Spin wave dispersion and magnons from short range order in tapiolite (FeTa₂O₆); a quasi-two-dimensional antiferromagnet

Hague, J. P.; Chung, E. M. L.; Visser, D.; Balakrishnan, G.; Clementyev, E.; Paul, D. McK. and Lees, M. R. (2005). Spin wave dispersion and magnons from short range order in tapiolite (FeTa₂O₆); a quasi-two-dimensional antiferromagnet. Journal of Physics: Condensed Matter, 17(46) pp. 7227–7235.

DOI: https://doi.org/10.1088/0953-8984/17/46/007

Abstract

We describe neutron inelastic scattering measurements of spin waves in the quasi-two-dimensional antiferromagnet FeTa₂O₆. The intrinsic spin wave dispersion for a single domain is deduced from linear spin wave theory and used to determine the intra-plane exchange coefficients. Almost dispersion-free excitations are observed along the c* direction perpendicular to the magnetic planes. This justifies the neglect of inter-plane coupling, which merely stabilizes the ordered configuration below TN. Spin wave theory yields values for the nearest neighbour and next nearest neighbour exchange constants on perpendicular diagonals of J = −0.040(9) meV, J'/J = 0.20(0), J''/J = 0.23(9), and the anisotropy parameter D = 0.31(8) meV. The large value of D shows an Ising-like spin configuration to be consistent with the experimental data. Measurements of the temperature dependence of the scattering show that magnetic excitations persist to at least 20 K (over two times TN) due to the extensive 2D short range order. This is a highly unusual result, and has consequences for the understanding of two-dimensional spin systems.

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