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  Yokaichiya, F.; Krimmel, A.; Tsurkan, V.; Margiolaki, I.; Thompson, P.; Bordallo, H.N.; Buchsteiner, A.; Stüßer, N.; Argyriou, D.N.; Loidl, A.: Spin-driven phase transitions in ZnCr₂Se₄ and ZnCr₂S₄ probed by high-resolution synchrotron x-ray and neutron powder diffraction. Physical Review B 79 (2009), p. 064423/1-10

10.1103/PhysRevB.79.064423

Abstract:
The crystal and magnetic structures of the spinel compounds ZnCr₂S₄ and ZnCr₂Se₄ were investigated by high resolution powder synchrotron and neutron diffraction. ZnCr₂Se₄ exhibits a first order phase transition at T_N=21~K into an incommensurate helical magnetic structure. Magnetic fluctuations above T_N are coupled to the crystal lattice as manifested by negative thermal expansion. Both, the complex magnetic structure and the anomalous structural behavior can be related to magnetic frustration. Application of an external magnetic field shifts the ordering temperature and the regime of negative thermal expansion towards lower temperatures. Thereby, the spin ordering changes into a conical structure. ZnCr₂S₄ shows two magnetic transitions at T_N1=15~K and T_N2=8~K that are accompanied by structural phase transitions. The crystal structure transforms from the cubic spinel-type (space group Fd\={3}m) at high temperatures in the paramagnetic state, via a tetragonally distorted intermediate phase (space group I4₁/amd) for T_N2 < T < T_N1 into a low temperature orthorhombic phase (space group I m m a) for T < T_N2. The cooperative displacement of sulfur ions by exchange striction is the origin of these structural phase transitions. The low temperature structure of ZnCr₂S₄ is identical to the orthorhombic structure of magnetite below the Verwey transition. When applying a magnetic field of 5~T the system shows an induced negative thermal expansion in the intermediate magnetic phase as observed in ZnCr₂Se₄.