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Orbital occupation, atomic moments and magnetic ordering at interfaces of manganite thin films

We have performed x-ray linear and circular magnetic dichroism experiments at the Mn L2,3-edge of the La0.7Sr0.3MnO3 ultra thin films. Our measurements show that the antiferromagnetic (AF) insulating phase is stabilized by the interfacial rearrangement of the Mn 3d orbitals, despite the relevant magnetostriction anisotropic effect on the double-exchange ferromagnetic (FM) metallic phase. As a consequence, the Mn atomic magnetic moment orientation and how it reacts to strain differ in the FM and AF phases. In some cases a FM insulating (FMI) phase adds to the AF and FM. Its peculiar magnetic properties include in-plane magnetic anisotropy and partial release of the orbital moment quenching. Nevertheless the FMI phase appears little coupled to the other ones.


I. INTRODUCTION
Interfaces obtained by assembling insulating, non-magnetic perovskite oxides can show unexpected properties such as high conductivity and ferromagnetism 1,2 . Oxygen vacancies 3 , epitaxial strain 4,5 , the so-called "polarization catastrophe" from interface-generated dipoles 6 and electronic reconstruction at the interface 7,8 are all at play in perovskite oxides and their individual roles are still far from being understood. An important case is the interface of manganites thin films with other different oxides, providing tunnel junctions for a number of manganite-based devices, such as spin valve or spin injectors. In this context, several authors have investigated the properties of ultra-thin manganite films on various substrates and it has been found at the double exchange (DE) magneto-transport properties are strongly depressed below a critical thickness 9 . To explain such a behavior, nanoscale inhomogeneities with coexisting clusters of different stable phases have been extensively investigated, but it is still uncertain if and how this acts on the suppression of the magneto-transport properties of ultrathin films 10 . Chemical composition, strain and oxygen stoichiometry are considered the main parameters influencing the disorder-driven phase separation 11 . In thin films of La 0.7 Sr 0.3 MnO 3 (LSMO) the existence of intrinsic inhomogeneities has been explained in terms of structural macroscopic distortions induced by the strain with the substrate which favors preferential orbital occupation of the e g Mn orbitals 12,13 . While in the case of LSMO films grown on LaAlO 3 substrate the suppression of the DE magneto-transport properties is a "bulk" effect caused by strong in-plane compressive epitaxial strain, in the case of the LSMO films grown on SrTiO 3 (weak tensile strain) and NdGaO 3 (almost unstrained) the same phenomenon is a pure "interface/surface" effect 14 . Broken symmetry at the interfaces to the substrate and to the vacuum drives the orbital reorganization in ultrathin LSMO films, thus favoring the occupation of the e g (3z 2 −r 2 ) versus the e g (x 2 −y 2 ) orbitals among the otherwise energy degenerate Mn 3d states at the Mn 3+ sites, as shown in Fig. 1. In addition, it has been recently reported 15 that structure and stoichiometry gradually change at the interface of ultrathin LSMO films on STO, with a resulting elongation of the interfacial out-of-plane lattice constant. Such structural modification is similar to the cooperative Jahn-Teller-like distortion induced by the in-plane compressive strain, which in turn favors the stabilization of the e g (3z 2 − r 2 ) orbitals. As a result, the disproportion in e g orbital occupation induces a coupling between neighboring Mn cations that is ferromagnetic (FM) along the c-axis (perpendicular to the surface) and antiferromagnetic (AF) in the ab plane 16 , eventually resulting in the stabilization of the C-type AF phase at low temperature 17 . However, such magnetic phase was not directly experimentally observed.
While the "bulk" magnetic properties have been largely investigated in manganite films as a function of the strain 18,19,20,21 , the microscopic origin of the magnetic properties at the films surface and at the interface between film and substrate have not yet been completely clarified.
In this respect experimental investigations by surface sensitive x-ray magnetic scattering of layered manganite single crystals 22  in soft x-ray absorption spectroscopy (XAS) by synchrotron radiation. Such techniques were already successfully employed to observe the directional coupling by exchange bias between the spins in the AF regions and those in the adjacent FM regions in different magnetic systems 24 . Therefore, XMLD and XMCD are the ideal techniques to study the arrangement where the magnetic contribution to XLD is strong, whereas the latter were taken above the Curie temperature, where the only contribution to XLD comes from the preferential orbital occupation. Moreover the interpretation given by Huijben et al. of the XLD is surprisingly opposite to that of numerous papers with experimental and theoretical contents 12,13,14,28 .
