The role of octahedral tilting in the structural phase transition and magnetic anisotropy in SrRuO3 thin film

Abstract

We present a stoichiometry-dependent structural phase transition in SrRuO3 film on SrTiO3 substrate. The oxygen stoichiometry in the films was varied by changing the oxygen partial pressure P(O2) during the deposition process. For SrRuO3 films with P(O2) ≥ 60 mTorr, they exhibited a pseudo-orthorhombic structure with in-plane uniaxial magnetic anisotropy. On the other hand for films with P(O2) ≤ 45 mTorr, the tetragonal SrRuO3 phase with a perpendicular uniaxial magnetic anisotropy was stabilized at room temperature. The big difference in the magnetic anisotropy of these two SrRuO3 phases was shown to be closely linked to their respective RuO6 octahedral rotation patterns: the RuO6 octahedra rotate differently along the two orthogonal in-plane directions in the pseudo-orthorhombic phase, whereas in the tetragonal phase only octahedral rotations around z-axis are present and the octahedral tilts along the in-plane axes are diminished. First-principles calculations show that such a suppression of the RuO6 octahedra tilting in the tetragonal phase arises from the oxygen vacancies at the octahedral apex (along z-axis). This work demonstrates that the stoichiometry plays an important role in determining the octahedral rotations and tilts in the perovskite materials, which may induce new phases with distinctively different structural symmetry and physical property.

Oxygen deficiency and cooling field driven vertical hysteretic shift in epitaxial SrRuO3/SrTiO3 heterostructures

“…It is known that the SRO films grown at low oxygen pressure may produce oxygen deficiencies and introduce charged defects, which would cause an effective unit cell volume expansion to compensate the Coulomb repulsion between the uncompensated charges. [18][19][20] This volume expansion, together with the inplane coherent compressive strain, enables the elongation of the c-axis lattice parameter of the film. XPS measurements indeed show that the Ru/O atomic ratio for the SRO films deposited at P O 2 ¼150, 50, and 10 mTorr is estimated to be 1:(2.97 6 0.05), 1:(2.85 6 0.05), and 1:(2.65 6 0.05), respectively, which confirms the oxygen deficiency-induced nature of out-of-plane lattice expansion.…”

“…Moreover, it has been demonstrated that the pseudocubic SRO films show in-plane uniaxial magnetic anisotropy, whereas the tetragonal films possess perpendicular uniaxial magnetic anisotropy. 19,20 With reducing oxygen pressure, the c-axis lattice parameter of the tetragonal SRO films increases, displaying more significant perpendicular uniaxial magnetic anisotropy. The applied in-plane cooling field disfavors the alignment of unpinned moments along the field direction, and hence, the relative fraction of in-plane pinned uncompensated moments was diminished after sweeping the hysteresis loop.…”

Size effects in epitaxial oxide thin films

“…[15] Due to strong hybridization between the ruthenium d-orbitals and the oxygen p-orbitals, the rotations of the oxygen octahedra in SrRuO 3 can have a large effect on the electronic and magnetic properties. [153] Calculation results including this interface coupling effect are shown blue in Fig. 7.7 (blue curve).…”

Review on Electronic Correlations and the Metal-Insulator Transition in SrRuO3

“…An STO substrate (usually [001]) exerts a compressive strain on the SRO thin film because the lattice constant of cubic STO is smaller than that of pseudo-cubic SRO, and this SRO/ STO model system shows some interesting phenomena. For example, an orthorhombic-to-pseudo-orthorhombic or orthorhombic-to-tetragonal transition of SRO emerges as the system reaches the ultrathin and low-oxygen-vacancy limit (Chang et al, 2011;Lu et al, 2013aLu et al, , 2013bVailionis et al, 2008). Notably, two possibilities exist for the direction of the pseudo-orthorhombic or tetragonal c axis: pseudocubic [100] or [001].…”

“…One easy approach to observe the (tetragonal) anisotropy is to evaluate the (directional) magnetization curves. If the pseudo-orthorhombic and cubic c axes are parallel, the inplane spontaneous magnetizations are oppressed, whereas the magnetization along the pseudo-cubic [001], which is the magnetic easy axis of SRO, remains magnetically ordered (Lu et al, 2013a(Lu et al, , 2013b. Given that the ferromagnetism of SRO relies on the Ru-O-Ru channel and that the Ru 4d and O 2p hybridization is important (He et al, 2010), the electrical and magnetic anomalies are reasonably speculated to arise from the interfacial strain and the structural phase transitions (Vailionis et al, 2008).…”