Elastic and antiferromagnetic anomalies in Pr0.48Ca0.52MnO3 as determined by resonant ultrasonic spectroscopy

The Pnma incommensurate phase transition in the perovskite Pr0.48Ca0.52MnO3 at ∼ 235 K is accompanied by shear strains of up to ∼ 2.5% (from neutron diffraction) and changes in the shear modulus of up to ∼ 40% (from resonant ultrasound spectroscopy, RUS), indicating strong coupling between the structural order parameter and strain. In contrast, the antiferromagnetic (AFM) ordering transition at ∼ 180 K displays no detectable static strain, implying that there is either no coupling or only very weak coupling between the magnetic order parameter and strain. Conventional analysis of RUS data, based on measurements of resonance peak frequencies and peak widths, also failed to detect any anomaly in elastic or anelastic properties due to the AFM ordering. A new approach to the analysis the RUS data, based on autocorrelation and convolution of the entire spectra, however, has revealed that the AFM order does indeed affect the elastic behaviour in an unexpected manner. The new analysis shows, firstly, that dynamical fluctuations of the charge density ordering at T > Tc = 237 K lead to an increase of the RUS amplitude and of the spectral convolution function. Secondly, fluctuations and convolution effects peak at the transition point and decrease with decreasing temperatures. Below 180 K the stripe structure is essentially static. Finally, AFM ordering leads to an increase in the damping of the elastic resonances.