Magnetic domains in itinerant metamagnets

Experimental results of the metamagnetism in Sr$_2$Ru$_2$O$_7$ give strong evidence for a new type of quantum critical behavior, namely a quantum critical end point [S. A. Grigera {\it et. al.}, Science {\bf 294}, 329 (2001)]. Our study shows that this behavior can be caused by band structure effects, e.g. by the vicinity of a van Hove singularity. Based on a mean field analysis, a phase diagram for the multi-layer ruthenates is developed as a function of magnetic field and band filling, showing the presence of a quantum critical end point which terminates a first-order phase boundary. At the end point, the Fermi level of the majority-spin band is precisely located at the Van Hove singularity. Thus, the system is rather susceptible to disorder effects. We investigate the possible appearance of Condon-domain like structures in a metamagnetic system. Condon-domains are the result of the coexistence of two magnetic phases at a first-order phase transition and may influence the motion of carriers via domain-wall scattering. We discuss the physical consequences and propose new test experiments.