Hidden weak ferromagnetism in slightly doped antiferromagnets

The slightly doped Hubbard model at finite temperature is analyzed in the spin-wave expansion. It is found that in d=2 weak uniform magnetization perpendicular to the staggered magnetization is present for any finite doping fraction. In d=3 there is no net magnetization for doping fractions smaller than some critical one. We suggest that this mechanism can be responsible for recent experimental observations of in-plane spin canting in ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$.