  We report neutron diffraction measurements of the magnetic structures in two
pyrochlore iridates, Yb2Ir2O7 and Lu2Ir2O7. Both samples exhibit the
all-in-all-out magnetic structure on the Ir4+ sites below TN~ 150,K, with a low
temperature moment of around 0.45 muB/Ir. Below 2\,K, the Yb moments in
Yb2Ir2O7 begin to order ferromagnetically. However, even at 40 mK the ordered
moment is only 0.57(3)muB/Yb, well below the saturated moment of the ground
state doublet of Yb3+ (1.9 muB/Yb), deduced from magnetization measurements and
from a refined model of the crystal field environment, and also significantly
smaller than the ordered moment of Yb in Yb2Ti2O7 (0.9 muB/Yb). A mean-field
analysis shows that the reduced moment on Yb is a consequence of enhanced phase
competition caused by coupling to the all-in-all-out magnetic order on the Ir
sublattice.
