Magnetism when you least expect it

The picture of magnetism that has prevailed over the years has centered around the existence of localized electrons and their ordering leading to different types of magnetic order. Recently several unconventional examples of magnetism have been emerged such as p-band magnetism in alkali metal oxides - a system with no traditional magnetic ions in them. Unfortunately high magnetic ordering temperatures were not possible and this was traced to an orbital ordering transition [1]. While orbital ordering are usually found to reinforce one type of magnetic order over another, in the present case one found it led to the loss of any possibility of high magnetic ordering temperature. In another example, we consider the example of a 4d oxide SrTcO₃ where recent experiments [2] have found a Neel temperature of 1023 K, far higher than its 3d counterpart SrMnO₃. A detailed analysis [3] within a microscopic multiband Hubbard model reveals a rather surprising result of higher bandwidth and smaller Hund's exchange coupling strength resulting in higher magnetic ordering temperatures in "SrTcO₃-type" oxides.

[1] A.K.Nandy, Priya Mahadevan, P.Sen and D.D. Sarma, Phys. Rev. Lett. 105, 056403 (2010).

[2] Efrain E. Rodriguez, Frédéric Poineau, Anna Llobet, Brendan J. Kennedy, Maxim Avdeev, Gordon J. Thorogood, Melody L. Carter, Ram Seshadri, David J. Singh, and Anthony K. Cheetham^, Phys. Rev. Lett. 106, 067201 (2011).

[3] S. Middey, A.K. Nandy, P. Mahadevan and D.D. Sarma (submitted).