55 T at selected temperatures.

FIG. 3.

(a) Temperature dependence of the magnetization divided by the applied magnetic field M/H for Sr1−xCaxCoO3. (b) M/H for x = 1.0 as a function of temperature. (c) Magnetic field dependence of the magnetization for Sr1−xCaxCoO3 at 2 K. (d) Magnetic field dependence of the magnetization for x = 1.0 up to 55 T at selected temperatures.

Close modal

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 similar to 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).30–32 The FM behavior was also observed for x = 0.6–0.7, while the hysteresis loop was enlarged and the saturation moment was slightly diminished, probably due to the emergence of the orthorhombic phase. At x = 0.8, the M–H curve shows a nonlinear and hysteretic behavior, suggesting that an AFM state tends to predominate over the competing FM state as the magnetic ground state. The magnetization curve of x = 1 shows AFM-like linear behavior without saturation up to 7 T, unlike the other compounds showing hysteretic and FM-like behavior. To gain more insight into the magnetism of the compound with x = 1, we performed magnetization measurements under high fields up to 55 T at various temperatures, as shown in Fig. 3(d). The M–H curve at 4.2 K was almost saturated above 40 T, with the moment of 1.6 μB/Co. The abrupt increase above 9 T can be ascribed to a spin–flop transition possibly inherent to the non-collinear or helical spin structure. The magnetization of x = 1 at 40 T is slightly smaller than that of x = 0.4, which may be a sign of the partial emergence of the low-spin state (∼1.0 μB/Co). On the other hand, the effective Bohr magneton number Peff = 3.35–3.98 μB for all compounds estimated from the Curie–Weiss law M/H(T) = Cw/(T-θ), where Cw is the Curie constant (see Figs. S3 and S4 in the supplementary material), is close to the theoretical value of 3.87 μB expected for the intermediate spin state of Co4+ (t2g4eg1) with S = 3/2. Thus, it is likely that the spin state of Sr1−xCaxCoO3 remains to be the intermediate spin state for all compositions, even though the Co–O bond length slightly decreases upon the Ca substitution with x up to 0.6. Considering the fact that the spin state of cubic perovskites Sr1−xBaxCoO3 has also been found to be independent of the Ba content,25 it is presumable that the intermediate spin state in the Co4+-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, a 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.33 

FIG. 4.

(a) Temperature dependence of ρ and (b) T2 dependence of C/T for Sr1−xCaxCoO3. (c) Weiss temperature θ and (d) the ratio of the electrical resistivity at 2 K compared to that at room temperature, ρ2K300K, as a function of x. (e) Electronic specific heat coefficient γ (left axis) and Debye temperature θD (right axis) as a function of x. Black filled circles at x = 0 in (d) and (e) are the reported data of the polycrystalline sample.32 

FIG. 4.

(a) Temperature dependence of ρ and (b) T2 dependence of C/T for Sr1−xCaxCoO3. (c) Weiss temperature θ and (d) the ratio of the electrical resistivity at 2 K compared to that at room temperature, ρ2K300K, as a function of x. (e) Electronic specific heat coefficient γ (left axis) and Debye temperature θD (right axis) as a function of x. Black filled circles at x = 0 in (d) and (e) are the reported data of the polycrystalline sample.32 

Close modal

Figure 1(e) depicts the magnetic phase diagram of Sr1−xCaxCoO3 as a function of x and T, where the transition temperatures Tc and TN are determined by the M/H–T curves. The value of Tc gradually decreases with increasing x, and the two types of FM phases are found in the cubic (FM1) and orthorhombic (FM2) structure phases. The drop of Tc upon the change from FM1 to FM2 can be associated with the bandwidth narrowing caused by the first-order structural transition from the cubic to the orthorhombic phase. For x = 0.8, the AFM (HM) ground state emerges below the FM transition temperature. Finally, the FM transition was indiscernible for x = 1.0. This phase diagram suggests that the FM and AFM interactions are competing with each other in Sr1−xCaxCoO3, and the FM interaction seems to be reduced by the Ca substitution. Here, the suppression of Tc with increasing x in the cubic phase is inconsistent with the naive expectation that the nearest neighbor FM interaction (double exchange interaction) should be enhanced via the bandwidth broadening, accompanied by the decrease of the Fe–O bond length. In fact, the application of pressure for SrCoO3 was found to enhance TC.14 For the origin of this Tc suppression, two possible mechanisms can be considered: (i) Sr/Ca disorder similar to Ln1/2Ba1/2MnO3 (Ln = La–Tb), where the A-site disorder suppresses Tc in the vicinity of the multi-critical composition, where FM and AFM phases are competing with each other34,35 and (ii) local disorder of the orthorhombic structural distortion observed in the solid solution of perovskite-type oxides,36,37 which should play important roles in the variation of the magnetic ground state, as discussed below.

