Muon-spin-relaxation studies of magnetic order and dynamics of the n = 2 Ruddlesden-Popper phases Sr2RMn2O7 (R = Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho)

Zero field muon spin relaxation (mu SR) has been used to study the magnetic properties of n = 2 Ruddlesden-Popper phases Sr2RMn2O7, where R = Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho. The results show that the size of the lanthanid e ion is crucial in determining the magnetic state and dynamics of the syst em. Because muons are implanted throughout the bulk of the sample, impurity phases contribute only according to their volume fraction. Hence in the ca se of biphasic samples the data are dominated by the majority phase. Althou gh none of our samples has a ferromagnetic ground state, colossal magnetore sistance (CMR) is observed over a wide temperature range, 4 K<T<150 K, for both the Pr and Nd compounds. The mu SR results show that the magnetic tran sition in both these samples is broad. Ordered, but fluctuating, regions fo rm at similar to 150 K, the reported onset of CMR, with the fluctuation rat es gradually decreasing with temperature. Even at 5 K, fluctuations are sti ll observed. The ferromagnetic double exchange between Mn ions becomes weak er as the size of the lanthanide ion decreases. Sr2SmMn2O7 shows weak clust ering at a much reduced temperature of 30 K whereas Sr2EuMn2O7 shows spin-g lass-like behavior. For all lanthanide ions smaller than Eu no long range m agnetic ordering of the spins is observed and the observed relaxation rates follow an activated dependence. The technique allows us to extract the eff ective activation energy associated with the magnetic fluctuations of the l anthanide moments in samples with R = Sm, Eu, Gd, Tb, Dy, and Ho. CMR is on ly observed where mu SR measurements show a broad magnetic transition assoc iated with fluctuations. We therefore believe that these fluctuating ordere d regions are responsible for the extended temperature regime in which CMR has been observed in these nonferromagnetic n=2 Ruddlesden-Popper phases. [ S0163-1829(99)03041-6].