Oxygen vacancy formation, crystal structures, and magnetic properties of three SrMnO3−δ films

Abstract

The crystal structures and magnetic properties of the 40 nm brownmillerite SrMnO2.5 film, perovskite SrMnO3-δ film, and mixed-phase film have been systematically investigated. The features of the oxygen vacancy ordering superstructure in the brownmillerite SrMnO2.5 film are observed from HRSTEM as follows: the dark stripes with a periodicity of four (110) planes of the cubic perovskite appearing at an angle of 45° with the substrate-film interface and extra reflection spots in fast Fourier transformation patterns along the (001) plane. When annealing the brownmillerite SrMnO2.5 film under higher oxygen pressure, the top portion undergoes structure transition into perovskite SrMnO3-δ as seen in the mixed-phase film consisting of the perovskite SrMnO3-δ phase dominating at the top part and the brownmillerite SrMnO2.5 phase dominating at the bottom part. The magnetic properties and Mn valences of the brownmillerite SrMnO2.5 film indicate that this film, similar to the bulk, is antiferromagnetic with TN at 375 K. However, the strained tetragonal perovskite SrMnO3-δ film exhibits ferromagnetic behavior with Curie temperature at 75 K and a saturation magnetization of 2.5μB/Mn at 2 K. Moreover, the top perovskite SrMnO3-δ phase of the mixed-phase film also exhibits ferromagnetic behavior evidenced by the existence of the exchange bias. We propose that the ferromagnetic properties in both the perovskite SrMnO3-δ film and the top perovskite SrMnO3-δ phase in the mixed-phase film originate from Mn3+–Mn4+ double exchange coupling. However, the formation of Mn3+ differs for the two samples in that it is caused by oxygen vacancies in the former and the distribution of oxygen content across the film during annealing in the latter.

Self-consistent site-dependent DFT+ U study of stoichiometric and defective SrMnO3

“…Straindependent magnetic properties of SMO thin films are still an open question. Some theoretical and experimental results 13,26,27 have found SMO thin films to be AFM, while more recent experimental studies 28,29 report that SMO thin films have ferromagnetic (FM) properties. Ferromagnetism was explained to emerge from strain-induced changes of the Mn-O-Mn bond angles and the Mn 3+ -Mn 4+ double exchange coupling induced by the presence of oxygen vacancies.…”

Coexistence of different magnetic ordering in thin films of SrMnO3 studied by spin transport

“…14,23 However, in thin films, the effect of tensile strain, oxygen vacancies, and different growth environment promotes competing magnetic states with ferromagnetic, ferrimagnetic, and antiferromagnetic ordering. [17][18][19][20][21] In our case, the thinner sample S1 shows a strong ferromagnetic behavior that arises from the mixed valence states in Mn, prevalent in such competing magnetic phases, due to tensile strain and oxygen vacancies. [17][18][19][20] Owing to its ultrathin nature in sample S1, no structural characterization and quantification of the strain were possible.…”

“…[17][18][19][20][21] In our case, the thinner sample S1 shows a strong ferromagnetic behavior that arises from the mixed valence states in Mn, prevalent in such competing magnetic phases, due to tensile strain and oxygen vacancies. [17][18][19][20] Owing to its ultrathin nature in sample S1, no structural characterization and quantification of the strain were possible.…”

“…16,17 In addition, controlled tuning of different growth conditions can also lead to coexistence of different magnetically ordered phases for this end member of the manganite family, typical among them being a ferromagnetic insulating phase brought about by the structural distortion of the oxygen octahedra and leading to an oxygen deficient SrMnO 3Àd phase. 18 Moreover, first principles calculations have predicted that tensile or compressive strain can drive a series of magnetic phase transitions that couples with the ferroelectric order in SMO and were corroborated by experimental studies. 17,[19][20][21][22] In this work, we study the temperature dependence of magnetic phases that coexist in thin films of SrMnO 3 (SMO) using complementary techniques, (i) bulk magnetization studies using Superconducting Quantum interference device (SQUID), (ii) Surface sensitive SMR response, and (iii) bulk sensitive SSE response.…”

“…Magnetization vs Temperature (M-T) plots show an additional magnetic ordering around 40 K for both S1 and S2 as seen from the cusp like feature (black arrow) in the ZFC curve similar to previous reports. 18 Secondary phases of MnO x with mixed oxidation states of Mn (Mn 2þ , Mn 3þ , and Mn 4þ ) that could segregate during the deposition of ReMnO 3 are well documented in the literature. The competing magnetic phases are reported to be ferrimagnetic Mn 3 O 4 24-27 or c À Mn 2 O 3 28 with transition temperature around 40-46 K and antiferromagnetic MnO/a À Mn 2 O 3 with transition temperatures around 80-100 K. 29 Further, computationally both the cubic and hexagonal structures are shown to exhibit several magnetically ordered structures, depending on the interactions between the neighboring B site cations and with an AF ground state.…”