Neutron scattering and muon-spin spectroscopy studies of the magnetic triangular-lattice compounds A2La2NiW2 O12 (A=Sr,Ba)

We report on the geometrically frustrated two-dimensional triangular-lattice magnets A2La2NiW2O12 (A=Sr,Ba) studied mostly by means of neutron powder diffraction (NPD) and muon-spin rotation and relaxation (μSR) techniques. The chemical pressure induced by the Ba-for-Sr substitution suppresses the ferromagnetic (FM) transition from 6.3 K in the Ba compound to 4.8 K in the Sr compound. We find that the R3¯ space group reproduces the NPD patterns better than the previously reported R3¯m space group. Both compounds adopt the same magnetic structure with a propagation vector k=(0,0,0), in which the Ni2+ magnetic moments are aligned ferromagnetically along the c axis. The zero-field μSR results reveal two distinct internal fields (0.31 and 0.10 T), caused by the long-range FM order. The small transverse muon-spin relaxation rates reflect the homogeneous internal field distribution in the ordered phase and, thus, further support the simple FM arrangement of the Ni2+ moments. The small longitudinal muon-spin relaxation rates, in both the ferromagnetic and paramagnetic states of A2La2NiW2O12, indicate that spin fluctuations are rather weak. Our results demonstrate that chemical pressure indeed changes the superexchange interactions in A2La2NiW2O12 compounds, with the FM interactions being dominant.