Physical Review B (Condensed Matter and Materials Physics) -- 1 November, 1999 -- Volume 60, Issue 17, pp. 12286-12293

mSR studies of magnetic order and dynamics of the n=2 Ruddlesden-Popper phases Sr₂LnMn₂O₇, where Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy and Ho

R. I. Bewley, S. J. Blundell, B. W. Lovett, Th. Jestädt, , F. L. Pratt, K. Chow, and W. Hayes

Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom

P. D. Battle, M. A. Green, J. E. Millburn, M. J. Rosseinsky, L. E. Spring and J. F. Vente

Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, United Kingdom

Zero field muon spin relaxation (mSR) has been used to study the magnetic properties of n = 2 Ruddlesden-Popper phases Sr₂LnMn₂O₇, where Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy and Ho. The results show that the size of the lanthanide ion is crucial in determining the magnetic state and dynamics of the system. Because muons are implanted throughout the bulk of the sample, impurity phases contribute only according to their volume fraction. Hence in the case of biphasic samples the data are dominated by the majority phase. Although none of our samples has a ferromagnetic groundstate, colossal magnetoresistance (CMR) is observed over a wide temperature range, 4 K < T < 150 K, for both the Pr and Nd compounds. The mSR results show that the magnetic transition in both these samples is broad. Ordered, but fluctuating, regions form at ~150 K, the reported onset of CMR, with the fluctuation rates gradually decreasing with temperature. Even at 5 K, fluctuations are still observed. The ferromagnetic double exchange between Mn ions becomes weaker as the size of the lathanide ion decreases. Sr₂SmMn₂O₇ shows weak clustering at a much reduced temperature of 30 K, whereas Se₂EuMn₂O₇ shows spin glass like behaviour. For all lanthanide ions smaller than Eu no long range order of the spins is observed and the observed relaxation rates follow an activated dependence. The technique allows us to extract the effective activation energy associated with the magnetic fluctuations of the lanthanide moments in samples with Ln = Sm, Eu, Gd, Tb, Dy and Ho. CMR is only observed where mSR measurements show a broad magnetic transition associated with fluctuations. We therfore believe these fluctuating ordered regions are responsible for the extended temperature regime in which CMR has been observed in these non-ferromagnetic n = 2 Ruddlesden-Popper phases. © 1999 The American Physical Society

URL: http://publish.aps.org/abstract/prb/v60/p12286
PACS: 76.75.+i, 75.25.+z, 71.27.+a

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