Field-induced and impurity-induced magnetic order in La _2-xSr_xCuO₄ studied by the thermal conductivity and μSR

Our recent experimental works on the field-induced and impurity-induced magnetic order in La_2-xSr_xCuO₄ are reviewed. The muon-spin-relaxation (μSR) measurements of La_2-xSr _xCu_1-yM_yO₄ (M = Zn, Ni) have revealed that both Zn and Ni impurities develop the magnetic order around x = 0.115, though the effect of Zn on the development of the magnetic order is more marked than that of Ni. The field-induced magnetic order around x = 0.115 pointed out from the neutron scattering experiments has also been investigated by the thermal conductivity measurements in magnetic fields. Marked suppression of the thermal conductivity by the application of magnetic fields parallel to the c-axis has been observed in x = 0.10 and x = 0.13, which appears to be in good correspondence to the development of the magnetic order. In conclusion, not only impurities but also vortex cores in the CuO₂ plane seem to operate to pin the dynamical stripes of spins and holes, leading to the development of the magnetic order.