Master Thesis Defense by Christopher R. Andersen – Niels Bohr Institute - University of Copenhagen

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Master Thesis Defense by Christopher R. Andersen

Studying multiferroic tellurides

The making and investigations of the M3TeO6 family-type, M = Ni, Co, Mn

Multiferroicity is the coupling of the magnetic, ferroelectric and elastic orders in a solid. It is a phenomenon, which has obtained increasing interest from the beginning of the 21st century. It is especially the coupling between the magnetism and electric polarization, which is believed to be important for the development of future technology.

The telluride family, M3TeO6 with M = Co, Mn and Ni, has a great interest within this field, since they obtain different crystal structures and the origin of the magneto-electric coupling is believed to be different for the three compounds. A study of the magnetic behavior is therefore of great importance.

One way to study the magnetic behavior is indirectly by substituting some of the magnetic atoms with non-magnetic atoms. This has been done for the complex magnetic compound, Co3TeO6, which has a spin current causing the electric polarization. Two new phases, Co2ZnTeO6 and Co2MgTeO6, have successfully been synthesized and characterized and the compounds were not been reported in earlier literature. Co2ZnTeO6 has a second order phase transition at TN = 13.2(3) K with some frustration f ≈ 2.6 and appearance of short range magnetic ordering. It has an effective magnetic moment of µeff = 3.49 µB/Co, being close to the effective moment of a low spin only configuration and significantly lower than that of Co3TeO6.

Co2MgTeO6 has a crystal structure similar to that of Mn3TeO6. It does not get an incommensurate magnetic structure as Mn3TeO6, but a commensurate antiferromagnetic structure with k = (0 0 0.5) below a critical temperature of TN = 29.06(10) K with a second order phase transition. The spins are oriented along the a-b-plane. It shows almost no frustration with f ≈ 1.3 and it has an effective magnetic moment of µeff = 3.97 µB/Co similar to that of Co2ZnTeO6.

Another way to investigate a material is a direct study of the magnetic behavior of the compound, which has been done with neutron diffraction on 14 co-aligned Ni3TeO6 single crystals. The present study reveals a field-induced phase transition at high fields and the character of the phase transition is debated. The H-T phase diagram is reported. These studies reveal a magnetic behavior different from what has been reported in earlier literature.