We describe neutron scattering experiments performed to investigate the magnetic order and dynamics of half-doped La1.5Sr0.5CoO4. This layered perovskite exhibits a near-ideal checkerboard pattern of Co2+/Co3+ charge order at temperatures below ~ 800 K. Magnetic correlations are observed at temperatures below ~ 60 K but static magnetic order only becomes established at 31 K, a temperature at which a kink is observed in the susceptibility. On warming above 31 K we observed a change in the magnetic correlations which we attribute either to a spin canting or to a change in the proportion of inequivalent magnetic domains. The magnetic excitation spectrum is dominated by an intense band extending above a gap of approximately 3 meV up to a maximum energy of 16 meV. A weaker band exists in the energy range 20-30 meV. We show that the excitation spectrum is in excellent quantitative agreement with the predictions of a spin-wave theory generalized to include the full magnetic degrees of freedom of high-spin Co2+ ions in an axially distorted crystal field, coupled by Heisenberg exchange interactions. The magnetic order is found to be stabilized by dominant antiferromagnetic Co2+ -- Co2+ interactions acting in a straight line through Co3+. No evidence is found for magnetic scattering from the Co3+ ions, supporting the view that Co3+ is in the S = 0 state in this material.
@article{arxiv.0909.4964,
title = {Magnetic order and dynamics of the charge-ordered antiferromagnet La1.5Sr0.5CoO4},
author = {L. M. Helme and A. T. Boothroyd and R. Coldea and D. Prabhakaran and C. D. Frost and D. A. Keen and L. P. Regnault and P. G. Freeman and M. Enderle and J. Kulda},
journal= {arXiv preprint arXiv:0909.4964},
year = {2009}
}
Comments
17 pages, 10 figures. Accepted for publication in Phys. Rev. B