Neutron diffraction and ESR studies of pseudocubic Nd 0.75Ba 0.25MnO 3 and its unusual critical behavior above [formula omitted

Abstract

Results of structural neutron diffraction study and electron spin resonance (ESR) measurements are presented for insulating Nd 1− x Ba x MnO 3 ( x = 0.25 ) with the Curie temperature T C ≈ 129 K . 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 T JT≈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 * ≈ 143 K ( τ * ≈ 0.12 ⩽ τ < 1 , τ = ( T – T C ) / T C ) . 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*.