Study Of Magnetic Ordering And Electrical Transport Properties In The Perovskite Manganites La_1-yM_yMn_1-xFe_xO₃(M_y=Ba_0.15?Ba_0.40?Sr_0.40)

Perovskite manganites with ABO₃ structure are the typical magnetoelectric coupling materials,whose properties can be adjusted over a large-scale by cation doping at A and B sites.This provides a good material foundation and broad application prospects for constructing certain functional materials and devices.In conventional view,the magnetic ordering and electrical transport properties of these materials are explained using both the double exchange interaction?DE?model and the super exchange interaction?SE?model,where all oxygen ions are assumed as O^2-.However,we found no quantitative explanation for the dependence of the magnetic moment on the doping level at the A site of the perovskite manganites,using the DE and SE models.In fact,there are O^1-ions in addition to O^2-ions in an oxide,which has been proved by a large number of experiments and theoretical studies.This fact must have important influence on the magnetic and electrical transport properties of the material.Taking into account the existence of some O^1-ions in the oxide,our group proposed an O 2p itinerant electron model for magnetic oxides?IEO model?.Using the IEO model,several puzzles for the magnetic ordering of spinel ferrites and perovskite manganites can be explained,which had remained unclear for many years.In this paper,the samples with nominal composition La_1-xBa_x MnO₃?0?x?0.4?,La_0.85Ba_0.15Fe_x Mn_1-x-x O₃?0?x?0.2?,La_0.60Ba_0.40Fe_x Mn_1-x-x O₃?0?x?0.2?and La_0.60Sr_0.40Fe_xMn_1-x-x O₃?0?x?0.3?were prepared using sol-gel method.We studied the influence of the doping at the A and B sites on the magnetic and electrical transport behavior and the corresponding physical mechanism.The results are as follows:?1?For the polycrystalline samples La_1-xBa_xMnO₃?0?x?0.4?,it is found that when the calcination temperature reaches 973 K,nanocrystallite samples can be formed.As further increase of the calcination temperature,the crystallites in the samples begin to grow.When the calcination temperature reaches 1273 K,the crystallite grain size is close to or greater than100 nm.At this time,the influence of the surface effect on the magnetic properties of the samples can be neglected.It is also found that there is notable influence of the calcination temperature on the magnetic moment of the samples with no Ba doping.With the increase of Ba doping level x,the influence of the calcination temperature on the magnetic moment of the sample decreases gradually.When x?0.15,the calcination temperature has little effect on the magnetic moment of the sample.?2?For the Fe-doped samples La_0.85Ba_0.15Fe_xMn_1-xO₃?0.0?x?0.2?,La_0.60Ba_0.40Fe_x Mn_1-x-x O₃?0.0?x?0.2?and La_0.60Sr_0.40Fe_xMn_1-xO₃?0.0?x?0.3?,X-ray diffraction?XRD?analysis indicated that all the samples have rhombohedral structures with space group R 3 c.The magnetic moments of three series samples decrease with the increasing x.According to the IEO model,the magnetic moments of Fe³⁺cations are cant coupled antiferromagnetically with those of Mn³⁺cations,we therefore fitted successfully the curves of the magnetic moment of the sample versus the Fe doping level.?3?In order to confirm the canted magnetic structure between the cations in the B sublattice,magnetoresistances of the samples were measured.We found that the magnetoresistances of all the three series of samples increase gradually with decreasing temperature below a certain temperature,which indicated that there canted angle between the caions moments.At a temperature being far lower than the Curie temperature,the samples La_0.60Ba_0.40Fe_0.10Mn_0.90O₃andLa_0.60Sr_0.40Fe_0.15Mn_0.85O₃havethemaximum magnetioresistance values,45.5%and 45.2%?under 2.0 T magnetic field?,respectively,which are much higher than their magnetoresistance values,25.8%and 17.7%near the Curie temperature.This suggested that a little Fe doping can reduce the average canted angle between the cation magnetic moments.?4?A model with two channels of electrical transport?TCET?for perovskite manganites is proposed.Using this model,we fitted the experimental curves of the resistivity versus test temperature for single-crystalline samples La_1-xSr_xMnO₃?0.00?x?0.40?reported in a reference[Phys.Rev.B.51,14103]and polycrystalline samples La_0.6Sr_0.4Mn1-_xFe_xO₃?0.00?x?0.30?in present paper.In addition,we also investigated the influence of the content ratio of antiferromagnetic phase,the grain boundary scattering and the unit cell constant of the samples on the resistivity.