The discovery of charge and spin stripes in the high-T_c cuprate superconductors has motivated us to investigate other doped oxides possessing the K₂NiF₄ structure.  The single-layered cobalt oxides La_2-xSr_xCoO₄ are one such system with similar crystal structure.  Although no superconductivity has been found at any doping level, compounds in this cobaltate family do exhibit charge stripe ordering and other phenomena similar to those found in the cuprates. 

 In order to understand the static and dynamic properties of the charge stripe phases in the cobaltates by neutron scattering, large single crystals are needed (1-3).  We will report the optimisation and growth of large La_2-xSr _x CoO₄ (x=0.0 to 0.9) crystals by the floating-zone technique in an image furnace. Crack-free single crystals of diameter ~10 mm and length up to 80 mm have been obtained. One of the major issues associated with this system was the oxygen non-stoichiometry.  To over come this problem, we have post annealed the as-grown crystals in CO:CO₂ atmosphere.  The phase-purity of the crystals has been checked by powder x-ray diffraction, and information on the crystal quality has been obtained by x-ray Laue and neutron diffraction. We will give an account of our investigations into the conditions needed to obtain the highest quality crystals. We will also provide some preliminary results on the spin and charge ordering in the crystals.

 References

1. A. T. Boothroyd, P. Babkevich, D. Prabhakaran, and P. G. Freeman, Nature 471 (2011) 341–344.

2. P. Babkevich, D. Prabhakaran, C. D. Frost, and A. T. Boothroyd, Phys. Rev. B 82 (2010) 184425.

3. L.M. Helme, A.T. Boothroyd, R. Coldea, D. Prabhakaran, C.D. Frost, D.A. Keen, L.P. Regnault, P.G. Freeman, M. Enderle, and J. Kulda, Phys. Rev. B 80 (2009) 134414.

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1. Transition-metal monoxide single crystal growth by the floating-zone technique