Spin state and spectroscopic modes of multiferroic BiFeO ${}_3$

Journal Article · Fri Apr 19 00:00:00 EDT 2013 · Physical Review. B, Condensed Matter and Materials Physics

DOI:https://doi.org/10.1103/PhysRevB.87.134416· OSTI ID:1076905

Fishman, Randy Scott ^[1]; Haraldsen, Jason T. ^[2]; Furukawa, Nobuo ^[3]; Miyahara, Shin ^[4]

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1. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Materials Science & Technology Division
2. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
3. Aoyama Gakuin Univ., Sagamihara, Kanagawa (Japan). Dept. of Physics and Mathematics
4. Pohang Univ. of Science and Technology (POSTECH), Pohang, Gyeongbuk (Korea, Republic of). Asia Pacific Center for Theoretical Physics

Spectroscopic modes provide the most sensitive probe of the very weak interactions responsible for the properties of the long-wavelength cycloid in the multiferroic phase of BiFeO₃ below T_N ≈ 640 K. Three of the four modes measured by terahertz (THz) and Raman spectroscopies were recently identified using a simple microscopic model. While a Dzyaloshinskii-Moriya (DM) interaction D along [-1, 2, -1] induces a cycloid with wave vector (2π/α) (0.5 + δ, 0.5, 0.5 - δ) (δ ≈ 0.0045), easy-axis anisotropy K along the [1, 1, 1] direction of the electric polarization P induces higher harmonics of the cycloid, which split the $${\Psi}_{1}$$ modes at 2.49 and 2.67 meV and activate the $${{\Phi}}_{2}$$ mode at 3.38 meV. However, that model could not explain the observed low-frequency mode at about 2.17 meV. We now demonstrate that an additional DM interaction D' along [1, 1, 1] not only produces the observed weak ferromagnetic moment of the high-field phase above 18 T but also activates the spectroscopic matrix elements of the nearly degenerate, low-frequency $${\Psi}_{0}$$ and $${\Phi}_{1}$$ modes, although their scattering intensities remain extremely weak. Even in the absence of easy-axis anisotropy, D' produces cycloidal harmonics that split $${\Psi}_{1}$$ and activate $${\Phi}_{2}$$. However, the observed mode frequencies and selection rules require that both D' and K are nonzero. This work also resolves an earlier disagreement between spectroscopic and inelastic neutron-scattering measurements.

• Research Organization: Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Materials Sciences & Engineering Division; USDOE National Nuclear Security Administration (NNSA); Ministry of Education, Culture, and Technology, Japan (MEXT); Max Planck Society (MPG); Korea Ministry of Education, Science and Technology (MEST)
• DOE Contract Number: AC05-00OR22725
• OSTI ID: 1076905
• Journal Information: Physical Review. B, Condensed Matter and Materials Physics, Vol. 87, Issue 13; ISSN 1098-0121
• Publisher: American Physical Society (APS)
• Country of Publication: United States
• Language: English