Approaching the Ground State of the Kagomé Antiferromagnet

Abstract

Y 0:5 Ca 0:5 BaCo 4 O 7 contains kagomé layers of Co ions, whose spins are strongly coupled, with a CurieWeiss temperature of ÿ2200 K. At low temperature, T 1:2 K, our diffuse neutron scattering study with polarization analysis reveals characteristic spin correlations close to a predicted two-dimensional coplanar ground state with staggered chirality. The absence of three-dimensional long-range antiferromagnetic order indicates negligible coupling between the kagomé layers. The scattering intensities are consistent with high spin S 3=2 states of Co 2 in the kagomé layers and low spin S 0 states for Co 3 ions on interlayer sites. Our observations agree with previous Monte Carlo simulations indicating a ground state of effectively short range, staggered chiral spin order. DOI: 10.1103/PhysRevLett.98.067201 PACS numbers: 75.25.+z, 61.12.ÿq, 75.40.Cx, 75.50.Ee The topology of many crystal structures has an important influence on the collective behavior of interacting magnetic moments. In lattices with triangular networks the antiferromagnetic (AF) coupling between all spins cannot be satisfied simultaneously owing to geometrical frustration that strongly reduces the ordering temperature and disturbs the settling of the system into a long-range ordered state. Exotic phenomena such as spin-ice and spinliquid phases can emerge from magnetic interactions that are incompatible with the underlying crystal geometry. A hallmark of frustration is the large degeneracy of complex, noncollinear ground states of finite entropy and the appearance of chiral correlations [1]. According to the MerminWagner theorem [2], in low dimensions, the AF order that would reveal the ground state properties is suppressed at finite temperatures. The famous case of spins on the twodimensional kagomé lattice that are simply AF coupled to only nearest neighbor comprises high geometrical frustration and low dimensionality and still challenges theoretical understanding as well as experimental observations and analysis.Here, we present diffuse neutron scattering results with polarization analysis on a new compound Y 0:5 Ca 0:5 BaCo 4 O 7 , in which the Co spins (S 3=2) in noninteracting kagomé layers appear to realize ideally the kagomé AF with only 2D nearest neighbor interactions, which allows an unprecedented approach to its unusual ground state properties.Considering the classical Heisenberg model of AF coupled nearest neighbor spins on the kagomé lattice, the ground state is highly degenerate and characterized by competing chiral spin structures. The spins have a relative orientation of 120 degrees in each triangular unit, so that the local sum of spins is zero. However, there are two competing structures that show either uniform or staggered chirality, see Fig. 1. According to predictions for the classical Heisenberg AF the staggered chiral structure is favored [3,4], because the existence of local zero energy modes, so-called weathervane defects, which are common spin rotations on hexagons, cause a larger degeneracy; such entropical selecti...

spinvert: a program for refinement of paramagnetic diffuse scattering data

“…[15], or do they follow the model of ferromagnetic correlations between Co1 spins identified in Ref. [39] for YBaCo 4 O 7 ?…”

“…We consider spin correlations in the compound Y 0.5 Ca 0.5 BaCo 4 O 7 [15]. Based on powder diffuse scattering data, this compound was originally suggested to realise a kagome-lattice model similar to that studied in Section 5 [15].…”

“…Most such approaches essentially involve fitting a simple form for the radial spin correlation function to the powder data. In these methods, the connectivity of the crystal structure is not considered; instead, either the magnitude of the spin correlations is fitted for a one-dimensional list of atomic separations (see, e.g., [14,15]), or the diffuse features are fitted to a broad peak-shape function to estimate the spin correlation length (see, e.g., [16]). However, the crystal structure plays no less significant a role in paramagnets than in ordered magnets: indeed, in a paramagnet the crystal structure completely determines the symmetry and periodicity of the diffuse scattering pattern [17].…”

Spin-liquid phase and order by disorder of classical Heisenberg spins on the swedenborgite lattice

“…The magnetic Co atoms in this structure reside on the so-called swedenborgite lattice and at present there exist examples of systems that order magnetically and some that do not [15][16][17][18][19][20][21][22]. In this Rapid Communication, we undertake an in-depth study of a minimal model for these compounds and show evidence that both classical spin-liquid phases as well as entropic order-by-disorder are potentially existent in these systems.…”

“…The ratio J 2 /J 1 can be altered experimentally by appropriate chemical substitutions of the ions on the triangular layers. Actual compounds show a large spread in the realized ratio J 2 /J 1 from 0 J 2 /J 1 1 [21,29] [30]. Compounds with small J 2 /J 1 might also be used to realize an effective 2d kagome system in a temperature range where J 2 is not relevant yet.…”