Spin dynamics in the triangular lattice compound α-SrCr2O4

Orateur : Manila SONGVILAY (LLB Saclay)

Two-dimensional triangular antiferromagnets have lead to a large number of theoretical and experimental studies as they combine low-dimensionality and frustration, which result into novel exotic states. α-ACr2O4 (A = Ca, Sr, Ba) is a family of quasi-2D triangular antiferromagnets, involving S=3/2 spins associated with chromium ions. These compounds present an orthorhombic crystal structure with pseudo-hexagonal symmetry, with stacked distorted triangular layers of edgesharing CrO6 octahedra, formed by two inequivalent Cr3+ sites. Among this family, we have studied α-SrCr2O4, which undergoes a magnetic transition below TN ∼ 43 K. The magnetic ordering corresponds to an incommensurate helical structure, close to the classical 120° structure [3]. Spin excitation spectra have been mapped out by means of inelastic neutron scattering and the spin Hamiltonian that best describes the data was determined, revealing a set of four different intra-plane exchange couplings. These four exchange couplings are associated with four different distances between nearest-neighbouring Cr3+, revealing the lattice distortion characteristic of this family [4]. The study further shows that α-SrCr2O4 can be placed as an intermediate case between the isotropic triangular lattice model and the parent compound α-CaCr2O4 reported by Toth et al. [1], as α-SrCr2O4 is less distorted than the Ca counterpart. In addition, α-SrCr2O4 has a reduced magnetic moment in the ordered state and the inelastic neutron scattering data shows singularly broad magnetic excitations ; both features could be a sign of strong quantum fluctuations affecting the ground state, as observed in α-CaCr2O4.

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