Interestingly, Huijben et al. 27 have also reported that the spin to orbital-ordered coupled insulator phase develop at the interface. Therefore, the microscopic origin of the magneto- LSMO films were grown by pulsed laser deposition with in situ reflection high energy electron diffraction (RHEED). Film thickness was controlled at the level of a single unit cell by the intensity oscillations of the RHEED specular spot. Additional details on the growth technique are given in ref. 29 . The crystallographic and transport properties of the investigated samples, obtained by x-ray diffraction and electrical measurements 12,13,14 , are reported in Table I. The out-of-plane (ǫ zz ) and in-plane (ǫ xx ) strains are defined as the percentage variation of the out-of-plane and in-plane lattice parameters of films relative to the bulk LSMO values. From Table I  Linear and circular dichroism measurements were carried out at the ID08 beam line of the along the photon propagation direction of the sample magnetization vector, and, at the Mn L 2,3 edge, the effect is very strong (Fig. 2a). Thanks also to its exceptional sensitivity and at different temperatures ranging from 300K to 10K. The spectra are normalized to the sum of the XAS L 3 peak height signals.

III. RESULTS
The orbital contribution to XLD in Mn 3+ is caused by the anisotropy in the bonding and is strictly related to the occupation of the e g (3z 2 − r 2 ) or e g (x 2 − y 2 ) orbitals. Because the magnetic order vanishes above the magnetic order temperature, XLD measurements per-  formed at room temperature are only sensitive to the preferential orbital occupation. Room temperature XLD curves of Fig. 4b and Fig. 5b are typical of the e g (3z 2 − r 2 ) preferential occupation induced by the interface symmetry in very thin films 14 , whatever the sign of the mismatch between film and substrate. Furthermore, below the magnetic transition temperature, by applying a magnetic field parallel to the incident photon beam , it is possible to suppress selectively the FM contribution to the XMLD spectrum thus singling out the AF contribution. In such a geometry, if the applied field is strong enough, magnetization in the FM system is forced to align along the direction of the incident beam. Therefore, the FM contribution to the XMLD is suppressed because the spin system is orthogonal to both the V and H polarization directions. On the other hand, spin orientation in the AF phase is not affected by an external field, so that no major changes in the XMLD spectra under external field are expected for the AF phase.
Normalized XLD spectra as a function of temperature for LSMO films grown on STO and LAO are reported in Fig. 4b and Fig. 5b, taken with or without the applied magnetic field B = 1 T. Such a field is strong enough to saturate the magnetization of the FM phase, as demonstrated by the hysteresis loops reported in Fig. 2b and Fig. 3b. The normalized XLD spectra for a 10 u.c. thick LSMO film on STO is shown in Fig. 4b, with (bottom panel) or without (top panel) applied magnetic field. As reported in Table I, the 10 u.c. thick film on STO has a depressed T MI and becomes metallic only below 275 K, because of the proximity to the critical thickness for the suppression of the magnetotransport properties (about 7 u.c. for films grown on STO and NGO) 14 . XLD spectrum measured at 300K, above the magnetic transition temperature, is scarcely influenced by the application of a 1 T magnetic field, as expected. The intensity and shape of the XLD curves changes dramatically when the film is cooled below the magnetic ordering temperature. Large intensity changes of the XLD spectra with temperature reveal the magnetic dependence of the dichroic signal. Furthermore, below the ordering temperature, the application of a 1 T field along the X ray beam direction results in a full reversal of the XLD curves relative to the zero field case.
Such an effect is a consequence of the field induced suppression of the FM contribution to the XLD signal which is left with the AF contribution alone. The substantial presence of FM phase even in a sample with reduced T MI is confirmed by the XMCD measurements of Fig. 2a and b. The observed changing with temperature of the XLD shape is related to the different easy-axis orientation, i.e. local magnetic moment preferential orientation, of the prevalent magnetic phase at the corresponding temperature.
A different behavior is shown in fig.5b for the 30 u.c. thick film on LAO. In this case, the sizeable in-plane compressive epitaxial strain induced by the substrate strongly affects the magneto-transport properties 13,14 , thus resulting in an insulating behaviour over the whole temperature range. The XLD spectra at B = 0 T and B = 1 T are similar at all tempera-tures indicating that the signal comes here mainly from an AF phase. Although, the XMCD measurements of Fig. 3a and b demonstrates the presence of a sizeable FM contribution even in this sample, in spite of the depressed magneto-transport properties. The domains dispersion of such FM metallic phase is supposed to be below the percolation limit for the charge transport because of the insulating character of the 30 u.c. LSMO film on LAO. The additional presence at the interface of the Mn 4+ -rich FM insulating (FMI) phase 13,33,34 has to be also considered.
