Ni impurity effect on antiferromagnetic order in hole-doped L _a2xSr_xCuO₄

The doping effect of magnetic impurities on the lightly hole-doped antiferromagnetic state was investigated by neutron diffraction techniques using single crystals of La_1.99Sr_0.01Cu_1-yNi _yO₄ (0.01 ≤ y ≤ 0.10). Dilute Ni-doping between y = 0.02 and 0.05 dramatically changes the Q-dependence of the antiferromagnetic Bragg intensity, corresponding to a switch of spin structure from the La ₂CuO₄ type (Cu type) to the La₂NiO₄ type (Ni type) triggered by a spin rotation in CuO₂ planes. Upon Ni-doping, the Néel temperature (T_N) increases and rapidly approaches that of nondoped La₂CuO₄ in the Cu-type phase due to the strong localization of doped carriers, whereas T_N gradually decreases in the Ni-type phase. More interestingly, between the Cu- and Ni-type structures, there exists an intermediate region where the spin direction is between those of Cu- and Ni-types. In this region, we found an unusual temperature dependence of magnetic intensities, which suggests a temperature dependence of the spin direction. These unusual behaviors are discussed in terms of doped carrier itinerancy.