  We report a detailed study of the temperature and magnetic-field dependence
of the spin susceptibility for a single crystal of La(1.875)Ba(0.125)CuO(4).
  From a quantitative analysis, we find that the temperature-dependent
anisotropy of the susceptibility, observed in both the paramagnetic and
stripe-ordered phases, directly indicates that localized Cu moments dominate
the magnetic response. A field-induced spin-flop transition provides further
corroboration for the role of local moments. Contrary to previous analyses of
data from polycrystalline samples, we find that a commonly-assumed isotropic
and temperature-independent contribution from free carriers, if present, must
be quite small. Our conclusion is strengthened by extending the quantitative
analysis to include crystals of La(2-x)Ba(x)CuO(4) with x=0.095 and 0.155. On
the basis of our results, we present a revised interpretation of the
temperature and doping dependence of the spin susceptibility in
La(2-x)(Sr,Ba)(x)CuO(4).
