  We theoretically investigate the Heisenberg antiferromagnet on a triangular
lattice doped with nonmagnetic impurities. Two nontrivial effects resulting
from collective impurity behavior are predicted. The first one is related to
presence of uncompensated magnetic moments localized near vacancies as revealed
by the low-temperature Curie tail in the magnetic susceptibility. These moments
exhibit an anomalous growth with the impurity concentration, which we attribute
to the clustering mechanism. In an external magnetic field, impurities lead to
an even more peculiar phenomenon lifting the classical ground-state degeneracy
in favor of the conical state. We analytically demonstrate that vacancies
spontaneously generate a positive biquadratic exchange, which is responsible
for the above degeneracy lifting.
