Magnetic Ground State of the Ideal Triangular-Lattice Antiferromagnets Tuned by the Inter-layer Interactions

Neutron inelastic scattering and diffraction techniques have been applied to investigate both structures and spin wave excitations of the tri-perovskite Ba$_{\mathrm{3}}$\textit{MM'}$_{\mathrm{2}}$O$_{\mathrm{9}}$, which is antiferromagnetic with the equilateral-triangular lattice M layers. Although the magnetic structure of the system is non-collinear 120\textdegree in ab-plane, the c-axis canting appears by increasing the spin momentum from $S$(Co$^{\mathrm{2+}}) \quad =$ 1/2 to $S$(Mn$^{\mathrm{2+}}) \quad =$ 5/2. Our measurements clearly show that the magnetic ground state and excitations could be modified by the inter-layer interaction, which is strongly interfered by the type and staggering method of the nonmagnetic $M'$O6 clusters. In addition, the lattice-related quantum phenomena were discussed, such as the temperature-dependent acoustic mode in Ba3NiNb2O9, and the pressure-effect on the magnetic dynamics of the Fermi-liquid-like ground state, Ba3NiSb2O9(6H-B).