Abstract
A detailed study of the properties of the ferromagnetic to paramagnetic transition in polycrystalline manganites La1−xCaxMnO3 (x=0.2, 0.3, 0.4) is presented. The first order transition in La0.7Ca0.3MnO3 is bordered by second order transitions in the neighboring La0.8Ca0.2MnO3 and La0.6Ca0.4MnO3 compositions. Analysis of the Landau–Lifshitz coefficients obtained from Arrott plots showed that while b(T) is uniformly negative in La0.7Ca0.3MnO3, it changes from positive to negative values in different magnetic field ranges for La0.8Ca0.2MnO3 and La0.6Ca0.4MnO3, indicating that the behavior cannot be described within a single model under the application of a strong field. The Kouvel–Fisher procedure performed on the samples with continuous transitions over different ranges of fitting field confirmed tricritical exponents in La0.6Ca0.4MnO3 but revealed that the critical exponents obtained for La0.8Ca0.2MnO3 depend strongly on the choice of field range, shifting from values consistent with short range (3D Heisenberg/3D Ising) interactions to those approaching the tricritical mean field model. This observation is attributed to the influence of magnetic field on the coexistence of energetically close double-exchange and superexchange ferromagnetic interactions in La0.8Ca0.2MnO3.
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