Single crystals of ${\mathrm{Mg}}_{1-x}{\mathrm{Al}}_x{\mathrm{B}}_2$ have been grown at a pressure of 30 kbar using the cubic anvil technique. Precipitation free crystals with $x<0.1$ were obtained as a result of the optimization of an already developed ${\mathrm{MgB}}_2$ crystal growth procedure. A systematic decrease of the $c$-axis lattice constant with increasing Al content, when the $a$-axis lattice constant is practically unchanged, was observed. Variation of the critical temperature on Al content in ${\mathrm{Mg}}_{1-x}{\mathrm{Al}}_x{\mathrm{B}}_2$ crystals was found to be slightly different than that one observed for polycrystalline samples, since even a very small substitution of 1%–2% of Al leads to the decrease of $T_c$ by about 2–3 K. X-ray and high resolution transmission electron microscopy investigations indicate the appearance of the second precipitation phase in the crystals with $x>0.1$. This is in a form of nonsuperconducting ${\mathrm{MgAlB}}_4$ domains in the structure of a superconducting ${\mathrm{Mg}}_{1-x}{\mathrm{Al}}_x{\mathrm{B}}_2$ matrix. Resistivity and magnetic investigations show the slight increase of the upper critical field, $H_{c2}$, for $H\Vert c$ for the samples with small $x$, a significant reduction of the $H_{c2}$ anisotropy at lower temperatures, and a decrease of the residual resistance ratio value for Al substituted samples as compared to those of unsubstituted crystals. Superconducting gaps variation as a function of Al content, investigated with point contact spectroscopy for the series of the crystals with $T_c$ in the range from 20 to 37 K, does not indicate on the merging of the gaps with decreasing $T_c$ down to 20 K. It may be related to an appearance of the precipitation phase in the ${\mathrm{Mg}}_{1-x}{\mathrm{Al}}_x{\mathrm{B}}_2$ structure.

• Received 11 November 2004

DOI:https://doi.org/10.1103/PhysRevB.71.174506