Magnetic tunnel junctions with perpendular anisotropy

具有垂直各向异性的磁性隧道结显示出极小的自旋转移扭矩切换(STT)电流密度。此外，温度、电场和薄膜厚度对磁化各向异性和磁化强度的转换有很大的影响。特别是，电场的影响还没有被很好地理解，它为具有极小功耗的设备开辟了道路。使用CoFeB电极和MgO势垒的隧道结，我们发现开关电流密度最低可达100kA/cm2。显然，也可以实现非物理的值(&lt；10 A/cm？)。然而，对于获得这些值所必需的条件，事实证明，所有三个因素(自旋极化电流密度、温度和电场)都改变了磁性，从而改变了开关行为。这是申请继续资助的动机：在该项目的进一步过程中，我们希望通过系统地改变隧道势垒以及由Co、Fe和CoFe(B)制成的电极之一以及非磁性对电极的厚度来实现这三种效应。特别是，电场的影响将通过使用厚栅栏进行研究，这种栅栏只允许较小的隧道电流，并防止加热。我们计划在同步加速器上应用磁光克尔效应和圆、线X射线二向色性来详细研究这种效应。我们希望对这个问题有更多的发现，在多大程度上电子密度的变化和/或对界面铁磁/绝缘体的轨道矩的影响是造成这种影响的原因。使用这些输入，我们想要实现尺寸小于200纳米的完整隧道结，允许结合所有三种效应，从而实现高数据稳定性和低开关功率(FJ范围)。在第二篇博士论文中，我们计划增加到目前为止允许研究这些效应的极少数材料。特别是，具有小电子密度的铁磁和亚铁磁性材料是该项目的重点。我们的目标是半导体铁氧体和Mn-Ge系统。对于这两个材料类别，我们已经取得了初步成果。这两类材料都非常有希望，因为铁氧体是半导体型的，而对于Mn-Ge系统来说，垂直各向异性和低金属电子密度是可以预期的。对于这些材料，我们计划首先进行非常薄的外延薄膜的制备和电场对磁性的影响的基础研究。后者应该用霍尔效应和反常霍尔效应来研究。在这些研究之后，应该实现磁隧道结，允许在比CoFeB系统更厚的薄膜厚度上产生电场效应。

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.