Evolution of magnetic phases and orbital occupation in(SrMnO3)n/(LaMnO3

Abstract

The magnetic and electronic modifications induced at the interfaces in (SrMnO3)n/(LaMnO3)2n superlattices have been investigated by linear and circular magnetic dichroism in the Mn L2,3 x-ray absorption spectra. Together with theoretical calculations, our data demonstrate that the charge redistribution across interfaces favors in-plane ferromagnetic (FM) order and eg(x 2 − y 2 ) orbital occupation, in agreement with the average strain. Far from interfaces, inside LaMnO3, electron localization and local strain favor antiferromagnetism (AFM) and eg(3z 2 − r 2 ) orbital occupation. For n = 1 the high density of interfacial planes ultimately leads to dominant FM order forcing the residual AFM phase to be in-plane too, while for n ≥ 5 the FM layers are separated by AFM regions having out-of-plane spin orientation.PACS numbers: 75.47. Lx, 78.70.Dm, 72.10.Di, 73.21.Cd, Interfaces between different transition metal oxides (TMO) have been widely demonstrated to be sources of interesting and unexpected electronic and magnetic properties.Metallic conductivity arises, for example, at the interface between two insulators, such as LaAlO 3 /SrTiO 3 [1] and LaTiO 3 /SrTiO 3 [2], while ferromagnetism (FM) occurs at the interface between the antiferromagnet (AFM) CaMnO 3 and the paramagnet CaRuO 3 [3]. 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. In the particular case of (SMO) n /(LMO) 2n the La:Sr ratio is 2:1, in analogy with the optimal composition of La 2/3 Sr 1/3 MnO 3 (LSMO). In such a case, the metal-insulator transition (MIT) and the magnetic properties depend on the thickness of the constituent blocks [4,5,6,7], although in a non trivial way. Indeed, saturation magnetization does not linearly decrease with n [4] and both fast and viscous spin populations are present, the latter associated to FM/AFM pinning [8]. Therefore, the development of the FM metallic phase at the interfaces is well established and the coexistance of the FM and AFM phases was inferred. However, the knowledge of the mutual dependence of the AFM and FM phases with n is still uncertain, but it could open further perspectives in the control of the low dimensional magnetic properties, thus in the engineering of the TMO magnetic heterostructures. In addition, as the role of interfacial Mn e g electrons is known to be important, the influence of strain and reduced dimensionality on the transpor...

Ubiquitous long-range antiferromagnetic coupling across the interface between superconducting and ferromagnetic oxides

“…5). We remind here that by using circularly polarised photons, the X-ray magnetic circular dichroism (XMCD) spectra provide information on the spin and orbital magnetic moments [23][24][25] , whereas X-ray linear dichroism (XLD) can be used to determine the orientation of the empty 3d orbitals 26,27 . In this way even tiny magnetic moments and orbital anisotropies can be detected.…”

Interface ferromagnetism in a SrMnO3/LaMnO3superlattice

“…The Tdependent polarization contrast in the SL may also be substantial; for instance, on closely related SL (Ref. 47), in-plane e g (x 2 − y 2 ) occupation and FM near LMO interfaces and out-of-plane e g (3z 2 − r 2 ) orbital occupation and AFM in the middle of LMO layers was inferred from XMLD and XMCD measurements. Polarization-resolved scattering measurements in a magnetic field with π and σ incident light and scattered beam polarization analysis are necessary to separate different magnetic and orbital contributions to scattering at the Mn L 3,2 edges.…”

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

“…Due to the effect of strain, the magnetic solution in LM O turns out to be compatible with experimental results in superlattices. 15 The interface is still ferromagnetic with metallic large polaron features. In the figure A/B/C refers to magnetic orders and character of charge carriers inside LM O (A), at interface (B), inside SM O (C).…”

“…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. 15 As a consequence, one can infer that LM O blocks are subjected to compressive strain (−2.2%) and SM O blocks to tensile strain (+2.6%). For the case of LM O block, the resulting higher occupancy of 3z 2 − r 2 enhances the out-ofplane ferromagnetic interaction owing to the larger electron hopping out-of-plane.…”