Inducing Ferrimagnetism in Insulating Hollandite Ba1.2Mn8O16

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• 作者：Amber M. Larson ; Pouya Moetakef ; Karen Gaskell ; Craig M. Brown ; Graham King ; Efrain E. Rodriguez
• 刊名：Chemistry of Materials
• 出版年：2015
• 出版时间：January 27, 2015
• 年：2015
• 卷：27
• 期：2
• 页码：515-525
• 全文大小：616K
• ISSN：1520-5002

文摘

Magnetic insulators are functional materials with potential applications in spintronics and multiferroics. The hollandites A_xM₈O₁₆, 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 鈥渇olded鈥?triangular lattice. We have tuned the hollandite Ba_xMn₈O₁₆ from a complex antiferromagnet with N茅el temperature (T_N) = 25 K to a ferrimagnet with Curie temperature (T_C) = 180 K via partial cobalt substitution for manganese. Both Ba_xMn₈O₁₆ and Ba_xCo_yMn_8-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 T_C of 180 K. On the basis of neutron diffraction measurements, we provide the first solution of the magnetic structure of Ba_xMn₈O₁₆, 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 Ba_xCo_yMn_8鈥?i>yO₁₆ is explained as arising from uncompensated spins aligned along the (200) crystallographic planes.