Coexistence of ferromagnetic and antiferromagnetic spin correlations in La1.2Sr1.8Mn2O7

We have investigated the antiferromagnetic (AF) spin correlations which coexist with the ferromagnetic spin excitations close to the Curie temperature T-C approximate to 128 K in the bilayer manganite La1.2Sr1.8Mn2O7 that shows colossal magnetoresistance (CMR) behavior, by inelastic neutron scattering both on a cold triple-axis and a time-of-flight neutron spectrometer. The scattered neutron intensity at Q=(0.5,0,0) could be detected at T=120 K. This intensity grows rapidly to become maximum at about T approximate to 128 K. Upon increasing the temperature, the intensity decreases very slowly and can be easily detected up to T=200 K. The temperature variation of AF correlations strongly suggests that they are of intrinsic origin. The scattered neutron intensity due to the AF correlations decreases under applied magnetic field in a very similar way as the resistivity, suggesting that the AF correlations are relevant to the CMR effect