Magnetic order, spin-induced ferroelectricity and structural studies in multiferroic Mn1 xCoxWO4

Abstract

The manuscript summarizes the work done during the last three years in the Institut Laue Langevin (Grenoble) in collaboration with the Instituto de Ciencia de Materiales de Barcelona, the Universitat Autònoma de Barcelona and the Commissariat à l’énergie atomique et aux énergies alternatives (Grenoble). Since ferroelectricity was found to be coupled to the magnetic order in TbMnO3, there has been a renewed interest in multiferroics, both for fundamental reasons and for possible technological applications. About five year ago, it was discovered that the electric polarization that emerges in MnWO4 is coupled to a cycloidal magnetic structure. This material has three magnetic states: AF3 (13.5 K < T < 12.5 K), moments order collinearly along a direction u within the ac plane with a sinusoidally modulated amplitude and an incommensurate propagation vector k = (-0.214, 1 , 0.457); AF2 (12.5 K < T < 6.8 K), it presents an additional magnetic component along b, the propagation vector be- ing the same. Thus, the magnetic ordering is an elliptical cycloidal spin structure within the ub plane. In this phase the electric polarization arises along the b axis and therefore, this is the multiferroic phase. AF1 (T < 6.8 K), the magnetic structure is collinear (along u) with k = (¼, ½, ½). The succession of magnetic structures is induced by a strong magnetic competition which turns into magnetic frustration. The application of external fields or chemical substitution could unbalance the subtle equilibrium between the magnetic interactions. The work on the thesis was initiated by a publication in the literature (on polycrystalline samples) which showed that a subtle substitution of manganese by cobalt induces changes and stabilizes the multiferroic structure at low temperatures. The thesis presents a study of the crystal and magnetic structures, and electric polarization of Mn1-xCoxWO4 (x =0, 0.05, 0.10, 0.15 and 0.20) at zero and non-zero magnetic field (up to 12 T). We have studied the interplay between the magnetic structure and the electric polarization. The superspace formalism, used to analyze the incommensurate mangetic structures of the pure MnWO4 compound, showed the relation between the mangetic atoms in each phase in a very simple way. The Co-doping gives rise to new magnetic structures and some of them are multiferroics. Regarding the electric polarization associated to those new magnetic structures, depending on the structure the polarization is oriented along a different direction. Therefore, we have obtained a chemical control of the orientation of the electric polarization. Moreover, we have found weak but clear spin-lattice coupling in some compositions. Concerning the measurements under field, several types of transitions have been observed: continuous rotations of the rotation plane of the spins (and continuous rotation of the associated polarization), discontinuous transitions... To achieve our purposes bulk magnetometry, pyroelectric measurements, neutron- and synchrotron-diffraction experiments have been carried out on single crystals. The manuscript is organized in the following way: (i) Part I, Introduction: multiferroics, magnetic structures, the studied materials and the experimental techniques used are introduced. At the end the interest and objectives of the thesis are enumerated. (ii) Part II, Magnetic, ferroelectric and structural properties of Mn1-xCoxWO4 under zero magnetic field. (iii) Part III, Magnetic-field-induced transitions. (iv) Part IV, Summary and Conclusions. (v) Part V, Appendices.

Date of Award	6 Feb 2013
Original language	English
Supervisor	Vassil Hristov Skumryev (Director), Eric Ressouche (Director) & José Luís García Muñoz (Director)