Neutron diffraction and ESR studies of pseudocubic Nd0.77Ba0.23MnO3 and its anomalous critical behavior above TC

Structural neutron diffraction study and electron spin resonance (ESR) measurements are presented for insulating Nd0.77Ba0.23MnO3 with the Curie temperature TC≈124 K. Its pseudocubic structure exhibits the definite distortions to a low symmetry. A detailed analysis of the data is given for a Pbnm space group in the temperature range 4.2–500 K. A systematic transformation of the crystalline structure, which is accompanied by the peculiarities in the temperature variations of structural parameters, is found. The ESR data are in agreement with the predictions for 3D isotropic ferromagnets in the paramagnetic critical region at τ∗≈0.2≤τ<1,τ=(T−TC)/TC. The main disagreement observed below τ∗ is that the ESR linewidth reveals no the characteristic critical enhancement. The anomalous critical behavior, which is found in the present and our previous studies in this range, is consistent with the development of some structural changes. Different temperature treatments (slow/fast cooling/heating, with/without external magnetic field) of the sample, which usually reveal the possible hysteretic phenomena, have a little effect on the ESR spectra. These results present a challenge to the understanding of the magnetic phase transition in these manganites.