Growth and characterization of large $(\mathrm{Y},\mathrm{La}){\mathrm{TiO}}_3$ and $(\mathrm{Y},\mathrm{Ca}){\mathrm{TiO}}_3$ single crystals

The Mott-insulating rare-earth titanates ($R{\mathrm{TiO}}_3$, with $R$ being a rare-earth ion) are an important class of materials that encompasses interesting spin-orbital phases as well as ferromagnet-antiferromagnet and insulator-metal transitions. The growth of these materials has been plagued by difficulties related to overoxidation, which arises from a strong tendency of ${\mathrm{Ti}}^{3+}$ to oxidize to ${\mathrm{Ti}}^{4+}$. We describe our efforts to grow sizable single crystals of ${\mathrm{YTiO}}_3, {\mathrm{Y}}_{1-x}{\mathrm{La}}_x{\mathrm{TiO}}_3$ ($x\le 0.25$), and ${\mathrm{Y}}_{1-y}{\mathrm{Ca}}_y{\mathrm{TiO}}_3$ ($y\le 0.35$) with the optical traveling-solvent floating-zone technique. We present sample characterization via chemical composition analysis, magnetometry, charge transport, neutron scattering, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism to understand macroscopic physical property variations associated with overoxidation. Furthermore, we demonstrate a good signal-to-noise ratio in inelastic magnetic neutron scattering measurements of spin-wave excitations. A superconducting impurity phase, found to appear in Ca-doped samples at high doping levels, is identified as TiO.

Figure 1

Picture of a large ${\mathrm{YTiO}}_3$ single crystal grown with the TSFZ method. The sectioning used for characterization is indicated.

Figure 2

Magnetic-field dependence of the magnetic moment per Ti ion for the ${\mathrm{YTiO}}_3$ samples, obtained along the $c$ axis at $T=5\mathrm{K}$. The samples were zero-field cooled and measured in an increasing field. Inset: Temperature dependence of the field-cooled $c$-axis magnetization, obtained in an applied magnetic field of 50 G. The magnetization values are normalized to those at 20 K.

Figure 3

(a) Temperature dependence of the resistivity for the ${\mathrm{YTiO}}_3$ samples. (b) Arrhenius plot for the resistivities in (a), along with the corresponding activation energies.

Figure 4

(a) Ti $L_{2,3}$-edge XAS results for four ${\mathrm{YTiO}}_3$ samples. The data measured at the lowest energy were subtracted, and the net intensity was subsequently normalized at the highest peak. A ${\mathrm{Ti}}^{4+}$ reference spectrum obtained from a ${\mathrm{SrTiO}}_3$ (STO) sample is displayed as well; it has been shifted vertically for clarity, and the dashed black lines correspond to its peak positions. (b) XMCD intensities at the Ti $L_{2,3}$ edge at 6 K, in the ferromagnetic phase. The data were normalized by the intensity of the largest Ti $L_{2,3}$-edge XAS peak.

Figure 5

Temperature dependence of the AC susceptibility obtained with an excitation frequency of 3.63 kHz for samples cut from a large ${\mathrm{Y}}_{1-x}{\mathrm{La}}_x{\mathrm{TiO}}_3$ ($x=0.22$) single crystal.

Figure 6

Pictures of comounted single crystals of (a) ${\mathrm{Y}}_{1-x}{\mathrm{La}}_x{\mathrm{TiO}}_3$ ($x=0.2$) and (b) ${\mathrm{Y}}_{1-y}{\mathrm{Ca}}_y{\mathrm{TiO}}_3$ ($y=0.1$) for inelastic magnetic neutron scattering measurements.

Figure 7

Temperature dependence of the AC susceptibility obtained with an excitation frequency of 3.63 kHz for samples from near the beginning (B) and end (E) of the growth of a large ${\mathrm{Y}}_{1-y}{\mathrm{Ca}}_y{\mathrm{TiO}}_3$ ($y=0.15$) single crystal.

Figure 8

(a) Reciprocal space map of the XRD intensity in the ($0KL$) plane for a ${\mathrm{Y}}_{1-y}{\mathrm{Ca}}_y{\mathrm{TiO}}_3$ (YCTO) single crystal with $y=0.35$. Apart from the strong Bragg peaks at even integer valued $K$ and $L$, powder rings due to a polycrystalline impurity phase are visible. The intensity has been normalized to the peak value at (002)${}_{\text{o}}$. (b) Integrated XRD intensity as a function of the scattering angle $2\theta$ for the same $y=0.35$ crystal as in (a). The calculated $2\theta$ values along with those for the TiO impurity phase are indicated as dashed lines. (c) Temperature dependence of the superconducting volume fraction $4\pi \chi$ obtained from zero-field-cooled magnetization in an applied $c$-axis magnetic field of 50 G (left axis) and of the sample resistance $R$, normalized to the normal-state value $R_n$ (right axis). The onset temperatures are nearly indistinguishable. The data are for a $y=0.4$ sample. Temperature dependence (d) of the normalized resistance in different applied magnetic fields and (e) of the critical magnetic field for the same sample as in (c).

Figure 9

(a) Neutron diffraction rocking scans across the (020)${}_{\text{o}}$ Bragg peak for large single crystals of ${\mathrm{Y}}_{1-x}{\mathrm{La}}_x{\mathrm{TiO}}_3$ and ${\mathrm{Y}}_{1-y}{\mathrm{Ca}}_y{\mathrm{TiO}}_3$. The intensities are normalized to the peak values. The solid lines are Gaussian fits to the data. (b) Inelastic neutron scattering measurement of spin waves at (001)${}_{\text{o}}$. The high-temperature ($T>T_C$) paramagnetic scattering was subtracted from data taken in the ferromagnetically ordered state at 1.5 K. Note that some of the data are scaled, as indicated. The error bars correspond to one standard deviation. The solid lines are guides to the eye.