Most of your garden-variety magnets are made with elements in the middle of the periodic table. However, physicists have recently found magnetic properties coming from unexpected elements, such as oxygen. In a new development, Riki Oja of Aalto University, Finland, and collaborators report in Physical Review Letters on the first observation of permanent magnetism at the interface between two nonmagnetic oxides.
The classic ferromagnet is an element like iron or cobalt, in which the 3d orbital is only partially filled with electrons. In certain situations, the spins of these d electrons can align with each other to give the material an overall magnetic moment. Although less common, elements with unfilled p orbitals, like oxygen, can exhibit a similar spin alignment. This “non-d” orbital arrangement is called d0 ferromagnetism.
Previous work has found d0 ferromagnetism in doped oxides and oxides with defects. The team decided to look at a different class of oxygen compounds, called perovskites. The unique cubic structure of these materials sometimes gives rise to interesting properties, including magnetoresistance and ferroelectricity. To generate ferromagnetism, the team investigated combinations of two normal perovskites: SrTiO3 with either KNbO3, NaNbO3, or KTaO3. In certain pairings, electrons on metal d orbitals or electron holes on oxygen p orbitals become confined to the two-dimensional interface between the oxides. The researchers showed with computer simulations that hole-type interfaces could generate a spin alignment of the electron holes. On the experimental side, for several of the oxide combinations the team observed ferromagnetic hysteresis loops at room temperature. The discovery could lead to new routes towards multiferroics. – Michael Schirber