The effect of the external field on La-NMR spectra has been studied on La-based high-Tc cuprates La2-xSrxCuO4(x ≈ 0.115) and La2-xBaxCuO4(x=0.125), where the magnetic ordering in Cu-3d spin exists at low temperature. In the magnetically ordered state, the broadening in the resonance line was observed, reflecting the hyperfine field at the La-site contributed by the Cu-3d spins. By applying a high magnetic field of 15 T, this broadening in La2-xSrxCuO4 showed a significant shrink, which is interpreted as the spin-flop in the canted-spin system with the Dzyaloshinski-Moriya interaction. A model is proposed to explain both this spin-flop and the previously-reported anomalous field dependence of the structural instability in this system.
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