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The spin dynamics of the S=1 Ni-chains in mixed-spin antiferromagnets Pr_2 BaNi O_5 and Nd_x Y_2-x Ba Ni O_5 is described in terms of a simpleGinzburg-Landau Lagrangian coupled to the sublattice of rare-earth ions. Withinthis framework we obtain a theoretical explanation for the experimentallyobserved coexistence of Haldane gap excitations and long-range magnetic order,as well as for the increase of the Haldane gap energy below the Neel point. Wealso predict that the degeneracy of the Haldane triplet is lifted in themagnetically ordered phase. The theoretical results are consistent with theavailable experimental data.Comment: 4 pages, 1 figure, submitted to PRL An alternative derivation of main  results and new references adde

physical review. b, condensed matter

Coexistence of Haldane-gap excitations and long-range antiferromagnetic order in mixed-spin nickelates<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>R</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">BaNiO</mml:mi></mml:mrow><mml:mrow><mml:mn>5</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Coexistence of Haldane-gap excitations and long-range antiferromagnetic order in mixed-spin nickelatesR2BaNiO5