  Oxide double perovskites wherein octahedra formed by both 3d elements and
sp-based heavy elements give rise to unconventional magnetic ordering and
correlated quantum phenomena crucial for futuristic applications. Here, by
carrying out experimental and first principles investigations, we present the
electronic structure and magnetic phases of Ba2MnTeO6, where Mn^2+ ions with S
= 5/2 spins constitute a perfect triangular lattice. The magnetic
susceptibility reveals a large Curie- Weiss temperature -152 K suggesting the
presence of strong antiferromagnetic interactions between Mn^2+ moments in the
spin lattice. A phase transition at 20 K is revealed by magnetic susceptibility
and specific heat which is attributed to the presence of a sizeable inter-plane
interactions. Below the transition temperature, the specific heat data show
antiferromagnetic magnon excitations with a gap of 1.4 K. Furthermore, muon
spin-relaxation reveals the presence of static internal fields in the ordered
state and provides strong evidence of short-range spin correlations for T > TN.
The DFT+U calculations and spin-dimer analysis infer that Heisenberg
interactions govern the inter and intra-layer spin-frustrations in this
perovskite. The inter and intra-layer exchange interactions are of comparable
strengths (J1 = 4.6 K, J2 = 0.92 J1). However, a weak third nearest-neighbor
ferromagnetic inter-layer interaction exists (J3=-0.04 J1) due to
double-exchange interaction via the linear path Mn-O-Te-O-Mn. The combined
effect of J2 and J3 interactions stabilizes a three dimensional long-range
magnetic ordering in this frustrated magnet.
