In this study, we conducted a comprehensive investigation of the magnetic, electronic, and structural properties of high-quality polycrystalline Y-type ferrite Ba₂Fe₁₄O₂₂ throughout a wide temperature range [1]. This compound with a mixed valence state of Fe²⁺ and Fe³⁺ is expected to display a rich variety of physical phenomena because of the interplay of charge, spin, and orbital degrees of freedom. Previous studies have been impeded by the difficulties in synthesizing phase-pure samples since ferrimagnetic impurities like BaFe₂O₄ and Fe₃O₄ can contaminate Ba₂Fe₁₄O₂₂. We were able to synthesize high-purity polycrystalline samples that are ideal for studying the material’s intrinsic characteristics by fine-tuning solid-state reaction conditions.

A ferrimagnetic transition was detected at 662 K, which is somewhat higher than the values reported in previous literature [2,3], according to magnetic measurements. Dissimilarities in oxygen stoichiometry are probably to blame for this disparity. The finding of a precipitous decrease in magnetization at 160 K points to the existence of a phase transition. The magnetic moment per formula unit drops from 9.4 ${\mu }_{\mathrm{B}}$ before of the transition to 8.8 ${\mu }_{\mathrm{B}}$ below it. Magnetization curves above the transition and below 160 K are also significantly different.

Above the transition, the magnetization curve indicates planar-type collinear ferrimagnetism with saturation occurring at around 20 kOe. In contrast, the magnetization grows ferromagnetically at lower temperatures; it keeps growing linearly all the way up to 70 kOe, though. We can infer a non-collinear magnetic structure from this unsaturated field-linear behavior near high fields.

Electrical conductivity measurements indicate semiconducting behavior with moderate conductivity at room temperature ($\rho \approx$33 Ω·cm). On cooling, conductivity gradually diminishes and shows a drop near 160 K, suggesting carrier localization. The conductivity exhibits Arrhenius-type behavior at lower temperatures, with two distinct activation energies: 98 meV between 105–125 K, and 50 meV below 105 K. This implies a reduction in carrier population and mobility at low temperatures.

Figure 1 illustrates the powder X-ray diffraction pattern obtained using Cu-Kα₁ radiation on a SmartLab diffractometer with a He closed-cycle refrigerator system. The results revealed a structural phase transition at 160 K from a high-temperature rhombohedral to a low-temperature triclinic crystal system. The site-selective localization of Fe²⁺ ions is thought to be the cause of this structural change. Owing to the larger ionic radius compared to Fe³⁺, Fe²⁺ ions preferentially occupy octahedral sites, particularly those with an up-spin alignment in the magnetic structure. The magnetic moment per formula unit decreases from 9.4 ${\mu }_{\mathrm{B}}$ above the transition to 8.8 ${\mu }_{\mathrm{B}}$ below it, consistent with the theoretical value assuming Fe²⁺ localization among specific octahedral sites.

In summary, this study elucidates the intrinsic temperature-dependent properties of Ba₂Fe₁₄O₂₂ by combining structural, magnetic, and electronic measurements. The first order structural transition at 160 K, linked to Fe²⁺ site-selective localization, leads to a reduction in magnetic moment and a marked change in magnetic anisotropy. These findings highlight the intricate coupling between charge, spin, and lattice degrees of freedom in mixed valence hexaferrites.

References

• [1] T. Waki et al., J. Phys. Soc. Jpn. 94 (in press).
• [2] M. A. H. Farhat and J. C. Joubert, J. Magn. Magn. Mater. 62, 353 (1986).
• [3] X. Zhang and J. Zhang, Mater. Lett. 269, 127642 (2020).

Authors

• T. Waki^a, R. Sobajima^a, J. Yamaura, Y. Tabata^a, and H. Nakamura^a
• ^aDepartment of Materials Science and Engineering, Kyoto University