Metamagnetism of itinerant electrons in multi-layer ruthen- ates

– The problem of quantum criticality in the context of itinerant ferroor metamagnetism has received considerable attention [S. A. Grigera et. al., Science 294, 329 (2001); C. Pfleiderer et. al., Nature, 414, 427 (2001)]. It has been proposed that a new kind of quantum criticality is realised in materials such as MnSi or Sr2Ru2O7. We show based on a mean-field theory that the low-temperature behaviour of the n-layer ruthenates Srn+1RunO3n+1 can be understood as a result of a Van Hove singularity (VHS). We consider a single band whose Fermi energy, EF , is close to the VHS and deduce a complex phase diagram for the magnetism as a function of temperature, magnetic field and EF . The location of EF with respect to the VHS depends on the number of layers or can be tuned by pressure. We find that the ferromagnetic quantum phase transition in this case is not of second but of first order, with a metamagnetic quantum critical endpoint at high magnetic field. Despite its simplicity this model describes well the properties of the uniform magnetism in the single, double and triple layer ruthenates. We would like to emphasise that the origin of this behaviour lies in the band structure. Introduction. – The issue of metamagnetism in itinerant electron systems was studied both theoretically [1, 2] and experimentally [3] long ago. Recently, the investigation of metamagnetism in two compounds, MnSi [4] and Sr3Ru2O7 [5,6], has revived interest in this phenomenon, considering it from a new point of view. It has been suggested that these systems might display a new type of quantum criticality, connected with a so-called quantum critical end point (QCEP), in the vicinity of which the Landau Fermi-liquid theory of metals breaks down. In fact the field Hm(T ) initiating the metamagnetic transition, the abrupt increase of the magnetisation, defines a line of first-order transitions in the field (H)-temperature (T) plane without any symmetry-breaking. The first-order line ends in a critical end point (Hc, Tc). A QCEP occurs if Tc is suppressed to zero as a function of an additional parameter, such as the pressure or the chemical composition of the material. While theoretical results concerning the properties of the QCEP have been obtained recently on the basis of phenomenological, low-energy field theories [7], to our knowledge no discussion of the microscopic origin of such a QCEP has yet been provided. In the following,