Magnetic and structural studies of G-phase compound Mn$_6$Ni$_{16}$Si$_7$

Transition metal compounds with complex crystal structures tend to demonstrate interesting magnetic coupling resulting in unusual magnetic properties. In this work, the structural and magnetic characterization of a single crystal of the Ni-Mn-Si based G-phase compound, Mn$_6$Ni$_{16}$Si$_7$, grown by the Czochralski method, is reported. In this structure isolated octahedral Mn$_6$ clusters form a f.c.c. lattice. As each octahedron consists of eight edge-sharing equilateral triangles, the possibility for geometric frustration exists. Magnetization and specific heat measurements showed two magnetic phase transitions at 197 K and 50 K, respectively. At 100 K neutron diffraction on powder samples shows a magnetic structure with k = (001) in which only four of the six Mn spins per cluster order along $<100>$ directions giving a two dimensional magnetic structure consistent with intra-cluster frustration. Below the 50 K phase transition the Mn spins cant away from $<100>$ directions and a weak moment develops on the two remaining Mn octahedral sites.