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We have investigated the magnetic diffuse scattering from the colossal magnetoresistive (CMR) bilayer manganite La 1.2Sr1.8Mn2O7 above the magnetic phase transition temperature. La 1.2Sr1.8Mn2O7 is ferromagnetic with a Curie temperature of about 128 K. It belongs to the n = 2 Ruddlesden-Popper phase with a modified bilayer perovskite structure in the I4/mmm space group. The magnetic diffuse scattering above Tc shows rod-like intensity distribution along c* indicating the quasi-2D bilayer character of the system. The strong magnetic correlations of the Mn atoms in the a-b plane and also that between the two planes of the bilayers persist even at room temperature, which is about 2.3 x Tc. The diffuse magnetic scattering has been successfully modelled by using only two exchange interactions in the quasistatic mean-field approximation. By fitting the intensity data to mean-field model calculations, using a least-squares fitting procedure, we have been able to determine the inplane exchance interaction Ja and interplane exchange interaction Jc of the bilayer. The exchange interactions Ja and Jc, obtained from the least squares fit, are Ja = 2.789(2) and Jc = 1.395(6) meV at room temperature. The interactions so determined are somewhat smaller than that (Ja = 4.8(1), Jc = 1.7(1) meV) determined by us from the measurements of the spin-wave dispersions at 1.5 K. The reduction of the exchange interactions at higher temperature is partly due to their renormalization at higher temperature. However, the anisotropy of the orbital occupancy may contribute to the reduction of the ratio Ja/Jc at higher temperature.

Diffuse magnetic scattering aboveTcin quasi-2D La1.2Sr1.8Mn2O7

Diffuse magnetic scattering aboveT_cin quasi-2D La_1.2Sr_1.8Mn₂O₇