Investigation by spherical neutron polarimetry of magnetic properties in BaCo2(AsO4)2

The cobaltite BaCo2(AsO4)2 (BCAO) is a good realization of the quasi-2D system in which the Co2+ ions form a magnetically frustrated honeycomb lattice with magnetic moments mainly confined in the basal planes. Below , the ground state structure is described by an incommensurate propagation vector k{approximately equal to}(0.265,0,-1.33) and the magnetic excitation spectrum displays a resolution-limited energy gap at q=0. No magnetic excitations emerging from q=k could be detected, contrary to what would be expected for a simple helimagnetic ground state. In order to understand these puzzling results, we have recently re-investigated the magnetic structure and excitations in BCAO using spherical neutron-polarization analysis.