Vol 50, No 9 (2019)

We study the effects of 10% Cr substitution in Mn sites of Bi\(_{0.5}\)Sr\(_{0.5}\)MnO\(_3\) on the antiferromagnetic (AFM) (\(T_{\text {N}} \sim \) 110 K) transition using structural, magnetic and electron paramagnetic resonance (EPR) techniques. Field cooled (FC) and zero field cooled (ZFC) magnetization measurements done from 400 K down to 4 K show that the compound is in the paramagnetic (PM) phase till 50 K where it undergoes a transition to a short-range ferromagnetic phase (FM). Electron paramagnetic resonance measurements performed in the temperature range of 300 K to 80 K conform with the magnetization measurements as symmetric signals are observed owing to the paramagnetic phase. Below 80 K, signals become asymmetric. Electron paramagnetic resonance intensity peaks at \(\sim \) 110 K, the decreasing intensity below this temperature confirming the presence of antiferromagnetism. We conclude that below 50 K the magnetization and EPR results are consistent with a cluster glass phase of BSMCO, where ferromagnetic clusters coexist with an antiferromagnetic background.

A method for the determination of number of spins from EPR spectra with a high level of noise is proposed. The method is based on a convolution of the experimental spectrum with the spectrum of the same shape characterized by a high signal-to-noise ratio. It was shown that the convolution technique is rather robust to the presence of additive noise in examined EPR spectrum.

The extraction of macromolecules from nano-self-assembled material can be used as a laboratory model for enhancing oil recovery in reservoirs. By combining Darcy’s law and Poiseuille equation, an improved nuclear magnetic resonance (NMR) permeability model, suitable for macromolecular flow in mesopores is obtained. The calibration coefficients in the Coates equation are expressed in terms of the physical parameters of pore throat ratio r_b/r_t, tortuosity, and thickness of bond film in the improved model. The results show that the proportion of irreducible fluid to total fluid obtained through NMR characterization can reflect the variation tendency of irreducible macromolecule and water. By simplifying the pores of the extracted samples, the thickness model of irreducible macromolecule and water is established with the total thicknesses calculated as 1.482 nm, 1.585 nm, 1.674 nm, and 1.834 nm. The corresponding permeability results obtained from the NMR characterization (K_NMR) are 7.39 mD, 6.02 mD, 5.27 mD, and 6.25 mD. The permeability results obtained from mercury intrusion experiment (K_HG) are 5.10 mD, 4.73 mD, 5.82 mD, and 5.56 mD, and those from the Darcy flow experiment (K_D) are 4.1 mD and 5.19 mD. The absolute deviation between K_NMR and K_HG varies from 0.69 to 2.29 mD, while that between K_NMR and K_D is 1.58 mD. This method can be applied to the enhanced recovery of shale oil.