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Mapping spin-wave dispersions in stripe-ordered La 2 − x Sr x Ni O 4 ( <…

2005; American Physical Society; Volume: 72; Issue: 6 Linguagem: Inglês

10.1103/physrevb.72.064437

1550-235X

Hyungje Woo, A. T. Boothroyd, Kenji Nakajima, T. G. Perring, Christopher Frost, P. G. Freeman, D. Prabhakaran, K. Yamada, J. M. Tranquada,

Advanced Condensed Matter Physics

Using the MAPS spectrometer at the ISIS spallation source, we have measured the magnetic excitations of single-crystal samples of stripe-ordered ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ni}{\mathrm{O}}_{4}$ with $x=0.333$ and 0.275. The full two-dimensional spin-wave dispersions were obtained using incident energies of 60 and 160 meV. To analyze the excitations, we have evaluated a spin-only Hamiltonian describing diagonal, site-centered stripes in the linear spin-wave approximation. Besides the superexchange energy $J$ within antiferromagnetic domains, we have considered effective exchange couplings ${J}_{1}$ and ${J}_{2}$ across a charge stripe coupling second-neighbor Ni sites along Ni-O bond directions and along the plaquette diagonal, respectively. From least-squares fits of the model to the measurements on the $x=1∕3$ sample at $T=10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, we find that the dispersions are well described by a model using just $J$ and ${J}_{1}$, but not $J$ and ${J}_{2}$. Consistent with an analysis of previous measurements, we find that $J$ is about 90% of the superexchange energy of undoped ${\mathrm{La}}_{2}\mathrm{Ni}{\mathrm{O}}_{4}$ and ${J}_{1}∕J\ensuremath{\approx}0.5$. The excitations observed for $x=0.275$ are surprisingly similar to those for $x=1∕3$, despite the differing magnetic-ordering wave vectors; the main difference is a broadening of the excitations for $x=0.275$. For both samples, we find that one spin-wave branch has a gap of $\ensuremath{\sim}20\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, confirming a previous observation for $x=1∕3$. We discuss the possible origin of this gap.