Neutron diffraction, magnetization, and ESR studies of pseudocubic Nd{sub 0.75}Ba{sub 0.25}MnO{sub 3} and its critical behavior above T{sub C}

Results of structural neutron diffraction study, magnetization, and electron spin resonance (ESR) measurements are presented for insulating Nd{sub 1-x}Ba{sub x}MnO{sub 3}(x=0.25) with the Curie temperature T{sub C}{approx_equal}129 K. Detailed analysis of the data is performed by using Pbnm space group in a temperature range 4.2-300 K. The compound is found to exhibit the Jahn-Teller (JT) transition at T{sub JT}{approx}250 K. The character of the coherent JT distortions and their temperature evolution differ from those of the x=0.23 manganite. The field cooled magnetization data are in reasonable agreement with the predictions for a three-dimensional (3D) isotropic ferromagnet above T{sub C}. These measurements, however, reveal a difference between the field cooled and zero field cooled data in the paramagnetic region. The ESR results also correspond with behavior of a 3D isotropic ferromagnet above T*{approx_equal}143 K[{tau}*{approx_equal}0.12{<=}{tau}<1,{tau}=(T-T{sub C})/T{sub C}]. The T-dependence of the ESR linewidth is found to be proportional to [T{chi}(T)]{sup -1}, where {chi}(T) is the susceptibility. This uncritical behavior results from the anisotropic spin interactions that can be attributed to the Dzyaloshinsky-Moria (DM) coupling. The critical enhancement is not observed. It can be explained by the strong uncritical contribution to the linewidth, and suppression of the critical enhancement by a magnetic field.more » 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 manganites suggests change in its character from second to first order at T*. The conventional free energy including the magnetization and magnetic field is not found to describe this first order transition. This suggests that the charge, orbital, and JT phonon degrees of freedom, in addition to magnetization, may be the critical variables, the unusual character of the transition being determined by their coupling. The unconventional critical behavior is attributed to an orbital liquid metallic phase that begins to coexist with the initial orbital ordered phase below T*.« less