The effect of the magnetic field on the crystal structural and the magnetic ordering at low temperatures of La2-xSrxCuO4 (x ∼ 1/8) has been studied using the ultrasonic and NMR measurements. An increase of the sound velocity Vs below Tud2 (∼ 10 K at H = 0) is drastically enhanced by applying the field, suggesting that the magnetic field promotes the structural phase transition to the tetragonal phase. A broadening of the resonance line of La-NMR in the magnetically ordered state below TN (∼ 35 K at x = 0.115) shows a significant shrink by applying a high magnetic field of 15 T, 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 field-dependent structural instability described above in this system.
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