High-field magnetic structure and critical phenomena in MnWO4

Complete text of publication follows. The application of an external magnetic field parallel to the easy direction of the magnetic ground state AF1 induces two successive phase transitions, first at 2 T into the intermediate phase AF2 with an elliptically modulated structure and at 13.5 T into a high-field phase HF [1]. By neutron diffraction on a single crystal at 14.5 T we have found the magnetic structure of HF: it is a spin-flop phase with the same magnetic unit cell as AF1, k = (-1/4, 1/2, 1/2), and a collinear spin arrangement, but the magnetic moments are switched perpendicular to the applied field. The field and temperature dependence of the propagation vector k = (kx, 1/2, kz) in the incommensurate phases AF2 and AF3 have been studied on single crystals, especially near phase boundaries. Intensity, position and half width of the (-102)- reflection show critical behaviour, indicating that the coexistence point of HF, AF2 and the paramagnetic region at 14.5 T and about 12 K might be a Lifshitz-point. (author)