Growth And Transport Properties Of SrFeO_3-δ Single Crystals

The SrFeO_3-δ（0<δ≤0.5）compounds exhibit various structural phases due to their oxygen vacancies,and accordingly,there are abundant physical properties in the system.Among them,the tetragonal phase SrFeO_2.85 is occupied in the iron structure with an average valence of Fe^3.5+in its crystal structure,and it exhibits charge-ordering of Fe^3.5+→Fe³⁺+Fe⁴⁺at low temperature where resistivity rises abruptly.However,in the electrical transport test of the compound,after charge ordering,the resistivity again shows metallicity with the decrease of temperature,and the cause of the abnormal insulator-mental transition is still unknown.In order to study the intrinsic physical mechanism of the aberrant transformation in detail,this paper analyzes the magnetic and electrical transport of SrFeO_2.85 single crystal and finds the relationship between magnetic phase transition and electrical transport anomaly.In this paper,the SrFeO_3-δsingle crystal samples were grown by optical floating zone method.The phase of tested sample was determined by powder XRD.The crystal quality was characterized by Laue backscattering and the orientation of the crystal was completed.Since the oxygen content is decisive for the physical properties of the compounds,we determined the oxygen content of the sample by means of iodometric titration.During the titration process,we found that the usual iodometric titration operation could not obtain the correct oxygen content due to the strong oxidizing of Fe⁴⁺.For this reason,we improved the titration process and gave the correct way of iodometric titration with oxygen content in high-valent compounds.Through the analysis of the titration results,we found that there is a phenomenon of uneven oxygen content in the grown sample.In order to obtain the single crystal sample with uniformed oxygen content for the final test,we used high oxygen pressure annealing to increase oxygen content.According to the magnetic test,the antiferromagnetic ordering temperature of the SrFeO_2.85 single crystal is 69.5 K.The significantly hysteresis phenomenon in the FCC/FCW curve and the resistivity curve shows that the compound has a structural transition at 69 K.Below the antiferromagnetic ordering temperature,we find that the insulator-metal transition phenomenon of resistivity changes with temperature is due to the orbital selective Mott phase transition through the anisotropy of the resistivity of the ab plane and c-axis of the SrFeO_2.84 single crystal.By testing the magnetoresistance of SrFeO_2.85,we found that there is a colossal magnetoresistance effect corresponding to the charge ordering is depressed by the external magnetic field.In order to understand the transport properties of the tetragonal phase SrFeO_2.85 more deeply,we also tested its Hall effect.From the relationship between Hall resistivity and magnetic field,we found that there is an anomalous Hall effect in the compound,indicating the existence of spin-orbit coupling in the material.From the temperature dependence of Hall resistivity at 5 T,the negative Hall resistivity,suggesting the presence of the nontrivial topological spin texture,is clearly found.By comparing the relationship between resistivity and temperature of SrFeO_2.74.74 and SrFeO_2.89 single crystals,we excluded the anomalous insulator-metal transition that occurred in the tetragonal phase due to the possibility of percolation of the cubic metal phase.And also found that the anomalous resistivity at 112 K is the character of tetragonal phase and it is corresponded to the origin of charge ordering.