⁵⁵Mn NMR study of the A-type antiferromagnetic and metallic manganites la_1-xSr_xMnO₃ (0.5 ≤ × ≤ 0.6)

The microscopic magnetic and electronic properties Of La _1-xSr_xMnO₃ (0.5 ≤ × ≤ 0.6) were investigated using ⁵⁵Mn nuclear magnetic resonance (NMR). The NMR spectral intensity showed a large decrement as x increases above the boundary (x - 0.52) of ferromagnetic (FM) and A-type antiferromagnetic (AFM) phases. The spectrum of the samples shows a coexistence of two different phases, i.e., a metallic major phase and an insulating minor phase. The spin-spin relaxation time strongly depends on the resonance frequency, indicating that the Shul-Nakamura process by spin-wave excitation is the dominant relaxation mechanism. This type of frequency dependence of the spin-spin relaxation time was not observed in the CE-type AFM manganite Nd_0.5Sr _0.5MnO₃. The A-type AFM spin structure and the Shul-Nakamura relaxation process appears to be a unique characteristics of La_1-xSr_xMnO₃ (0.54 ≤ × ≤ 0.6) that is related to the 2D-like spin coupling in a nearly tetragonal crystal structure.