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Magnetism of a material depends on two objects; (i) magnetic moment and (ii) mediators (delocalized electrons) that couple them. Almost all the magnetic materials consist of elements having partially filled f (rare-earths: Eu, Gd, Tb, Dy etc.) or d (transition metals: Fe, Co, Ni etc.) bands among the highest occupied energy bands. Small radial extension of f-orbitals in rare-earths makes them highly localized leading to finite magnetic moment and coupling occurs via the interaction of these moments with the conduction electrons. The d electrons have both; magnetic moment due to strong electron-electron Coulomb repulsion and itineracy that mediates magnetic coupling.
Various recent studies discovered some materials those exhibit ferromagnetism (Curie temperature ~ 600 K) although they do not contain magnetic elements. On the other hand, it is known for many decades that the presence of magnetic element in a material does not ensure magnetism of that material. In this talk, I will present some of our high resolution photoemission results that help to explore the origin of such effects. It appears that low density of vacancies/defects in small band gap semiconductors can lead to high temperature ferromagnetism due to disorder induced effects. In the reverse case, Kondo effect helps to compensate the magnetic moment leading to absence of magnetic order.
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