College Park, Maryland      June 6 - 10 , 2004

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W2-C4 (2:30 PM): Spin structure and dynamics in the half-doped La_1.5Sr_0.5CoO₄ (Invited)

I. A. Zaliznyak, J. M. Tranquada, G. Gu (Physics Department, Brookhaven National Laboratory), R. W. Erwin (National Institute of Standards and Technology), Y. Moritomo (Center for Integrated Research in Science and Engineering (CIRSE), Nagoya University, Nagoya 464-8601, Japan)

Magnetic order and spin dynamics in the charge-ordered phases of the doped, strongly correlated transition metal oxides such as superconducting cuprates, magnetoresistive manganites, etc., are at the focus of the modern condensed matter research. The interplay of charge, spin and/or orbital degrees of freedom in these systems results in non-trivial ground states and leads to many fascinating physical properties, such as a magnetic field driven metal-insulator transition, etc. In order to understand the relative importance of the various interactions it is imperative to explore the properties of different materials in the strongly correlated oxides family. Here we report a study of the charge and spin opder and the spin dynamics in the half-doped cobaltate, La_1.5Sr_0.5CoO₄, by elastic and inelastic neutron scattering. We find a short-range correlated, glassy charge order (CO) which occurs at a very high temperature, T_c = 825(25)K. It is of the checkerboard type, which is not unexpected at half doping, and is essentially two-dimensional. This ordering does not seem to depend on the spin order, which is incommensurate and occurs only below T_s ~ 30 K. The spin system, on the contrary, is essentially defined by the charge-ordered ground state, where Co³⁺ ions are in the S=1, intermediate spin (IS) state. As a result, these ions seem to be quenched to the S^z = 0 state at low T by the strong single-ion anisotropy, and do not participate in the static magnetic ordering. However, they bridge the magnetic Co²⁺ ions and introduce a diagonal, frustrating coupling between them. Peculiar coupling of the Co²⁺ and Co³⁺ spin systems leads to a very unusual spin dynamics, where a well-defined 2D spin waves with bandwidth ΔSW ~ 15 meV are followed by a broad continuum scattering at E > 20 meV.

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