Ising lattices with +/- J interactions and uniform magnetic field are
calculated looking for energy, magnetization and correlations at zero
temperature. All magnitudes present discontinuities that occur at rati
onal numbers for the magnetic held measured in units of J. We interpre
t this as a manifestation of pinning due to the complex topology of th
ese lattices. (C) 1998 Elsevier Science B.V. All rights reserved.