Field induced antiferromagnetism and ${}^{17}$O Knight shift anomaly in La${}_2$CuO${}_4$

We address the effect of the field induced antiferromagnetism in paramagnetic state of the cuprate weak ferromagnet La${}_2$CuO${}_4$. The planar oxygen ${}^{17}$O Knight shift is shown to be an effective tool to inspect the effects of Dzyaloshinsky-Moriya coupling in cuprates in an external magnetic field. Field induced antiferromagnetism and anisotropic antiferromagnetic contribution to ${}^{17}$K explain the anomalies observed in ${}^{17}$O NMR in La${}_2$CuO${}_4$. The experimental observation of antiferromagnetic contribution to the ${}^{17}$O Knight shift provides probably the only way to find out the problem of the sense of Dzyaloshinsky vector in cuprates.