Physica B 312–313 (2002) 763–765 Spin correlations and magnetoresistance in the bilayer manganite La 1:2 Sr 1:8 Mn 2 O 7 S. Rosenkranz a,b, *, R. Osborn a , L. Vasiliu-Doloc c,d , J.W. Lynn c , S.K. Sinha d , J.F. Mitchell a a Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA b Department of Physics, University of Illinois, Chicago, IL 60607, USA c NIST Center for Neutron Research, NIST,Gaithersburg, Maryland 20899, USA d Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA Abstract We have studied the magnetic correlations in the x = 40% hole doped bilayer manganite La 1:2 Sr 1:8 Mn 2 O 7 using neutron scattering. The in-plane correlations obey standard two-dimensional scaling above T C B113 K with a crossover towards three-dimensional critical behavior close to T C ; consistent with quasi two-dimensional critical fluctuations. This suggests that conventional magnetism drives the phase transition while simultaneously destroying the charge correlations observed in the paramagnetic region. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Manganites; Spin correlations; Magnetoresistance Colossal magnetoresistance in optimally doped man- ganese oxides is believed to involve a strong coupling among spin, charge, and lattice degrees of freedom [1]. However, the nature of the ferromagnetic and con- comitant insulator–metal transition has yet to be clearly established. Studying the nature of the transition and the interplay among correlations is greatly facilitated in the layered manganites because the reduced dimensionality increases the importance of fluctuations and extends the temperature region of significant correlations. For the 40% bilayer compound La 1:2 Sr 1:8 Mn 2 O 7 ; inelastic neu- tron scattering measurements of the spin-wave excita- tions yield a ratio of the nearest neighbor exchange constants J between spins within the same plane and J 0 between spins in different bilayers of J =J 0 B150 [2,3], comparable to the anisotropy in the transport properties [4]. This quasi-two-dimensionality is also observed in the paramagnetic diffuse scattering above T C ; which appears in the form of rods perpendicular to the planes [5,6]. Here, we present our detailed investigation of the temperature dependence of the in-plane correlation length x and static susceptibility wðq ¼ 0Þ; which are related to the width and the height of these rods, respectively, using neutron scattering. The experiments were performed at the NIST Center for Neutron Research on the BT2 triple-axis spectrometer operating in two-axis mode with a fixed incident energy 13.7meV and horizontal collimations of 60 0 -20 0 -20 0 ; full-width at half- maximum (FWHM). Pyrolytic graphite was used both as monochromator and filter against higher order contamination. In this two-axis mode, the diffuse scattering is, in the quasistatic approximation, proportional to the wave- vector-dependent susceptibility SðQÞ = T * w T ðqÞ; where q ¼ Q  t; Q is the momentum transfer and t denotes a reciprocal lattice vector of the magnetic structure. Scans were taken around the magnetic Bragg reflection at Q = ½h02; where the nuclear Bragg contribution is extremely weak. The validity of the quasistatic approximation was verified by compar- ing the results of these scans with those obtained from scans at Q = ½1 þ h; 0; 1:83 for which an optimal energy integration is obtained because, given the experimental conditions, the scattered wavevector is parallel to the c-axis [7]. ARTICLE IN PRESS *Corresponding author. Department of Physics, University of Illinois, Chicago, IL 60607, USA. Tel.: (312) 413-1844; fax: (312) 996-9016. E-mail address: sro@uic.edu (S. Rosenkranz). 0921-4526/02/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0921-4526(01)01224-8