Strain Effect In SrCoO_x Thin Films

Transition metal oxide materials have many interesting properties to be applied in high temperature superconductivity,ferroelectric storage and giant magnetoresistance.SrCoO_x is a typical transition metal oxide with strong spin-phonon coupling.In theory,strain can play a role in the control of magnetic and oxygen vacancies in SrCoO_x.Up to now,no experimental results can prove the theoretical calculation results.At the same time,the coupling mechanism in strain,oxygen vacancy,magnetism and conductivity of this strong electron correlation system is still not revealed.Herein,the SrCoO_x target was prepared by solid-state reaction,and then,the films were deposited by pulsed laser deposition(PLD).The stress was applied into SrCoO_x films by the lattice mismatch between thin films and substrates.We chose the substrates including YAl O₃(110),La Al O₃(001),LSAT(001),STO(001)and Mg O(001).The X ray diffraction and selected area electron diffraction results indicating the SrCoO_x films grown by PLD are in brownmillerite structure(BM-SCO).The strain effects in manipulating the BM-SCO thin films can be summarized:(1)Tensile stress would change the epitaxial crystal orientation of BM-SCO films.At the same time,the direction of oxygen vacancy tunnels would also be changed.In this case,chemical activity of thin films is controlled;(2)From the results of X-ray photoelectron spectroscopy,the relationship between strain and oxygen content can be deduced:with the increasing of compressive strain,the oxygen content of BM-SCO thin films increase from 2.56 to 2.64,however,the oxygen content of tensile strained film is only 2.51.(3)The conductivity of the films is also positive correlated to the oxygen content.The overlap of Co3d and O2p orbitals is enhanced,which result in good conductivity of thin films and make SrCoO_x film be more suitable as a cathode material for fuel cell.(3)The BM-SCO thin films show weak ferromagnetism when they are in compressive strain.The strain effects of perovskite SrCoO_x(P-SCO)thin films are also necessary to be studied.Firstly,the BM-SCO films grown by PLD.Then,the annealing and Na Cl O solution oxidation methods are tried to form P-SCO thin films respectively.For annealing method,the oxygen content of annealed films was less than 2.75,which indicating no perovskite structure in this case.After oxidation in Na Cl O solution,the structure of SrCoO_x thin films transit from brownmilleritep to perovskite.In perovskite structure,the saturation magnetization of this film can be 2.5?_B/Co,and the resistance of films is reduced,and the film obtain the metal behavior.Theoretical calculations indicate that the P-SCO film will transit from"ferromagnetic metal"to"antiferromagnetic insulator ferroelectric"under stress.The strain effects in manipulating the P-SCO thin films can be summarized:(1)P-SCO film is antiferromagnetic under tensile strain which induced by Mg O substrates;(2)The compressive strain would increase the formation energy of oxygen vacancies and slow down the reduction rate of P-SCO thin films in Na Cl O solution.In summary,we prepared of BM-SCO and P-SCO thin films by PLD and solution oxidizing technique.The strain-manipulated effects of SrCoO_x thin films were studied by using the lattice mismatch between the substrate and SrCoO_x film.We also made some preliminary discuss on the coupling relationship in strain,structure,ferromagnetism and oxygen vacancies.