Among various parent compounds of iron pnictide superconductors, EuFe 2 As 2 stands out due to the presence of both spin density wave of Fe and antiferromagnetic ordering (AFM) of the localized Eu 2+ moment. Single crystal neutron diffraction studies have been carried out to determine the magnetic structure of this compound and to investigate the coupling of two magnetic sublattices. Long range AFM ordering of Fe and Eu spins was observed below 190 K and 19 K, respectively. The ordering of Fe 2+ moments is associated with the wave vector k = (1,0,1) and it takes place at the same temperature as the tetragonal to orthorhombic structural phase transition, which indicates the strong coupling between structural and magnetic components. The ordering of Eu moment is associated with the wave vector k = (0,0,1). While both Fe and Eu spins are aligned along the long a axis as experimentally determined, our studies suggest a weak coupling between the Fe and Eu magnetism.

We have studied a EuFe 2 As 2 single crystal by neutron diffraction under magnetic fields up to 3.5 T and temperatures down to 2 K. A field induced spin reorientation is observed in the presence of a magnetic field along both the a and c axes, respectively. Above critical field, the ground state antiferromagnetic configuration of Eu 2+ moments transforms into a ferromagnetic structure with moments along the applied field direction. The magnetic phase diagram for Eu magnetic sublattice in EuFe 2 As 2 is presented. A considerable strain (∼0.9%) is induced by the magnetic field, caused by the realignment of the twinning structure. Furthermore, the realignment of the twinning structure is found to be reversible with the rebound of magnetic field, which suggested the existence of magnetic shape-memory effect. The Eu moment ordering exhibits close relationship with the twinning structure. We argue that the Zeeman energy in combined with magnetic anisotropy energy is responsible for the observed spin-lattice coupling.PACS numbers: 74.70. Xa, 75.30.Kz, 75.80.+q The recent discovery of iron pnictide superconductors has triggered extensive research on their physical properties and mechanism of high temperature superconductors [1][2][3]. All iron pnictides are found to be of layered structure in nature. For undoped iron pnictides, the chains of parallel Fe spins within the FeAs layers couple antiferromagnetically in the ab plane of the orthorhombic lattice with an antiparallel arrangement along the c axis [4][5][6]. This antiferromagnetic (AFM) order in the parent compounds is likely due to a spin-densitywave (SDW) instability caused by Fermi surface nesting [7]. Similar to the high T c cuprate superconductors, the undoped iron pnictides are not superconducting under ambient pressure and show an antiferromagnetic SDW order. Upon carrier doping, the magnetic order is suppressed and superconductivity emerges concomitantly [8,9]. EuFe 2 As 2 is a peculiar member of the iron arsenide AFe 2 As 2 family since the A site is occupied by Eu 2+ , which is an S-state (orbital angular momentum L = 0) rare-earth ion possessing a 4f 7 structure with the total electron spin S = 7/2. Here we report a single crystal neutron diffraction measurement on EuFe 2 As 2 under a magnetic field up to 3.5 T. The spin reorientation of Eu moments is observed upon an applied magnetic field parallel to both a and c axes of the orthorhombic structure, while the Fe SDW order persists at high magnetic fields. Interestingly, the application of a magnetic field changes the twinning population in EuFe 2 As 2 and the redistribution of the domain population is found to be associated with the evolution of the magnetic order of Eu moments, which indicates the existence of a giant spin-lattice coupling effect. A single crystal of EuFe 2 As 2 was grown by the Sn-flux method [10]. It was in shape of a platelet with approximate dimensions of 5 × 5 × 1 mm 3 . Single crystal neutron scattering measurements were performed on the thermal neutron two-axis diffractometer...

Single crystals of NASICON-type material Li 1+x Ti 2-x Al x (PO 4) 3 (LATP) with 0 ≤ x ≤ 0.5 were successfully grown using long-term sintering techniques. Sample material was studied by chemical analysis, single crystal X-ray and neutron diffraction. The Ti 4+ replacement scales very well with the Al 3+ and Li + incorporation. The additional Li + thereby enters the M3 cavity of the NASICON framework at x, y, z ~ (0.07, 0.34, 0.09) and is regarded to be responsible for the enhanced Li + conduction of LATP as compared to Al-free LTP. Variations in structural parameters, associated with the Ti 4+ substitution with Al 3+ + Li + will be discussed in detail in this paper.

The floating-zone method with different growth ambiences has been used to selectively obtain hexagonal or orthorhombic DyMnO(3) single crystals. The crystals were characterized by x-ray powder diffraction of ground specimens and a structure refinement as well as electron diffraction. We report magnetic susceptibility, magnetization and specific heat studies of this multiferroic compound in both the hexagonal and the orthorhombic structure. The hexagonal DyMnO(3) shows magnetic ordering of Mn(3+) (S = 2) spins on a triangular Mn lattice at T(N)(Mn) = 57 K characterized by a cusp in the specific heat. This transition is not apparent in the magnetic susceptibility due to the frustration on the Mn triangular lattice and the dominating paramagnetic susceptibility of the Dy(3+) (S = 9/2) spins. At T(N)(Dy) = 3 K, a partial antiferromagnetic order of Dy moments has been observed. In comparison, the magnetic data for orthorhombic DyMnO(3) display three transitions. The data broadly agree with results from earlier neutron diffraction experiments, which allows for the following assignment: a transition from an incommensurate antiferromagnetic ordering of Mn(3+) spins at T(N)(Mn) = 39 K, a lock-in transition at T(lock-in) = 16 K and a second antiferromagnetic transition at T(N)(Dy) = 5 K due to the ordering of Dy moments. Both the hexagonal and the orthorhombic crystals show magnetic anisotropy and complex magnetic properties due to 4f-4f and 4f-3d couplings.

The recent surge of interest in phase change materials GeTe, Ge2Sb2Te5, and related compounds motivated us to revisit the structural phase transition in GeTe in more details than was done before. Rhombohedral-to-cubic ferroelectric phase transition in GeTe has been studied by high resolution neutron powder diffraction on a spallation neutron source. We determined the temperature dependence of the structural parameters in a wide temperature range extending from 309 to 973 K. Results of our studies clearly show an anomalous volume contraction of 0.6% at the phase transition from the rhombohedral to cubic phase. In order to better understand the phase transition and the associated anomalous volume decrease in GeTe we have performed phonon calculations based on the density functional theory. Results of the present investigations are also discussed with respect to the experimental data obtained for single crystals of GeTe.