Magnetic properties of perovskite Ca_1−xSr_xFeO₃

The perovskite Fe oxides with the formula AFeO₃ (A: Ca or Sr) contain high-valence Fe⁴⁺.^1,2 A perovskite-type CaFeO₃ undergoes a metal-insulator transition and structural transition from orthorhombic to monoclinic at 290 K.⁵ Mössbauer spectroscopy indicated that CaFeO₃ undergoes charge disproportionation 2Fe⁴⁺ → Fe^(4−δ)+ + Fe^(4+δ)+(δ : temperature dependence) below 290 K.⁶ 

In contrast, SrFeO₃ has a perovskite cubic lattice structure and displays metallic conductivity. The magnetic structure of SrFeO₃ also has helical magnetic order below 134 K.³ Saturated magnetic moment reported for SrFeO3 at 38 T, has a value of 3.5 μ_B.⁴ 

Alloys of Ca_1−xSr_xFeO₃ containing Fe⁴⁺ with unusual valence have been synthesized under high oxygen pressure and their resistivity and magnetism have been investigated.⁷ 

Here, we report the magnetic properties of perovskite Ca_1−xSr_xFeO₃ prepared by ozone oxidation method. We observed the magnetic susceptibility and the magnetization of Ca_1−xSr_xFeO₃ under high magnetic field H ∼ 60 T. Hence, we also report the magnetic behavior of Fe in Ca_1−xSr_xFeO₃.

First, we synthesized polycrystalline Ca_1−xSr_xFeO₃ with an oxygen deficiency. The raw materials were mixed and calcined twice at 1073 K and 1273 K in air. Finally, an ozonizer was used to obtain oxygen-stoichiometric Ca_1−xSr_xFeO₃.

Magnetic susceptibility was measured between 5 and 300 K in a field of 1 kOe using a superconducting quantum interference device (SQUID) magnetometer. High magnetic field magnetization measurement up to 60 T was performed using a pulse magnet with duration of about 6 ms at the Kindo Laboratory, Institute for Solid State Physics, University of Tokyo.

The temperature dependence of the magnetic susceptibility is shown in Fig. 1. The Neel temperature, T_N, indicating antiferromagnetic transition in all the compositions, is observed and is almost constant.

In the temperature range above T_N, a bending of the magnetic susceptibility corresponding to the charge disproportionation transition temperature T_CD is observed. T_CD shifted towards low temperature with increase in the doping amount of Sr and was observed until x = 0.6. This result is consistent with the fact that charge disproportion is not observed in SrFeO₃,⁸ indicating that it disappears near the composition with x = 0.6.

Figure 2 depicts the dependence of T_N and T_CD on composition. Since the antiferromagnetic transition temperature is independent of composition, it is shown that all the compositions of Ca_1−xSr_xFeO₃ have a helical magnetic structure. We confirmed that it has a crystal structure in which the cubic and orthorhombic phases are mixed around x = 0.6. From these results, it is concluded that the antiferromagnetic interaction of helical magnetism is not directly related to the change of charge disproportionation.

T_CD decreases towards the boundary between the orthorhombic and cubic phases on Sr substitution. It is clear that the charge disproportionation occurs in orthorhombic crystal structure and disappears in cubic. Thus, the decrease of T_CD strongly suggests that it is related to the deformation of the crystal structure from orthorhombic to cubic. This phenomenon might be caused by a change in the inclination of the oxygen octahedron surrounding the Fe atom.

Magnetization of the samples as a function of applied magnetic fields up to H ∼ 60 T at 4.2 K is presented in Fig. 3. All compositions exhibited metamagnetic behavior with hysteresis similar to that reported in a previous study.⁹ Magnetization curve with respect to magnetic field shows antiferromagnetic behavior due to the presence of helical magnetism. In the Ca-rich region, magnetization does not reach saturation but saturates in the Sr-rich region with x = 0.8 and 1.0. In the Sr-rich region, that is, under the cubic structure, magnetization exhibits a forced ferromagnetic order with a value of ∼3.2 μ_B at high magnetic fields. The metamagnetic transition in each magnetization curve indicates that there is an intermediate stable state, a ferrimagnetic state, in which the helical magnetism is deformed. In addition, the magnitude of the high magnetic field magnetization (H = 60 T) does not show a monotonic composition dependence on the doping amount of Sr. It has a minimum at x = 0.4; it decreases from x = 0.0 to 0.4 and then increases until saturation magnetization at x = 1.0. It is thought that this behavior against the Sr doping amount is related to the behavior of electric resistivity⁷ but the origin is not clearly understood.

In order to understand these magnetic and electronic behaviors of the charge disproportionation Fe, it is helpful to know the magnitude of saturation magnetization of the Ca-rich samples, it is necessary to perform experiments at higher magnetic fields. Furthermore, these results are thought to be closely related to the crystal structure and a detailed study of the tetravalent or charge disproportionation Fe in the oxygen octahedron is necessary.

We synthesized perovskite Ca_1−xSr_xFeO₃ by low temperature heat treatment with ozone oxidization and measured magnetic susceptibility at H = 1 kOe and magnetization under high magnetic field H ∼ 60 T.

T_CD decreased with Sr substitution and this charge disproportionation occurred in orthorhombic crystal structure while disappeared in cubic. It is possible that this charge disproportionation can be caused by a change in the inclination of the oxygen octahedron surrounding the Fe atom.

Magnetization of Ca_1−xSr_xFeO₃ exhibited metamagnetic behavior with hysteresis. This metamagnetic behavior is thought to be due to the existence of an intermediate ferrimagnetic stable state derived from the presence of helical magnetic order. In the Ca-rich orthorhombic region, the magnetization does not reach saturation, and in the Sr-rich cubic region with x = 0.8 and 1.0, the saturation magnetizations show forced ferromagnetic order with value of ∼3.2 μ_B at high magnetic field.