Study of the CMR manganites La0.82Ca0.18MnO3 and La0.85Sr0.15MnO3 by μSR method

The magnetic structures of La0.87Ca0.18MnO3 and La0.85Sr0.15MnO3 polycrystals have been studied by the mu SR method at PNPI. The magnetic susceptibility for these samples has been also measured. The obtained magnetic phase transition temperatures (T-C = 266 K, T-N = 165 K for La0.82Ca0.18MnO3 and T-C = 185.5 K, T-N = 105 K for La0.85Sr0.15MnO3) are in agreement with the published data. The muon precession frequencies are found in both samples at zero external magnetic field below T-C. The critical exponents obtained from the best fit of the temperature dependence of these frequencies (beta = 0.383 +/- 0.009 for La0.82Ca0.18MnO3 and beta = 0.387 +/- 0.015 for La0.85Sr0.15 MnO3) are equal to the theoretical value for 3D Heisenberg system. The local magnetic field's dispersion is of the same order of magnitude as the average value of the fields. Evidence for the second frequency in La0.82Ca0.18MnO3 at T less than or equal to 150 K caused, probably, by the separation of magnetic phases is the object of future investigation. The nature and separation of the magnetic phases is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.