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arXiv · 1105.1044

Antiferromagnet with two coupled antiferromagnetic sublattices in a magnetic field

Alexey V Sizanov·Arseny V Syromyatnikov
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Abstract

We discuss the magnon spectrum of an antiferromagnet (AF) in a magnetic field $h$ consisting of two interpenetrating AF sublattices coupled by the exchange interaction at T=0. One-ion easy-plane anisotropy is also taken into account. We calculate using the 1/S expansion the gap in the spectrum which is a manifestation of the order-by-disorder effect in this system and the optical magnon mode splitting. Both of these phenomena originate from the inter-sublattice interaction. We calculate also the gap value at $h\approx h_c$ in the leading order of the small parameter $(h_c-h)/h_c$ using the magnon Bose-Einstein condensation treatment, where $h_c$ is the saturation field. By comparing results obtained within these two approaches we conclude that the 1/S expansion gives a qualitatively correct result at $h\sim h_c$ even at large one-ion anisotropy but it overestimates the gap value. The application is discussed of these results to the actively studied AF of the considered type $\rm NiCl_2$-$\rm 4SC(NH_2)_2$ (DTN).

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Alexey V Sizanov, Arseny V Syromyatnikov. 2011-05-05. Antiferromagnet with two coupled antiferromagnetic sublattices in a magnetic field. https://doi.org/10.1088/0953-8984%2F23%2F14%2F146002

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