There is currently a renewed interest in weakly ferromagnetic crystals such as iron borate (FeBO3) and haematite (a-Fe203) firstly, because they could be promising materials for data storage and processing systems in modern microelectronics and secondly because they display a range of novel non-linear phenomena. Such crystals are characterized by a strong interaction between the magnetic and elastic subsystems. So far, there have been preliminary studies on domain wall motion and non-linear effects using neutron diffraction and magneto-optic techniques. I wish to extend this work by taking advantage of the high resolution diffraction facilities of the ESRF to study the domain wall dynamics under an ultrasonic excitation and/or the application of an ac magnetic field. The pulsed nature of the x-ray source will also allow stroboscopic techniques to be employed for solving problems in magnetoacoustics and in the physics of non-linear interactions in solids.