The compound La2-2xSr1+2xMn2O7,x= 0.30-0.40, consists of bilayers of ferromagnetic metallic MnO2sheets that are separated by insulating layers. The materials show colossal magnetoresistance-a reduction in resistivity of up to two orders of magnitude in a field of 7 T-at their three-dimensional ordering temperatures,TC= 90-126 K, and are the layered analogues of the widely studied pseudo-cubic perovskite manganites, R1-xAxMnO3(R = rare earth, A = Ca, Sr, Ba, Pb). Two distinct short-range orderings-antiferromagnetic fluctuations and correlated polarons, which are related to the magnetic and the lattice degrees of freedom respectively-have previously been discovered in La2-2xSr1+2xMn2O7,x= 0.40, and have each been qualitatively connected to the resistivity. Here, in a comprehensive study as a function of both temperature and magnetic field for the different hole-concentrations per Mn site ofx= 0.30 and 0.35, we show that antiferromagnetic fluctuations also appear at temperatures just aboveTC, and that the intensities of both the antiferromagnetic fluctuations and polaron correlations closely track the resistivity. In particular, forx= 0.35 we show that there is a simple scaling relation between the intensities of the antiferromagnetic fluctuations and the in-plane resistivity that applies for the temperatures and magnetic fields used in the experiments. The results show that antiferromagnetic fluctuations are a common feature ofLa2-2xSr1+2xMn2O7with ferromagnetic bilayers, and that there is a close connection between the antiferromagnetic fluctuations and polarons in these materials.
Keywords: charge carrier localization; magnetic phase transitions; short range order.
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