Effects of strain on orbital ordering and magnetism at perovskite oxide interfaces:LaMnO3/SrMnO3

Abstract

We study how strain affects orbital ordering and magnetism at the interface between SrMnO 3 and LaMnO 3 from density-functional calculations and interpret the basic results in terms of a three-site Mn-O-Mn model. Magnetic interaction between the Mn atoms is governed by a competition between the antiferromagnetic superexchange of the Mn t 2g core spins and the ferromagnetic double exchange of the itinerant e g electrons. While the core electrons are relatively unaffected by the strain, the orbital character of the itinerant electron is strongly affected, which in turn causes a large change in the strength of the ferromagnetic double exchange. The epitaxial strain produces the tetragonal distortion of the MnO 6 octahedron, splitting the Mn e g states into x 2 − y 2 and 3z 2 − 1 states, with the former being lower in energy, if the strain is tensile in the plane and opposite if the strain is compressive. For the case of the tensile strain, the resulting higher occupancy of the x 2 − y 2 orbital enhances the in-plane ferromagnetic double exchange owing to the larger electron hopping in the plane, causing at the same time a reduction in the out-of-plane double exchange. This reduction is large enough to be overcome by antiferromagnetic superexchange, which wins to produce a net antiferromagnetic interaction between the out-of-plane Mn atoms. For the case of the in-plane compressive strain, the reverse happens, viz., that the higher occupancy of the 3z 2 − 1 orbital results in the out-of-plane ferromagnetic interaction, while the in-plane magnetic interaction remains antiferromagnetic. Concrete density-functional results are presented for the ͑LaMnO 3 ͒ 1 / ͑SrMnO 3 ͒ 1 and ͑LaMnO 3 ͒ 1 / ͑SrMnO 3 ͒ 3 superlattices for various strain conditions.

Evolution of magnetic phases and orbital occupation in(SrMnO3)n/(LaMnO3Aruta1, Adamo2, Galdi3 et al. 2009Phys. Rev. B682500View full textAdd to dashboardCite

“…4) and equal to the average density. The strain does not act in opposite way on LMO and SMO, but the system responds as a whole to the average strain that is slightly tensile, and the e g (x 2 − y 2 ) orbitals get stabilized as in LSMO grown on STO [27,29] or in (LMO) 1 /(SMO) 1 SLs [12].…”

“…In the case of LMO/SMO SLs the two materials widely change their behavior with respect to bulk. So, while in bulk the e g levels of SMO are empty and LMO presents in-plane (x 2 −r 2 )/(y 2 −r 2 ) orbital order, at SMO/LMO interfaces strain and electronic reconstruction are expected to modify orbital population and ordering in both components [11,12]. We have determined the orbital and magnetic properties of (SMO) n /(LMO) 2n SLs with n = 1, 5, 8, by measuring x-ray linear dichroism (XLD) and magnetic circular dichroism (XMCD) in x-ray absorption spectra (XAS) at the Mn L 2,3 edge.…”

“…In this context, strain driven spin-orbital coupled states arising in manganites make the interfaces between these compounds very interesting for engineering unique collective states. As a matter of fact, a certain amount of theoretical and experimental studies on superlattices (SLs) composed by the two AFM insulators, SrMnO 3 (SMO) and LaMnO 3 (LMO), appeared in literature during the last years [4,5,6,7,8,9,10,11,12]. The ordered sequence of the atomic layers in the digital SMO/LMO SLs [4,5], together with the electronic reconstruction arising from the interfacial Mn 3+ /Mn 4+ mixed valence, give rise to peculiar transport, magnetic and orbital properties, when different layering and strain conditions occur.…”

Electron-lattice and strain effects in manganite heterostructures: The case of a single interface

“…The epitaxial strain produces the tetragonal distortion of the M nO 6 octahedron, splitting the e g states into x 2 − y 2 and 3z 2 − r 2 states. 20 If the strain is tensile, x 2 − y 2 is lower in energy, while, if the strain is compressive, 3z 2 − r 2 is favored. In the case of n = 8 and three interfaces, 15 the superlattices grown on ST O are found to be coherently strained: all of them are forced to the in-plane lattice parameter of substrate and to an average out-of-plane parameter c ≃ 3.87Å.…”

“…Moreover, the effects of strain have been analyzed only within mean-field approaches. 20 In this paper we have studied phase diagrams, spectral and optical properties for a very large bilayer (LM O) 2n /(SM O) n (up to size of 48 planes relevant for a comparison with fabricated heterostructures) starting from a tight binding model. We have developed a correlated inhomogeneous mean-field approach taking into account the effects of electron-lattice anti-adiabatic fluctuations.…”