Optimum tunnel barrier in ferromagnetic–insulator–ferromagnetic tunneling structures

Abstract

Al 2 O 3 tunnel barriers I, formed by the oxidization of Al metal of various thicknesses between two ferromagnetic (FM) films were investigated to understand the influence of overlayer metal Al on the junction magnetoresistance (JMR). The optimum thickness of Al was observed to lie in the range of 1–1.6 nm to achieve good JMR in FM–I–FM junctions. Additionally, such junctions can be used to study the magnetic proximity effect in ferromagnet/normal metal bilayer systems.

Oxidation process of AlOx-based magnetic tunnel junctions studied by photoconductance

“…The junction resistance shows ͑roughly͒ an exponential increase with oxidation time, which is related to the growth of the barrier layer. 2 The TMR ratio, however, shows a maximum around an oxidation time of 105-120 s. Such a maximum in the TMR has also been observed by others, 2,6,7 and has been attributed to the full oxidation of the Al layer, without oxidizing the underlying magnetic electrode.…”

Evidence of a barrier oxidation dependence on the interfacial magnetism in Co/alumina based magnetic tunnel junctions

“…This is consistent with the paramagnetic nature of these regions at room temperature, and the formation of such regions has been used to explain the drop in TMR with increased barrier oxidation due to spin-flip scattering in the oxide. 12 In order to extract the magnetic moment of the unoxidized ͑i.e., metallic͒ portion of the Co layer, it is necessary to determine the normalization of the composite magnetizationaveraged absorption curve. To do this we performed a nonlinear least squares analysis on the composite curve by fitting three components: ͑1͒ a metallic Co absorption curve representing transitions to the unoccupied 3d states only; ͑2͒ a background function with a 2:1 step ratio at the L 3 and L 2 thresholds, approximating transitions to s and continuum states; and ͑3͒ a CoO absorption curve obtained from a reference standard.…”

“…The trend in TMR with oxidation time for this series was found to be similar to other reports. 11,12 XAS and XMCD measurements at the Co L 2,3 edge were performed on these operable MTJs by allowing x rays to strike an area of the sample containing the lower ferromagnetic electrode structure covered with the alumina barrier ͑see inset to Fig. 1͒.…”

Influence ofs−dinterfacial scattering on the magnetoresistance of magnetic tunnel junctions

“…Experimentally it was observed 5,7,8 that the TMR depends critically on the material of the insulating barrier and on the conditions of its preparation, in particular on the imperfections of the interfaces between the metal and the insulating layer 9,10 . On the other hand, the first theory suggested by M. Julliere expresses the TMR ratio in terms of the effective spin polarizations P 1 and P 2 of two magnetic layers via the expression TMR = 2P 1 P 2 /(1 + P 1 P 2 ), and thus predicts no dependence of the TMR on the parameters of the barrier.…”

“…with the use of the X-ray photoelectron spectroscopy, shows that alumina is amorphous and the obtained AlO x tunnel barrier deviates from the ideal Al 2 O 3 structure 10 . For a uniform coverage the Al film thickness is usually ranged from about 7 to 18Å, depending on a type of the ferromagnetic electrode 9 . There is a small range of Al thicknesses that yields to the best TMR ratio for a given oxidation condition.…”