  This article describes the Fermi surface properties of CeRu$_2$Si$_2$ and its
alloy systems CeRu$_2$(Si$_x$Ge$_{1-x}$)$_2$ and Ce$_x$La$_{1-x}$Ru$_2$Si$_2$
studied by the de Haas - van Alphen (dHvA) effect. We pay particular attention
to how the Fermi surface properties and the f electron state change with
magnetic properties, in particular how they change associated with metamagnetic
transition and quantum phase transition. After summarizing the important
physical properties of CeRu$_2$Si$_2$, we present the magnetic phase diagrams
of CeRu$_2$(Si$_x$Ge$_{1-x}$)$_2$ and Ce$_x$La$_{1-x}$Ru$_2$Si$_2$ as a
function of temperature, magnetic field and concentration $x$. From the
characteristic features of the magnetic phase diagram, we argue that the
ferromagnetic interaction in addition to the anitferromagnetic interaction and
the Kondo effect is responsible for the magnetic properties and that the
metamagnetic transitions in these systems are relevant to the ferromagnetic
interaction. We summarize the Fermi surface properties of CeRu$_2$Si$_2$ in
fields below the metamagnetic transition where the f electron state is now well
understood theoretically as well as experimentally. We present experimental
results in fields above the metamagnetic transitions in
CeRu$_2$(Si$_x$Ge$_{1-x}$)$_2$ and Ce$_x$La$_{1-x}$Ru$_2$Si$_2$ as well as
CeRu$_2$Si$_2$ to show that the Fermi surface properties above the metamagnetic
transitions are significantly different from those below in many important
aspects. We argue that the Fermi surface properties above the metamagnetic
transitions are not appropriately described in terms of either itinerant or
localized f electron. The experimental results in fields below the metamagnetic
transitions in CeRu$_2$(Si$_x$Ge$_{1-x}$)$_2$ and Ce$_x$La$_{1-x}$Ru$_2$Si$_2$
are presented to discuss the f electron state in the ground state. The Fermi
surface properties of ...
