Neutron diffraction and ESR studies of pseudocubic Nd0.75Ba0.25MnO3 and its unusual critical behavior above TC

Results of structural neutron diffraction study and electron spin resonance (ESR) measurements are presented for insulating Nd1−xBaxMnO3 (x=0.25) with the Curie temperature TC≈129K. Its pseudocubic structure reveals the definite distortions to a lower symmetry. Detailed analysis of the data is performed in the frame of Pbnm space group in a temperature range 4.2-300 K. The compound is found to exhibit the Jahn-Teller (JT) transition at TJT≈250 K. Character of the coherent JT distortions and their temperature evolution differ from those of the x=0.23 manganite. The ESR results correspond to behavior of a 3D isotropic ferromagnet above T*≈143K (τ*≈0.12⩽τ<1, τ=(T-TC)/TC). It is shown that an anisotropic exchange coupling of the Mn and Nd magnetic moments may give a substantial contribution in ESR linewidth masking its critical enhancement. The different temperature treatments (slow/fast cooling/heating with/without external magnetic field) of the sample reveal a temperature hysteresis of the ESR spectra below T* indicating an anomalous response in the paramagnetic region. The study of the magnetic phase transition in the x=0.23 and 0.25 NdBa manganites suggests a change in its character from the second to first order at T*. The conventional free energy including the magnetization and magnetic field failed to describe this first-order transition. The unconventional critical behavior is attributed to an orbital liquid metallic phase that begins to coexist with the initial orbital-ordered phases below T*.