Magnetic dispersion and anisotropy in multiferroic BiFeO₃

Based on neutron-scattering measurements of the low energy magnetic excitations, the exchange and anisotropy parameters of multiferroic BiFeO₃ have been determined for the first time.  These results will help to develop device applications for multiferroic BiFeO₃, which has attracted great interest because both the magnetic and ferroelectric transitions are well above room temperature. 

The low-energy excitations of BiFeO₃ were measured using newly-available single crystals and the recently-commissioned US-Japan Cold Neutron Triple-Axis Spectrometer CTAX at the High Flux Isotope Reactor (HFIR). As shown in Fig.1, zone-center peaks were observed at 1.1 and 2.5 meV.

Those peaks were successfully explained by a model that includes exchange interactions J₁ and J₂, a Dzaloshinsky-Moriya interaction D due to the broken inversion symmetry of the crystal, and a weak easy-axis anisotropy K along the electric polarization.  The dispersion of the spin excitations was used to fix J₁ and J₂.  For the observed periodicity of the multiferroic spin state and based on the observed low-energy peaks, this model determined that D = 0.16 meV and K = 0.0068 meV.  Those values yield zone-center peaks at 1.10 and 2.33 meV, in excellent agreement with the experimental observations. 

Results for the weak anisotropy K have implications for the manipulation of the spin state and electric polarization in multiferroic devices using BiFeO₃.

For more information, please contact Randy Fishman, fishmanrs@ornl.gov

M. Matsuda, R. S. Rishman, T. Hong, C. H. Lee, T. Ushiyama, Y. Yanagisawa, Y. Tomioka, and T. Ito, "Magnetic dispersion and anisotropy in multiferroic BeFeO₃," Phys. Rev. Lett. 109, 067205 (2012).