In order to strengthen the scenario outlined above, we have subtracted the orbital contribution to the XLD spectra. To do this we assumed that the orbital contribution to XLD is negligibly sensitive to the temperature, and plotted the difference between the XLD spectra mea-  Table I, the 50 u.c. thick film on STO (Fig. 6a) and the 100 u.c. thick film on LAO at B = 1 T (when the FM phase is suppressed) have the same behavior as for the films on the LAO substrate (and on the NGO substrate too, not shown here). As a consequence, in LSMO films grown on STO and NGO the C-type AF phase is stabilized regardless of the small in-plane tensile strain, which would rather be expected to favor the A-type AF phase 12,17,25,26 . Therefore, in thin films grown on STO and NGO the FM easy axis lays in the ab-plane whereas the AF easy axis is along the c-axis. The schematic drawings of the different easy axes directions are reported at the top of Fig. 6.
Further insight in the magnetic properties of very thin manganite layers can be obtained from XMCD measurements. The spin and orbital magnetic moments per atom, m s and m o , can in principle be quantified by applying the sum rules 35,36,37 . According to them, m s and m o are directly related to the dichroic difference intensities A and B (Fig.2a and 3a), which are the L 3 and L 2 areas, respectively, of the XMCD spectra:  and where C is the XAS energy integral over the L 2,3 edges. However, the quantitative analysis with the sum rules is demonstrated to fail in case of Mn atoms 38 , because of the mixing of the L 3 and L 2 core levels and of the contribution of the magnetic dipole term. In Fig. 7 we report the ratio of the orbital to spin moment (m o /m s ) as a function of the strain to qualitatively compare the investigated samples. It is also important to underline that the A and B values extracted from the XMCD data strongly depend on the energy range chosen for the integration of the L 3 and L 2 edges. Therefore, the determination of the orbital and spin moments is affected by a large uncertainty and can be carried out with the limited purpose of a comparison among homogenous measurements. In the inset of Fig. 7 fig.6. Using XMLD we found (see Fig. 6) that at the interfaces the AF C-type phase is nucleated by the stabilization of the 3z 2 − r 2 orbital due to the break of the symmetry along the c-axis. This also leads to preferential spin orientation out of the ab plane in the AF phase, irrespective of the strain induced by the substrate. On the contrary in the FM regions we found a preferential orbital occupation within the ab-plane for tensile strain (STO substrate) and out of the ab-plane for compressive strain (LAO).
As already reported in literature 21 , these results can be explained in terms of the positive magnetostriction which induces the FM easy-axis along the tensile strain direction. We can rule out the shape anisotropy contribution in agreement with previous reports 18 , because in our films the FM easy-axis in-plane orientation is not strictly dependent on the thickness of the film.
Moreover the evolution of m o /m s measured by XMCD reported in Fig.7  , a significant elongation of the octahedra along the z-direction takes place also in the FM phase, the e g (3z 2 − r 2 ) orbitals get preferentially occupied and the orbital moment quenching is partially lifted 18,40 because of the relevant contribution of the FMI phase. The non quenched m o value can also be explained in terms of the e g occupancy decrease, as in the case of Mn 4+ increasing of the interfacial FMI phase and the e g -band width narrowing 40 . The last being also related to the lattice distortions and the more ionic character of the FMI phase. All these effects cooperatively contribute to deviate the orbital moment from the quenching in the thinnest LSMO film on LAO.
Finally we note that the AF and FM phases have rather independent magnetic anisotropy.
From one side, the non quenched in-plane m o value is related to the FMI in-plane easy magnetic axis orientation. On the other side, in the FM metallic phase the magnetization orientation is determined by the strain induced magnetostriction. On the contrary, in the AF phase the preferential orbital occupation always leads to an out-of-plane spin orientation, irrespective of the strain conditions (Fig. 6). Therefore, in the phase separated state the exchange bias between the AF and FM regions can be considered negligible and the spin alignment decoupled in the case of LSMO films on STO. However, we can guess that the FM/AF exchange-bias coupling is very small also in the case of LAO, because the FMI phase is supposed to be in-plane oriented, thus orthogonal to both the metallic FM and the AF phases. In addition, despite the same spin orientation of the AF and the metallic FM phases and the higher coercitive fields, the presence of a relevant exchange bias should have induced an FM hysteresis loop shift.

V. CONCLUSION
We have experimentally determined the microscopic origin of magnetic anisotropy in phase separated LSMO thin films. The interfacial e g (3z 2 − r 2 ) orbital occupation favors the the C-type AF spin ordering. Thus, the easy-axis of the AF phase is preferentially oriented perpendicularly to the ab-plane for all the substrates, whatever the sign and the strength of the mismatch. On the contrary, in the FM phase the in-plane orbital magnetic moment is partially unquenched when the FMI content becomes relevant. In this case, the tetragonal distortion and the e g -band width narrowing relax the quenching of the orbital moment, giving rise to an effective spin-orbit coupling. This demonstrates that, in the magnetic coexisting phases, the spin-orbit to lattice coupling properties are different and magnetic anisotropy is quite independent.