This thesis is dedicated to the memory of my late professor A.R. Mackintosh — meant as a student’s tribute to his work of great insight. Through our unfortunately short acquaintance I owe Allan my interest in the field of solid state physics in general and rare earth magnetism in particular. I am grateful to my supervisors Jens Jensen from the Ørsted Laboratory, Niels Bohr Institute and Des McMorrow from the Solid state department, Risø National Laboratory. For their help and suggestions on various problems, I also wish to thank the people in the solid state department, my fellow students and my family. The work falls into three main parts, namely experiments, theory and calculations. On the other hand the examined system shows several different interesting features. I have chosen to start by supplying the background knowledge, occasionally mentioning the implications on the features. Afterwards I discuss each of the features individually. This procedure has led to some overlapping in the report, but on the other hand I found it the most instructive way.