Trinity College Dublin

Unexpected Ferromagnetism in a dielectric oxide

Following the research done by the group in ferromagnetic semiconductors [Ferromagnetism in Fe-doped SnO₂ thin films, J.M.D. Coey, A.P. Douvalis, C.B. Fitzgerald, M. Venkatesan, Applied Physics Letters 84, p1332-1334 (link)], the Magnetism Group has published another paper in Nature (click here for .pdf file). The subject of the paper is what has been termed as "d⁰ ferromagnetism", i.e. the phenomenon of magnetism is systems where there are supposedly no unpaired spins that can contribute to a magnetic moment. In most magnetic materials, the moment is due to spins that have no partner of opposite spin. Whether or not an unpaired electron is left is determined by the "Hund's rules". A lack of unpaired electrons should result in no magnetic moment.

The recent findings, however, contradict this theory. The samples investigated are thin films of HfO₂. Since neither Hf⁴⁺ or O^2- are magnetic ion and the d and f shells of the Hf⁴⁺ are either full or empty, one would not expect to see a moment due to the reasons given above. A moment is observed and it is also quite anisotropic. The proposed mechanism for this magnetism may be lattice defects and oxygen vacancies.

• Mike is elected to the US National Academy of Sciences
• Mike's 60th Birthday party
• Unexpected ferromagnetism in a dielectric oxide
• Michael Coey elected to Fellowship of the Royal Society