Inducing Ferrimagnetism in Insulating Hollandite Ba1.2Mn8O16

Abstract

Magnetic insulators are functional materials with potential applications in spintronics and multiferroics. The hollandites AxM8O16, which contain mixed-valent transition metals, have demonstrated ferromagnetism combined with insulating behavior and provide a new platform for exploring the effects of magnetic frustration due to their “folded” triangular lattice. We have tuned the hollandite BaxMn8O16 from a complex antiferromagnet with Néel temperature (TN) = 25 K to a ferrimagnet with Curie temperature (TC) = 180 K via partial cobalt substitution for manganese. Both BaxMn8O16 and BaxCoyMn8-yO16 were prepared by salt flux methods, and combined neutron and X-ray diffraction confirm a distorted hollandite-type structure for both oxides. X-ray photoelectron spectroscopy reveals that the Co2+ substitution drives the average Mn oxidation state from 3.7+ to nearly 4.0+, thereby changing its d-electron count. Magnetization and resistivity measurements show that the cobalt-doped hollandite is a ferrimagnetic insulator, with a high TC of 180 K. On the basis of neutron diffraction measurements, we provide the first solution of the magnetic structure of BaxMn8O16, which consists of a complex antiferromagnet with a large magnetic unit cell. Upon substituting cobalt for manganese, the magnetic structure changes dramatically, destroying the previously large magnetic unit cell and promoting ferromagnetic alignment along the hollandite tunnel direction. The observed hysteresis at base temperature for BaxCoyMn8–yO16 is explained as arising from uncompensated spins aligned along the (200) crystallographic planes.