Below, we discuss the possible origin of the AFM (HM) phase in terms of the change in the crystal structure and electronic states. As shown in Fig. 1, the unit cell volume decreases with increasing x, and the CoO6 octahedra are largely tilted: the average Co–O–Co bond angle is about ∼157°. First, the clear JT distortion is absent in the CoO6 octahedra. In order to quantify the relative distortion of the deviation of the octahedra, we define the Δd parameter, denoting the deviation of Co–O distances dn with respect to the average ⟨Co–O⟩ value as Δd=(1/6)n=1,6[(dnd)/d]2. For CaCoO3, Δd is evaluated to be ∼3 × 10−5, which is substantially smaller than that of the JT system LaMnO3 (∼5 × 10−3),38 but comparable to that of the non-JT system RCoO3 (∼5 × 10−5, R = Pr − Lu).39 

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 of less than 3 mΩ cm at room temperature. The ratio of the electrical resistivity at 2 K to that at room temperature, ρ2K300K, 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 T2 dependence of C/T is shown in Fig. 4(b). The Debye temperature θD and the electronic specific heat coefficient γ were evaluated with equation C/T = γ + (12/5)π4NRθD−3T2 (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 the 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 mJ mol−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)Cu3Co4O12.40 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 bandgap.41 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 mΩ cm) 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 Fe4+ 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 eg1 configuration and a negative p-d charge-transfer energy Δ.6,42 Assuming that this model, in which the pd hybridization plays an important role in the phase stability, holds for Sr1−xCaxCoO3 with the eg1 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 frustration43 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).42,44 The orthorhombic distortion not only reduces the FM double exchange interaction due to the band narrowing but also enhances a next-nearest-neighbor (NNN) superexchange interaction through the decreasing in Co–O(1)–O(1)–Co exchange path, where the t2g electrons play an important role.45 Given that the NNN superexchange interaction J2 is an AFM one, a non-collinear magnetic ground state is expected to manifest itself, reflecting the competition with the nearest-neighbor FM interaction. To get further insight into the magnetic ground states, microscopic measurements using single crystals are indispensable.

The present work establishes the structure and magnetic phase diagram of Sr1−xCaxCoO3 and demonstrates a great potential of the Co4+-containing perovskites as an interesting platform to explore a novel helimagnetic phase. Furthermore, this system can be an important reference to study topological helimagnetic phases in Fe4+-containing perovskites.9 

See the supplementary material for powder x-ray diffraction patterns with Rietveld refinement, thermogravimetric (TG) measurement, inverse magnetic susceptibility as a function of temperature, and effective Bohr magneton number as a function of Ca content x.

The authors appreciate M. Takano, H. Sakai, and J. Fujioka for the helpful suggestions. The authors thank M. Ashida, Y. Minowa, and M. Arai for experimental support. This work was partly supported by JSPS, KAKENHI (Grants Nos. 19K14652, 21H01030, and 22H00343), JST PRESTO Hyper-nano-space design toward Innovative Functionality (Grants No. JPMJPR1412), the Asahi Glass Foundation, the Murata Science Foundation, and the Mazda Foundation. The powder XRD measurement was performed with the approval of the Photon Factory Program Advisory Committee (Proposal No. 2018S2-006).

The authors have no conflicts to disclose.

Hidefumi Takahashi: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Writing – original draft (equal). Masaho Onose: Data curation (supporting); Formal analysis (equal). Yasuhito Kobayashi: Formal analysis (supporting). Takahiro Osaka: Data curation (equal); Formal analysis (equal). Maeda Soushi: Investigation (supporting). Atsushi Miyake: Formal analysis (supporting); Investigation (supporting). Masashi Tokunaga: Formal analysis (supporting); Investigation (supporting). Hajime Sagayama: Data curation (supporting); Investigation (supporting). Yuichi Yamasaki: Investigation (equal); Project administration (equal). Shintaro Ishiwata: Conceptualization (lead); Funding acquisition (lead); Supervision (lead); Writing – original draft (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Supplementary Material