Study Of Magnetic Ordering In The Perovskite Manganites Pr_0.6Sr_0.4Mn_1-xM_xO₃?M=Cr,Fe,Co,Ni?

The ABO₃ perovskite manganites Ln_1-xT_xMnO₃,where Ln represents a trivalent rare-earth element?Ln=La,Pr,Nd,...?and T represents a divalent alkaline-earth element?T=Sr,Ba,Ca,...?,have been extensively researched over the last 30 years due to their colossal magnetoresistance and potential applications in magnetic materials.In these compounds,the Ln and T cations with large ionic radii occupy the A sites,forming the A sublattice,and the Mn cations with small ionic radii occupy the B sites,forming the B sublattice.According to the traditional view,all the oxygen are O-2 anions in these compounds,Mn cations are mixed valence of trivalence and tetravalence,which magnetic and electrical transport properties are correlated via the super exchange interaction(SE,along the Mn³⁺-O2--Mn³⁺ ion chain)and double exchange interaction(DE,along the Mn³⁺-O2--Mn⁴⁺ ion chain).However,using the SE and DE models cannot fit the variation of the experimental magnetic moment of the sample with the doping level x.At the same time,there are disputes for the magnetic ordering when a part of Mn cations in these materials were replaced by transition metal cations.Therefore,it is necessary to further explore the magnetic ordering of perovskite manganese.Taking into account the requirement that a portion of the O ions be O1-anions,our group proposed an O2p itinerant electron model,in the process investigated the magnetic structures of?A?[B]2O4 spinel ferrites.In this paper,we explain the magnetic ordering of the manganites with an ABO₃ perovskite structure using the O2p itinerant electron model.In this paper,the samples with nominal composition Pr_0.6Sr_0.4M_xMn_1-xO₃?0.00?x?0.30,M=Cr,Fe,Co,Ni?were prepared by sol-gel method.We have investigated the X-ray diffraction spectra?XRD?spectra,X-ray photoelectron spectroscopy?XPS?,magnetocaloric curves and hysteresis loops at 10 K for the samples.In addition,the electric transport properties of the samples were studied,and we obtained the following conclusions:?1?All the samples had a single-phase orthorhombic structure with the space group Pbnm.The crystal cell volume v decreases with the doping level,x,for the Pr_0.6Sr_0.4M_xMn_1-xO₃?M=Cr,Co,Ni?,while v increases with the doping level,x,for the Pr_0.6Sr_0.4Fe_xMn?1-x?O₃.The volume averaged diameter of crystallites was found to be greater than or close to 100 nm for all the series samples,so that the surface effects were expected to be very weak.?2?We investigated the XPS of polycrystalline samples Pr_0.6Sr_0.4MnO₃,SrMnO₃ and Pr₂O₃.The O1 s spectra were fitted using two peaks with different binding energies?BE?,the lower BE peak is assigned to O2-ions,and the higher BE peak to O1-ions.The results show that the average valence of oxygen ions in the sample Pr_0.6Sr_0.4MnO₃ was ValO =-1.77.The average valence of oxygen ions of SrMnO₃ and Pr₂O₃ were ValO=-1.66 and ValO =-1.72 respectively.These results are close to that valence value,-1.63,of oxygen ions in BaTiO₃ calculated by Cohen [Nature 358,136?1992?] using density functional theory.The analysis showed that there are no Mn⁴⁺ cations in the sample Pr_0.6Sr_0.4MnO₃.?3?We observed two transition temperatures in the curves of magnetization???versus the temperature?T?for sample Pr_0.6Sr_0.4MnO₃.The higher transition temperature,TCM,is the Curie temperature of the sample.The lower transition temperature TCP occurs near TCP=60 K,an the magnetization???anomalously decreases,and the variation amplitude of magnetization????is related to the applied magnetic field.When ?0H=0.05 T,the value of ?? is higher than that when ?0H=0.01 T,but when ?0H=2.0 T,this anomalously change vanishes.?4?Using the O2p itinerant electron model to analyze the experimental results,we found that the magnetic moments of the Mn³⁺ cations show canted ferromagnetic coupling in the B sublattice.The magnetic moments of the Pr cations(including Pr2+ and Pr³⁺)in the A sublattice also show canted ferromagnetic coupling,however,the magnetic moment of the A sublattice is antiferromagnetic coupling to that of the B sublattice.The temperature TCP is the magnetic ordering temperature of the Pr cations,above which the magnetic moments of the Pr cations become disordered.The magnetic moment of the B sublattice is parallel to the direction of the applied magnetic field,the magnetic moment of the A sublattice is opposite to that of the applied magnetic field.This results in the cant angle between the Pr cations magnetic moments increases with increasing of magnetic field,the itinerant electrons along the O-Pr-O-Pr-O chains becoming more difficult.When ?0H?2.0 T,the Pr moments becoming disordered,the anomalous variation near TCP vanishes for the samples.?5?Compared with the average molecular magnetic moment of all the samples at 10 K,we observe that the magnetic moments of all the samples decrease with the increase of doping level,while the decrease of Fe doped samples is the fastest.According to the requirements of the O2p itinerant electron model,in the B sublattice,the magnetic moments of Cr³⁺ cations are canted ferromagnetic coupling with the magnetic moments of Mn³⁺ cations,the magnetic moments of Fe³⁺,Co³⁺,Ni³⁺ cations are canted antiferromagnetic coupling with the magnetic moments of Mn³⁺ cations.We fitted successfully the curves of the magnetic moments versus the doping level x.We found that the cant angle of magnetic moments monotonically increases with the increasing x for Cr-doped samples,while the cant angle of magnetic moments decreases firstly and then rapidly increases with increasing x for Fe?Co,Ni?-doped samples.?6?By investigated the electrical transport properties of the samples,we confirmed the assumption,that there are cant angles between the magnetic cations in the B sublattice,is reasonable.First,the magnetoresistance of the Cr-doped samples increases with the decrease of the temperature,namely,the magnetoresistance at the low temperature is larger than that near the Curie temperature.This is similar to that of the sample Pr_0.6Sr_0.4MnO₃.It is suggested that there are non-zero cant angles between the Mn and Cr cation moments at lower temperatures,an applied magnetic field can reduce the angle,increase the transition probability of the itinerant electrons and reduce the resistivity.Second,for the Fe-doped Pr_0.6Sr_0.4Mn_0.9Fe_0.1O₃ sample,which magnetoresistance variation is different from that of the sample Pr_0.6Sr_0.4MnO₃,there is magnetoresistance maximum value near Curie temperature,either above or lower which temperature magnetoresistance decreases rapidly.This is similar to the experimental result of the typical perovskite manganese La_0.85Sr_0.15MnO₃ sample,in which the magnetic moments are parallel each other.Therefore,the fitted result that the cant angle in the sample Pr_0.6Sr_0.4Fe_0.1Mn_0.9O₃ decreases to near zero,is reasonable.