Low-temperature evolution of the modulated magnetic structure in the ferroelectric antiferromagnet BiFeO3

High-resolution time-of-flight neutron diffraction studies of the magnetic ordering in the multiferroic BiFeO3 are presented. Our results show that the cycloidal modulated ordering proposed earlier [I. Sosnowska, T. Peterlin-Neumaier, and E. Steichele, J. Phys. C 15, 4835 (1982)] is stable between 10 and 295 K. The concept of the anharmonic character of the magnetic modulation in BiFeO3 that was used for the interpretation of NMR, Raman, and THz spectroscopy studies of BiFeO3 is discussed. The influence of the anharmonic modulation on the magnetic contributions to the BiFeO3 neutron diffraction patterns is presented. Our experimental data can be described by assuming anharmonic effects with m < 0.25. We propose a method for the anharmonicity strength evaluation based on neutron diffraction data.