Crystal growth and physical properties of helimagnetic oxides

This work is focused on two helimagnetic materials: Ba2CuGe2O7 and Cu3Nb2O8. Recent studies report a number of interesting anisotropic proper- ties [1, 2, 3]. Ba2CuGe2O7 melilite oxide shows a complex magnetic behaviour, indeed it is known that at low temperature the system undergoes a transition from a paramagnetic phase to an incommensurate antiferromagnetic cycloid spin structure. Applying a magnetic field, additional magnetic transitions take place, as for example a spin-cone phase [3]. Moreover, Ba2CuGe2O7 shows also multiferroic properties [1]. Several works report that the physical properties of melilite oxides mainly depend on the nature of the transition metal ion, thus interesting proper- ties could emerge in mixed melilite oxides. In this work Ba2MGe2O7 with M=Cu,Ni and Mn have been studied. Cu3Nb2O8 is an unusual helimagnetic compound that undergoes a series of magnetic ordering at low temperature. Development of electric polarization P has been reported at TN=25K corresponding to emergence of a non-collinear helicoidal ordering. P is oriented perpendicularly to the common plane of rotation of the spins. This observation cannot be reconciled with the conven- tional theory developed for cycloidal multiferroics [2]. The study of all these complex anisotropic phenomena requires the availability of good single crystals. In this thesis, an investigation on crystal growth conditions of Ba2MGe2O7 and of Cu3Nb2O8 will be presented [4, 5]. Single crystal samples are vital to study the physical properties exhibited by compounds which have high magnetic/ferroelectric anisotropy where significantly different behaviour is seen along different crystallographic directions. Preliminarily, high quality polycrystalline powders have been prepared for all compounds, indeed this is a critical point to grow pure crystalline samples. 1 Abstract 2 In this work the procedure to synthesize polycrystalline powders with high purity is reported. Moreover, by using powder X-ray diffraction and energy dispersive spectroscopy (EDS), the composition of the starting polycrystalline powder is checked. Successfully, the growth conditions to realize large and pure single crystals suitable for low temperature magnetometry and lattice dynamic studies are described. The chemical composition and the morphology of the crystals are investigated by X-ray diffraction and by scanning electron microscopy (SEM), with wave- length dispersive spectrometry (WDS). Furthermore, the excellent quality of the crystals is confirmed by rocking curve measurements. The X-ray Laue back reflection and electron backscattered diffraction (EBSD) techniques are used to orient single crystals specifically for selected experi- ments. To study the magnetic phase diagrams of grown crystals, magnetization measurement vs temperature is performed in the range 1.5 K<T<300K on both Ba2CuGe2O7 and Cu3Nb2O8 crystals. Moreover, low temperature (0.5 K<T<5 K) magnetometry study is performed on oriented single crystal of Ba2CuGe2O7. For the first time, a magnetic phase below 0.6 K is detected. Finally, this work reports lattice dynamics of Ba2CuGe2O7 helimagnet pro- vided by Infra-red and Raman spectra [6, 7]. [edited by author]