Magnetic tunnel junctions with perpendular anisotropy

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Magnetic tunnel junctions with perpendicular anisotropy show extremely small current densities for spin transfer torque switching (STT). In addition, the temperature, the electric field and the film thickness have a strong impact on the anisotropy and the switching of the magnetization. In particular the impact of the electric field, which is not yet well understood, opens paths towards devices with extremely small power consumption. Using tunnel junctions with CoFeB electrodes and MgO barriers, we have found a switching current density down to 100 kA/cm². Apparently unphysical values (< 10 A/cm²) can be realized as well. For the conditions that are necessary to obtain these values, however, it turned out that all three factors (spin polarized current density, temperature and electric field) modify the magnetic properties and thereby the switching behaviour.This is the motivation for the application for continuation of funding: in the further course of the project, we want to realise the three effects in pure form by systematically varying the thickness of the tunnel barrier and that of one of the electrodes made from Co, Fe and CoFe(B) and a non-magnetic counter electrode. In particular the impact of the electric field will be studied by using thick barriers which allow only small tunnel currents and prevent heating. We plan to apply the magnetooptical Kerr Effect and the Circular and Linear X-ray Dichroism at the synchrotron to study the effect in detail. We hope for findings about the question, in how far a change of the electron density and/or an impact on the orbital moments at the interface ferromagnet/insulator are responsible for the effect. Using these inputs, we then want to realise complete tunnel junctions with a size smaller than 200nm that allow to combine all three effects and thereby to achieve high data stability and low switching power (fJ-range).In a second PhD thesis, we plan to increase the so far very small number of materials that allow studying these effects. In particular ferro- and ferrimagnetic materials with small electron density are in the focus of the project. We target at semiconducting ferrites and the system Mn-Ge. For both material classes, we already achieved preliminary results. Both material classes are very promising, since the ferrites are semiconducting and for the Mn-Ge system a perpendicular anisotropy and a low metallic electron density can be expected. For these materials, we plan to first conduct basic studies of the preparation of very thin epitaxial films and of the impact of an electric field on the magnetic properties. The latter should be studied by the Hall Effect and the Anomalous Hall Effect. These studies should be followed by the realisation of magnetic tunnel junctions that allow for electric field effects at film thicknesses larger than for the CoFeB system.