Weak ferromagnetic ordering in brownmillerite Ca2Fe2O5 and its effect on electric field gradients

Brownmillerite Ca2Fe2O5 (CFO) exhibits a magnetic transition at TN ∼ 730 K. Many studies have reported the magnetic properties of CFO. However, the magnetic structure of CFO is still debated, i.e., whether the magnetic ordering is purely antiferromagnetic or weakly ferromagnetic, which originated fr...

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Bibliographic Details
Published in:Physical chemistry chemical physics : PCCP 2017, Vol.19 (46), p.31194-31201
Main Authors: Kagomiya, Isao, Hirota, Yuki, Kakimoto, Ken-ichi, Fujii, Kotaro, Shiraiwa, Masahiro, Yashima, Masatomo, Fuwa, Akio, Nakamura, Shin
Format: Article
Language:English
Subjects:
Antiferromagnetism
Brownmillerite
Calcium aluminum ferrite
Critical temperature
Crystal structure
Electric fields
Electric polarization
Electrical properties
Electrical resistivity
Ferromagnetism
Filing
Magnetic moments
Magnetic properties
Magnetic structure
Magnetism
Magnetization
Magnetoresistance
Magnetoresistivity
Mossbauer spectroscopy
Spectrum analysis
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Summary:Brownmillerite Ca2Fe2O5 (CFO) exhibits a magnetic transition at TN ∼ 730 K. Many studies have reported the magnetic properties of CFO. However, the magnetic structure of CFO is still debated, i.e., whether the magnetic ordering is purely antiferromagnetic or weakly ferromagnetic, which originated from canted magnetic moments. In addition, the reason for the CFO showing large magnetoresistance is still unclear. This study attempts to address the unresolved issues stated above by multiple investigations on the crystal structure, magnetization, and Mössbauer parameters. Based on the results of the investigation, we conclude that the CFO is not purely antiferromagnetic but weakly ferromagnetic. That is the reason for the disappearance of the spontaneous magnetization at the magnetic critical temperature TN. The Mössbauer spectroscopy shows that the magnetic moments slightly cant against the a-direction, resulting in the presence of a net magnetic moment along the c-direction under the space group of Pnma. A reason for the canted magnetic moments is due to the presence of the Dzyalosinskii–Moriya (DM) interaction. The electric field gradient (EFG) refined from the Mössbauer spectroscopy investigated at 287 K is larger than that at 750 K, which is higher than TN. This suggests that the EFG changes below TN. A local electric polarization induced by the DM interaction is a possible reason for the change in the EFG. As a result, strong correlations between the magnetic ordering and the electrical properties appear in the CFO. The Arrhenius plot of the total electrical conductivity showed a kink at TN, which is one of these strong correlations.
ISSN:1463-9076
1463-9084
DOI:10.1039/c7cp04520d