Study On Structure,Magnetism And Transport Properties Of NdMnO₃ And Nd Doped Perovskite

Manganese oxides with perovskite structure have been widely concerned due to their rich and interesting physical and chemical properties,such as Colossal-magnetoresistance effect,magnetocaloric effect,magnetic phase transition,exchange bias effect,etc.The synthesis of perovskite manganese oxide does not require much of the environment,and its preparation has the advantages of safety and convenience,which accelerates the popularization and practical application of related research.In this paper,perovskite manganese oxide NdMnO₃,and its Nd-site doped sample Nd_1-xSr_xMn O₃（x=0.2,0.3,0.4,0.5）and Nd_1-xBa_xMn O₃（x=0.2,0.3,0.4,0.5）compounds were prepared by high temperature solid-state reaction method.Focusing on the key points of crystal structure,magnetic phase transition,electronic transport,magnetocaloric effect and critical behavior,the physical properties of perovskite manganese oxide NdMnO₃and the influence of Nd-site doping were deeply studied,to investigate the physical mechanism and explore the application prospect of this series of samples.The main research contents and results of this paper are as follows:1.The single-phase NdMnO₃ compound was successfully synthesized by solid-state reaction method.The sample was single-phase orthogonal structure with the space group Pbnm（No.62）.The magnetic properties and magnetocaloric effect of NdMnO₃ were studied.At low temperatures around 12 K,Nd ordering is induced by the Mn ferromagnetic component with antiferromagnetically coupling with each other.Magnetization reversal occurs due to Mn moments reorientation induced by the ordering Nd moments,which leads to the phenomenon of the negative magnetization of NdMnO₃.Mn is in a high spin state acting on AFM interaction about 53 K and Mn FM-component ordering temperature is around 82 K within a-b plane.According to the critical behavior,the Kouvel-Fisher method is reliable,with the magnetic exchange distance as J（r）≈4.563,that is,somewhere between the 3D-Heisenberg model and the mean-field model.And three different extremum values of magnetic entropy change including positive and negative are obtained.Forμ₀ΔH=50 k Oe,the maximum entropy change（-ΔS_M）is 3.82 J/kg·K and the RCP is 105.9 J/kg at 10-15 K;At 80-85 K,the maximum entropy change（-ΔS_M）is 1.22 J/kg·K.Forμ₀ΔH=5 k Oe,the maximum entropy change（-ΔS_M）is-0.557 J/kg·K at about 8 K,due to the Nd moments ordering and the Mn moments orientation.Therefore,NdMnO₃ has good magnetic and magnetocaloric properties.In addition,NdMnO₃exhibits conductive behavior similar to that of a semiconductor,with relatively weak magnetoresistance（MR）.The maximum MR at a magnetic field of 6 T is only 8%,and the conductive mechanism in the paramagnetic temperature range is more consistent with the thermal activation model.2.Single-phase Nd_1-xSr_xMn O₃（x=0.2,0.3,0.4,0.5）polycrystalline samples were synthesized by high temperature solid-state reaction method.The x=0.2 and x=0.3 samples were single-phase orthogonal structure with the space group Pnma（No.62）.The x=0.4 and x=0.5 samples are single-phase orthogonal structure with the space group Imma（No.74）.With the increase of Sr doping concentration,the Curie temperature T_C is 152 K,217 K,283 K and291 K,respectively.The double exchange（DE）interaction leads to the increase of T_C and the spontaneous magnetization.However,for x=0.5 sample,in addition to the DE,there is also a charge-ordered AFM superexchange interaction,DE and SE both exist,so when the doping concentration increases to x=0.5,the T_C increases slowly（little change）and the spontaneous magnetization decreases.It is found that the difference between T_CW and T_C comes from the competition between polaron and double exchange.Below T_C,DE plays a dominant role,while above T_C,polaron appears to play a role,that leading to a series of complex magnetic behaviors.With the increase of Sr doping concentration,atμ₀ΔH=50 k Oe,the maximum magnetic entropy changes（-ΔS_M）of the samples near T_C reached 3.108 J/kg·K,4.315 J/kg·K,3.298J/kg·K and 2.433 J/kg·K,and maximum RCP values were 379.19 J/kg,280.48 J/kg,283.64J/kg and 262.81 J/kg,respectively.The critical behaviors of x=0.2,0.3 and 0.4 samples all conform to the mean-field model,while the critical behaviors of x=0.5 samples cannot be analyzed due to the existence of charge-ordered AFM phase,which does not conform to any classical model.In addition,the samples all show excellent magnetoresistance properties.When the applied magnetic field is 6 T,the maximum magnetoresistance of x=0.2,0.3,0.4 and 0.5samples near T_C is 69.4%,81.36%,45.2%and 60.33%,and the conduction mechanism of paramagnetic-section temperature accords with thermal activation（TA）model,variable range hoping（VRH）model,small polaron（SP）model and variable range hoping（VRH）model,respectively.The significant magnetoresistance and magnetocaloric effects greatly enhance its application value in the field of magnetic sensor and magnetic refrigeration.3.Polycrystalline samples of Nd_1-xBa_xMn O₃（x=0.2,0.3,0.4,0.5）were synthesized by high temperature solid state reaction method.The x=0.2 polycrystalline samples showed single-phase orthogonal structure with space group Pnma（No.62）;x=0.3 sample is single-phase orthogonal structure with space group Imma（No.74）;x=0.4 sample is orthogonal structure with space group Imma（No.74）,containing hexagonal Ba Mn O₃ impurity phase;x=0.5 sample is tetragonal system with space group P4/mmm（No.123）,containing hexagonal Ba Mn O₃ impurity phase.With the increase of Ba doping concentration,the Curie temperature T_C is 122 K,147 K,128 K/150 K and 128 K/151 K,respectively（the transition point of about150 K is resulted from Ba Mn O₃）.The double exchange（DE）interaction leads to the rise of T_Cand the increase of spontaneous magnetization.We found that the difference between T_CW and T_C comes from anisotropic spin interactions and could be attributed to Dzyaloshinshy-Moria（DM）coupling.With the increase of Ba doping concentration,whenμ₀ΔH=50 k Oe,the maximum magnetic entropy change（-ΔS_M）near T_C reached 2.588 J/kg·K,2.672 J/kg·K,3.754J/kg·K and 3.464 J/kg·K,and the maximum RCP was 491.72 J/kg,390.06 J/kg,270.26 J/kg and 235.57 J/kg,respectively.The critical behavior of x=0.2 and x=0.3 samples are consistent with the mean-field model,and the critical behavior of x=0.4 and x=0.5 samples cannot be analyzed due to the presence of impurity phase.In addition,the samples all show excellent magnetoresistance properties.When the applied magnetic field is 6 T,the x=0.2,0.3,0.4 and0.5 samples have the maximum magnetoresistance of 72.22%,88.32%,94.56%and 95.57%near T_C,respectively.For x=0.2 sample,the conduction mechanism in the range of paramagnetic temperature conforms to the thermal activation（TA）model,and the samples with other doping concentrations all conform to the variable range hoping（VRH）model.