Introduction

Hexaborides have been the subject of many research activities for many decades. In spite of their very simple crystallographic structure they show many very interesting properties which renewed the interest in these systems in recent years. In this work we will present nuclear magnetic resonance studies of the Alkaline-Earth Hexaborides SrB₆ and Ca_1-xLa_xB₆ with at temperatures between 0.08 K and room temperature.

The most puzzeling result is the very special temperature dependence of the spin-lattice relaxation rate 1/T₁ at the ¹¹B-sites which can't be explained by one of the known mechanisms. In this work we are going to give some evidence that the effect we are measuring is intrinsic to the structure for that Hexaborides.

We also measured ¹¹B NMR spectra. These spectra do not show anything unexpected. There is no Knight shift, not even in the slightly electron-doped compound Ca_1-xLa_xB₆.

Except of the quadrupole frequency we find no difference between the two compounds SrB₆ and Ca_1-xLa_xB₆.

All these results are presented and discussed in chapter .

An other important part of this work was to bring the flow cyostate up to work. A new computer and software environement had to be installed and configured. A lot about basic NMR theory but also about electronics and instrumental standards had to be learned. The experience I made during this time enters into the chapter and the appendix .

In chapter we give an introduction into the basic NMR theory and try to show how NMR can be used to probe microscopic properties of solids.

Personally this work was a completely new experience to me. I appreciated very much to work in a team and to contribute my part to a much larger project. On the other hand bringing my work to an end was like writing a story of which no one knows the end.

My part was first to perform measurements in SrB₆ at high temperatures, what involved a lot of technical work, and then to begin with the measurements in CaB₆. First we measured the spin-lattice relaxation rate of CaB₆, what was not a big problem because we could adapt our knowledge from the measurements in SrB₆ in the flow cryostate. Second we intended to determine the Knight shift and the quadrupole frequency as accurately as possible what caused again some technical problems.

The experimentalist's appoach to physics was new to me as well. It is difficult, maybe impossible to tell what the experimentalist's approachis. It is a feeling which might be compared with being a hunter or a detective in a world full of traps and pitfalls. It is the fascination of experienced physics.

I hope you will enjoy to read this work as much as I enjoyed to write it. If you are in a critical mood or simply not interested in details be aware of the appendix. It consists almost exclusively of details.