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Abstract

This work presents the single-crystal growth of the ilmenite titanates $M$TiO$_3$ ($M$ = Co, Ni) and the perovskite-nickelate LaNiO$_3$ and their detailed magnetic, structural and thermodynamic investigations. All the single crystals were grown using the floating-zone technique in a four-mirror and a high-pressure optical furnace respectively. Detailed characterization of the LaNiO$_3$ single-crystals solves the ambiguity concerning its magnetic ground state and emphasizes the importance of oxygen-vacancy defects in nickelates. The full magnetic phase diagram of the easy-plane type NiTiO$_3$ and CoTiO$_3$ is constructed for the first time using high-field dilatometry and magnetization measurements. The thermal expansion studies indicate strong magnetoelastic coupling in titanates and Grüneisen analysis yields the corresponding pressure dependencies. The comparison of neutron diffraction, dilatometry and dielectric data indicates the presence of significant magneto-electric coupling in NiTiO$_3$ and of magneto-dielectric coupling in CoTiO$_3$. Magnetostriction measurements in the easy-plane exhibit significant lattice changes in low-fields due to domain-rearrangement processes and furthermore show remarkable effects of changing the uniaxial pressure which is also attributable to domain effects. In addition, the crystal growth and magneto-structural characterization of MnTiO$_3$, Ni$_{0.25}$Mn$_{0.75}$TiO$_3$ and La$_4$Ni$_3$O$_{10}$ are presented.