Fluctuation-induced ferrimagnetism in sublattice-imbalanced antiferromagnets with application to SrCu${}_2$(BO${}_3$)${}_2$ under pressure

Abstract

We show that a collinear Heisenberg antiferromagnet, whose sublattice symmetry is broken at the Hamiltonian level, becomes a fluctuation-induced ferrimagnet at any finite temperature $T$ below the Néel temperature $T_{\mathrm{N}}$. We demonstrate this using a layered variant of a square-lattice $J_1$-$J_2$ model. Linear spin-wave theory is used to determine the low-temperature behavior of the uniform magnetization, and non-linear corrections are argued to yield a temperature-induced qualitative change of the magnon spectrum. We then consider a layered Shastry-Sutherland model, describing a frustrated arrangement of orthogonal dimers. This model displays an antiferromagnetic phase for large intra-dimer couplings. A lattice distortion which breaks the glide symmetry between the two types of dimers corresponds to broken sublattice symmetry and hence gives rise to ferrimagnetism. Given indications that such a distortion is present in the material SrCu${}_2$(BO${}_3$)${}_2$ under hydrostatic pressure, we suggest the existence of a fluctuation-induced ferrimagnetic phase in pressurized SrCu${}_2$(BO${}_3$)${}_2$. We predict a non-monotonic behavior of the uniform magnetization as function of temperature.

arXiv comment11 pages, 10 figures; extended discussion on application to SrCu2(BO3)2 + minor changes, v2 as published

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