Dünnschichtsysteme für den Einsatz in magnetischen Tunnelstrukturen

In recent years there has been increasing interest in the preparation and characterization of magnetic multilayer systems. The possibility of information storage based on magnetic properties exhibits a large application potential in mass storage devices. Especially the discovery of the GMR (giant magnetoresistance) has opened a large field of application because of its use e.g. for reading and writing of magnetic bits in hard disc read heads. One of the great challenges in magnetoelectronics is the use of the TMR (tunnelling magnetoresistance) e.g. in MRAM (magnetic random access memory) devices, which have crucial advantages over the presently used DRAMs (dynamical random access memory) because of their non-volatility and their low power consumption. The TMR effect plays on the spindependent tunnelling probability of electrons through an insulating barrier located between two ferromagnetic layers. The performance of TMR devices strongly depend on the structural, magnetic and electronic properties of the two ferromagnetic films. E.g. the question concerning the optimization of the preparation of the tunnelling barrier or concerning the electronic states contributing to the tunnelling process play an important role that is discussed within this thesis. In the first part of chapter "Ergebnisse und Diskussion", a growth process of the fully epitaxial Fe/MgO/Fe trilayer system based on Fe(110) electrodes is presented. The surface of the first Fe(110) electrode shows atomically flat terraces with monoatomic steps. MgO(111) barriers grow in a three dimensional growth mode. The second Fe(110) electrode shows a twinned structure that can be healed out by a subsequent annealing procedure. Transport properties show a MgO(111) barrier without defects. Current-voltage (I-U) characteristics of the Fe(110)/MgO(111)/Fe(110) trilayer system show a typical nonlinear shape. The tunneling barrier height Theta and width d estimated from the numerical fitting of the Simmons equation to the experimental data are Theta=0.5eV and d=1.5nm for a nominal MgO(111) thickness of 4nm. By means of Poole-Frenkel- and Glazman-Matveev analysis, the tunnelling barrier is tested for the presence of defects. Microstructured MTJs (metallic tunneljunctions) with tunnelling areas between 4µm2 and 400µm2 show a reproducible TMR of about 18-26% at room temperature and 45% at T=77K. The RxA products (the product of the tunnelling area and the tunnelling resistance) are determined to be (6±1)MOmegaµm2. Noise measurements depending on the frequency show the typical 1/f noise. First hints for the presence of a magnetic noise and a structure in the (I-U) characteristics are found at low temperatures. In the second part, a UV light assisted oxidation process for preparation of MTJs is presented in the Co/Al-oxide/Co trilayer system. Investigations are presented and discussed on two different oxidation systems and the influence of the UV light on the oxidation process is shown. With low power UV light (15W in-situ UV lamp) reproducible MTJs can be prepared by a shadow mask deposition process. TMR values of 33% at low temperature and 15% at room temperature are determined. But the oxidation times are found to be too long for any industrial application using this method. With high power UV light (100W ex-situ UV lamp) the microstructuring of MTJs that show reproducible properties is still problematic. Oxidation experiments by XPS (X-ray photoelectron spectroscopy) show both an influence of the direct or indirect UV irradiation on the film during the oxidation and the influence of the crystallinity of the lower ferromagnetic electrode. For both cases oxidation models are presented and discussed. Finally, a layer system for the investigation of Cu dilusions in the antiferromagnetic FeMn and the influence on the exchange bias effect is presented and discussed, respectively. A prove of the domain-state model is also found in FeMn/NiFe layer systems. For increasing dilusion concentration in the antiferromagnet a maximum of the exchange bias is detemined for a Cu concentration between 20% and 25%. A dependence of the maximum of the exchange bias on the structural quality of the antiferromagnetic film has been found and a clear influence of the domain structure on the characteristics of the exchange bias has been detected. Experiments with pure diluted antiferromagnetic films (without any ferromagnet) show a high agreement of the thermoremanent magnetization with the behaviour of the exchange bias in the (FeMn)1-x/Cux/Ni81Fe19 layer system. One can assume that the origin of the exchange bias can be found directly in the domain structure of the antiferromagnet.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2005

Author dc:creator

• Hauch, Jan Oliver

Contributors dc:contributor

• Güntherodt, Gernot

• info:eu-repo/classification/ddc/530
• Physik
• TMR (Tunnelmagnetwiderstand )
• MBE (Molekularstrahlepitaxie)
• UV Licht Oxidation
• Exchange Bias
• epitaktisch

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-208175%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:60255