Magnetic and structural properties of the Gd/Ni-Bilayer-System

Abstract

Rare earth elements have been the scope of intensive research for many of their different properties over the past decades. The high magnetic permanent moment of these elements aroused a lot of interest and research on their magnetic behavior or structure, which was already conducted in the early 1970s. But due to the challenging problems associated with the handling of rare earths like their strong reactivity with water and oxygen, strong alloying with other metals or their high content of impurities was keeping research only to a small number of involved groups and publications per year. Another issue is the non-epitaxial or even amorphous growth of nearly all rare earth elements if evaporated under UHV conditions. There are only a few substrates which allow an epitaxial growth of gadolinium for example. But still the search for materials with high magnetization or either high or low coercivity for magnetic data storage technology or magnetic sensing elements attract notice to this class of elements especially gadolinium. Gadolinium can be seen as the only room temperature ferromagnet among the rare earth materials due to the highest Curie temperature, which is 16°C. The low value of the coercivity limits its possible applications for data storage. Its permanent magnetic moment amounts to 7.98 µB, what exceeds for example the value of nickel by a factor of 12. Its magnetic moment is generated completely by the spin moment µs of the 4f-shell. The ferromagnetic behavior is a result of the indirect RKKY interaction between neighboring magnetic moments.The investigated bilayers represent an interesting system for exchange coupled thin magnetic films. Nickel as one of the classical ferromagnets has a much higher Curie temperature (631 K) but only a fraction of the atomic magnetic moment of gadolinium. In the temperature range from 300 K down to 5 K the system consists of a nickel layer with a nearly constant magnetic moment and a gadolinium layer whose magnetic behavior strongly depends on temperature, along with a high permanent moment and low coercivity. This leads to effects like a negative remanence for distinct compositions and a transition from the so called Ni-aligned state to the Gd-aligned state. Also the polarization of the Gd-moments at the interface, even at temperatures far beyond its Curie-point, could be demonstrated. The magnetic properties were investigated by by SQUID and XMCD. The high precision and accurate temperature control of the SQUID and the element-specific sensitivity of the XMCD-technique complemented each other.The structural properties of the bilayers were examined thoroughly by electron diffraction (MEED, LEED), Auger electron spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), Rutherford backscattering (RBS) and scanning tunneling microscopy (STM). The interplay between morphology, structure and magnetic properties was one of the key goals of this project and the close relationship was observed. The dependence on layer thickness, deposition temperature and substrate were determined for that and are summarized into a proposed model of the Gd/Ni-bilayer system at the end ot the thesis.