Sunday, November 15, 2015

Can graphite be ferromagnetic?

If people asked me this question last week, I would answer with an emphatic "No!", since then I discovered that my point of view about the matter was based on a prejudice based on an old article published by Heisenberg, which stated that only "d" or "f" electrons could spontaneously polarize. I learned this in the talk given by Prof. Pablo Esquinazi from the University of Leipzig, held in the 6th Workshop on Novel Methods for Electronic Structure Calculations, finished yesterday in La Plata, Argentina.

Magnetic ordering in these cases is a result of unpaired spins related with vacancies (if you still didn't read my point-of-view on things that do not exist, read here). The effect was predicted in graphene, but cannot be experimentally realized. The reason is that not all vacancies may ferromagnetically polarize, only those siting in particular sublattices do and there is no way to create an array of vacancies in graphene sitting only on a particular sublattice, because they are symmetrically equivalent. In graphite the layered structure breaks the symmetry and then the vacancy formation energy becomes different in different sublattices.

But the effect is not restricted to carbon, SiC may present the effect, as well as ZnO. Strange enough, not all oxides present the phenomenon. The effect is very weak and the Curie temperatures reach about room temperature, but it is exciting producing megnetism without Fe, Co, Ni, Cr, Gd, Dy and Tb.

Talking about the event, it s a very sympathetic meeting, with a small number of participants, mostly coming from south american countries, but I was gratefully surprised with the quality of the presentations. I must say I learned something with every talk I watched.