  Thermoelectric power measurements on the heavy fermion antiferromagnet
Ce(Ru0.92Rh0.08)2Si2 under magnetic field (H) show clearly that the
antiferromagnetic state below the critical field Hc ~ 2.8T can be fully
decoupled from the pseudo-metamagnetic crossover at Hm ~ 5.8T which occurs when
the magnetization reaches a critical value. By contrast to the weak field
variation of the Sommerfeld coefficient of the specific heat in the field
window from Hc to Hm, two electronic singularities with opposite signs are
detected in the thermoelectric power. The interplay between the magnetic
instability and the topological change of the Fermi surface is discussed and we
argue similarities to other field instabilities in various heavy fermion
compounds.
