Publikationsserver der RWTH Aachen University
Novel materials for magnetic tunnel junctions
Abstract
dc:descriptionThis thesis has been devoted to the understanding of spin polarized tunneling in magnetic tunnel junctions (MTJs). MTJs consist of two ferromagnetic layers separated by a thin (~2nm) insulating layer, usually amorphous Al2O3. The resistance of the structure depends on the relative orientation of the magnetizations of the ferromagnetic electrodes. The normalized change in resistance is called tunneling magnetoresistance (TMR). In simple models the TMR depends mainly on the degree of spin polarization (TSP) in the ferromagnetic metals. An important tool to directly measure this spin polarization is superconducting tunneling spectroscopy (STS). This technique uses a superconducting material in an applied magnetic field as an analyzer of the tunneling current from the ferromagnetic counter electrode. In a series of experiments to explore the relationship of TSP to magnetization the TSP of ferrimagnetic alloys formed from the transition metals (Co, Fe) and the heavy rare earths (Gd, Tb) was measured. For certain compositions negative TSP was measured, indicating a predominance of minority electrons in the tunneling current. The occurrence of negative TSP in these alloys can be explained using a simple model that takes into account different tunneling probabilities (due to differences in tunneling matrix elements) and spin polarizations for tunneling from different atomic sites. At the compensation point where the overall magnetization vanishes, a finite spin polarization is nonetheless preserved. This illustrates that the tunneling electrons can be spin polarized even in the absence of a net magnetization. MTJs with Co-Gd and Co-Fe counter electrodes have been fabricated that show either negative or positive TMR at room temperature depending on the composition. Using a thin Co-Fe interlayer between the barrier and the Co-Gd free layer, the TMR values as well as the thermal stability can be much increased. Furthermore, the dependence of magnetic anisotropy in Co-Gd and Co-Fe-Gd films was measured as a function of Gd content, layer thickness and thermal annealing time. In a second series of experiments the influence of the tunnel barrier on the TSP for otherwise the same ferromagnetic electrodes was explored. For crystalline tunnel barriers electron wave functions of different symmetries can decay at different rates across the barrier depending on its band structure. Furthermore, for ferromagnetic electrodes the decay rates can be very different for majority and minority spin polarized electrons due to the different symmetries of these spin polarized bands. This can lead to significant spin filtering and thus higher TSP than for amorphous tunnel barriers. In particular, large TMR values were theoretically predicted for perfectly epitaxial (100) oriented Fe/MgO/Fe MTJs. In the course of this thesis, high quality Fe/MgO and Co70Fe30/MgO tunneling structures have successfully been fabricated by reactive magnetron sputtering. Extraordinarily high TSP values were found of up to 74% and 85% for Fe/MgO and Co70Fe30/MgO junctions, respectively. These values are much higher than those in otherwise identical STS devices but with amorphous Al2O3 tunnel barriers (45% and 53%, respectively). Consistent with these very high TSP values very large TMR values were found exceeding 200% at room temperature and 300% at 4K.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2004
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Kaiser, Christian
- Contributors dc:contributor
-
- Güntherodt, Gernot
Subjects
dc:subject × 13Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- eng