Travelling-solvent floating-zone growth of the dilutely Co-doped spin-ladder compound Sr14(Cu, Co)24O41

We present here crystal growth of dilutely Co-doped spin-ladder compounds Sr14(Cu 1−x, Cox)24O41 (x = 0, 0.01, 0.03, 0.05, 0.1) using the Travelling Solvent Floating Zone (TSFZ) technique associated with an image furnace. We carried out detailed microstructure and compositional analysis. The microstructure of the frozen-in FZ revealed two bands: a lower band consisting of well-aligned single-crystalline stripes of the phase Sr14(Cu, Co)24O41 embedded in the eutectic mixture of composition SrO 18% and (Cu, Co)O 82%; and an upper band consisting of a criss-crossed pattern of these stripes. These analyses were also employed to determine the distribution coefficient of the dopants in Sr14Cu24O41. The distribution coefficient turned out to be close to 1, different from Sr2CuO3 reported previously where Co tend to accumulate in the molten zone. Direct access to the composition of the frozen-in zone eliminated any previous ambiguities associated with the composition of the peritectic point of Sr14Cu24O41; and also the eutectic point in the binary SrO-CuO phase diagram. The lattice parameters show an anisotropic variation upon Co-doping with parameters a and b increasing, c decreasing; and with an overall decrease of the unit cell volume. Magnetic susceptibility measurements were carried out on the pristine and the Co-doped crystals along the principal crystallographic axes. The spin susceptibility of the x = 0.01 crystal exhibits a strong anisotropy, which is in stark contrast with the isotropic behaviour of the pristine crystal. This anisotropy seems to arise from the intradimer exchange interaction as inferred from the anisotropy of the dimer contribution to the susceptibility of the Co-doped crystal. The Curie-tail in the magnetic susceptibility of Sr14(Cu 1−x, Cox)24O41 (x = 0, 0.01, 0.03, 0.05, 0.1) crystals (field applied parallel to the ladder) was found to scale with Co-doping - the scaling is employed to confirm a homogeneous distribution of Co in a x = 0.1 crystal boule.