  We successfully synthesized and characterized the triangular lattice
anitferromagnet Ba$_8$MnNb$_6$O$_{24}$, which comprises equilateral spin-5/2
Mn$^{2+}$ triangular layers separated by six non-magnetic Nb$^{5+}$ layers. The
detailed susceptibility, specific heat, elastic and inelastic neutron
scattering measurements, and spin wave theory simulation on this system reveal
that it has a 120 degree ordering ground state below T$_N$ = 1.45 K with
in-plane nearest-neighbor exchange interaction ~0.11 meV. While the large
separation 18.9 A between magnetic layers makes the inter-layer exchange
interaction virtually zero, our results suggest that a weak easy-plane
anisotropy is the driving force for the k$_m$ = (1/3 1/3 0) magnetic ordering.
The magnetic properties of Ba$_8$MnNb$_6$O$_{24}$, along with its classical
excitation spectra, contrast with the related triple perovskite
Ba$_3$MnNb$_2$O$_9$, which shows easy-axis anisotropy, and the iso-structural
compound Ba$_8$CoNb$_6$O$_{24}$, in which the effective spin-1/2 Co$^{2+}$
spins do not order down to 60 mK and in which the spin dynamics shows sign of
strong quantum effects.
