Spiral order induced by distortion in a frustrated square-lattice antiferromagnet

Abstract

In a strongly frustrated square-lattice antiferromagnet with diagonal coupling J ′ , for α = J/(2J ′ ) 1, an incommensurate spiral state with propagation vectorQ = (π ± δ, π ± δ) near (π, π) competes closely with the Néel collinear antiferromagnetic ground state. For classical Heisenberg spins the energy of the spiral state can be lowered as it adapts to a distortion of the crystal lattice. As a result, a weak superstructural modulation such as exists in doped cuprates might stabilize an incommensurate spiral phase for some range of the parameter α close to 1. An interplay between small distortion of the crystal lattice and the magnetic properties of the material is currently a subject of intense research. One problem which supplies strong motivation for such studies is that of stripe order in the lightly doped high-T c cuprates La 2−x Sr x CuO 4+y (LSCO) and in related nickelates [1,2]. These phases are always associated with a weak superstructural distortion of the original "stacked square lattice" structure of the un-doped parent material. Incommensurate magnetism in these compounds is usually interpreted in terms of a segregation of the doped charges into lines which separate the antiferromagnetic domains ("stripes") characteristic of the un-doped material. Although modulation of the crystal structure which is induced by charge-stripe segregation is often too small to be observed in experiment [1], it is clear that essential result of the stripe order for the spin system of cuprates is a periodic modulation of the exchange coupling in the Heisenberg spin Hamiltonian which describes their magnetic properties [3]. So far, though, only the simplest "average" consequence of the stripe superstructure, in the form of the effective weakening of exchange coupling in the direction perpendicular to the stripes, has been considered [4]. A similar problem, of an interplay between the spin order and the cooperative Jahn-Teller distortion accompanying the charge order, arises in the context of the charge-ordered phases in doped manganites [5].Because the low-energy magnetic properties of layered LSCO cuprates are believed to be adequately described by the two-dimensional (2D) Heisenberg spin Hamiltonian, this model has recently become a focus of intense research. Special attention was devoted to the frustrated square lattice, where in addition to the nearest-neighbor exchange interaction, J > 0, there is a diagonal coupling, J ′ > 0, such that α = J 2J ′ is close to 1. It was originally motivated by the predictions that non-Néel resonating valence bond states [6,7] and quantum-critical behavior [8] associated with the T = 0 order-disorder phase transitions which may occur in this case might be important for the physics of the superconductivity in cuprates.Despite RVB spin-liquid state and quantum criticality are strongly predicated upon the quantum nature of the spins (S=1/2 in cuprates), a semiclassical spin-wave theory appears to provide a surprisingly good guidance to the behavior of the frustrated square-lattice antif...

Effect of antifluorite layer on the magnetic order in Eu-based 1111 compounds, EuTAsF (T = Zn, Mn, and Fe)

“…Extensive theoretical studies show that collinear (½ ½ 0) phase is stable for J 2 / J 3 > 0.60–0.65, 47,48 thus explaining the experimental Eu 2+ order in the Zn and Mn compounds. In EuFeAsF, the critical value of J 2 / J 3 is approached, and incommensurate spiral states can become favourable, especially when they adapt to lattice distortions, 49 as present in EuFeAsF. Therefore, we conclude that the increasing J 3 and decreasing J 2 / J 3 ratio should be the main causes for the incommensurate order of the Eu 2+ spins in EuFeAsF as opposed to the Mn and Zn compounds.…”