Study of half-metallicity in LSMO G. Banacha,b,*, R. Tyera and W.M. Temmermana a Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK b Institute of Low Temperature and Structure Research, Polish Academy of Science, Wroclaw , Poland Elsevier use only: Received date here; revised date here; accepted date here Abstract Self-interaction corrected local spin density approximation calculations were performed for La(1-x)SrxMnO3 (LSMO) (x=0.7). A half-metallic state was obtained for LSMO with manganese configuration Mn+3, whilst Mn+4 gave rise to a metallic state with a negligible spin polarization at the Fermi level. Key words: Half metals, LSMO PACS: 75.47.Lx The half-metallic properties of La(1-x)SrxMnO3 (LSMO) valence band. This method was successfully applied to the are of great importance for applications in spintronics. The study of orbital order in LaMnO3 . tunnel magnetoresistance junction of LSMO/SrTiO3/LSMO We modeled cubic LSMO with a rigid band model and shows magnetoresistance in excess of 1500% . The also using a supercell of La4Sr2Mn6O18. In the rigid band electronic properties of LSMO, as described by band model the change in band filling (reduction by 0.3 theory, are nearly half-metallic [2, 3], reflecting the so- electrons) and the change in lattice constant are the only called transport half-metallic behavior . However the variables which describe the change of cubic LaMnO3 to fascinating electronic and magnetic properties of LSMO, LSMO. In the supercell approach we can also incorporate including colossal magnetoresistance (CMR), indicate that into the calculations charge ordering effects such as Mn+4 the electronic structure is more complex than the standard occurring in the vicinity of Sr and Mn+3 staying around La band theory picture . In particular, the electronic sites. structure is determined by the competition of double We find Mn+4 configuration to be the ground state in exchange and superexchange interactions, charge/orbital SrMnO3. The energy difference between Mn+4 and Mn+3 ordering instabilities and strong coupling with the lattice configurations decreases from 85 mRy to 70 mRy when deformations. increasing the lattice constant from 7.2 a.u. (corresponding In this work we discuss issues concerning the charge to pseudocubic SrMnO3) to 7.323 a.u. (corresponding to ordering and more specifically the distribution of Mn3+ and pseudocubic La0.7Sr0.3MnO3). In cubic LaMnO3 the ground Mn+4 in LSMO. We use the first principles self-interaction state has changed to Mn+3 configuration: 15 mRy separate corrected local spin density (SIC-LSD) approximation . this ground state from the Mn+4 excited state. In This method can determine the number of valence band comparison, for the Jahn-Teller distorted LaMnO3 structure states. Hence it can differentiate between Mn contributing 3 we find 20 mRy. Reducing the lattice constant by 1.5% to states (Mn+3) to the valence band with the remaining 4 Mn the same lattice constant as for LSMO, we find that in this states (3t2g+ and 1eg) localised well below the valence band case LaMnO3 becomes nearly tetravalent and less than 5 or Mn contributing 4 states (Mn+4) to the valence band with mRy separate the Mn+3 ground state from the Mn+4 excited the remaining 3 Mn states (3t2g) localised below the state. * Corresponding author. Tel.: +44 1925 603153; fax: +44 1925 603172; e-mail: G.Banach@dl.ac.uk. 2 Submitted to Elsevier Science Replacing 30% of La by Sr in a rigid band In this system we have one MnO2 plane with Mn+4, approximation results in a change of the ground state of sandwiched between two SrO planes, and all other MnO2 Mn+3 to Mn+4: 30 mRy separate these two configurations. planes are occupied by Mn+3. This supercell can be Actually the crossover from a Mn+3 ground state to a Mn+4 considered as a model of phase separation in LSMO, ground state already occurs at 10% Sr doping. The spin similarly to La0.7Ca0.3MnO3 described by Bibes . In this magnetic moment increases only slightly in comparison case the system is half-metallic (Fig. 1). Energetically with the LSD results: Mn spin magnetic moment is 3.14 mB unfavorable, however, the energy difference between this in SIC-LSD and 3.03 mB in LSD and total spin magnetic state and the ground state is reduced to 120 mRy, in moments are 3.34 mB in SIC-LSD and 3.31 mB in LSD. This comparison to the energy difference between all Mn having ground state is metallic and its electronic structure in the the Mn+3 configuration and the ground state (195 mRy). vicinity of the Fermi level is similar to the LSD one, i.e., it The gap is reduced from 0.68eV (the scenario with the is also a nearly half-metallic system. Only in the Mn+3 configuration of Mn+3 for all manganese atoms) to 0.54 eV. configuration is a half-metal obtained. It has a pseudogap Note that this pseudogap is constant for all MnO2 layers, of 1.6 eV. Mixed valent Mn+4/Mn+3 systems were modeled independently of their valency. with supercells. The energy states of these configurations In conclusion, it is found that half-metallicity is were such that E(Mn+4) < E(Mn+4/Mn+3) < E(Mn+3) and associated with Mn+3 valency. None of the supercells had the energy increased linearly between the tetravalent and this valency as the ground state configuration. However trivalent configurations as a function of the percentage of one could speculate that maybe at the surface Mn+3 could Mn+3. Supercells were also used to model the influence of be the stable configuration since the lower coordination charge order: LanSrMn(n+1)O3(n+1) with n=2, …, 7 and would favor the more localised state. Our study also LanSr2Mn(n+2)O3(n+2) with n= 4, and La3Sr3Mn6O18. For all indicates that charge ordering could be important. The these systems the ground state was Mn+4 and metallic. In model of a simple charge order scenario gave rise to a half- figure 1 we show density of states (DOS) of one of these metallic state. More complex supercells, where more Sr systems: La4Sr2Mn6O18. layers could phase separate, could plausibly lead to a half- metallic ground state. 100 (a) References 0  M. Bowen, private communication.  W.E.Picket, D.J. Singh, JMMM 172, 237 (1997).  E.A. Livesay et al., J. Phys. Cond. Matt. 11, L2711 (1999). 100 (b)  B. Nadgorny et al., Phys.Rev. B 63, 184433 (2001).  Y. Tokura, Y. Tamioka, JMMM 200, 1 (1999). 0  W.M. Temmerman et al., in Electronic Density Functional Theory: Recent Progress and New Directions, J.F. Dobson, G. DOS [Ry/unit cell/ spin ] Vignale, and M.P. Das (eds.), Plenum Press, New York and 100 (c) London, 1998.  R. Tyer et al., cond-mat/0303602. 0  M. Bibes et al., Phys. Rev. Lett. 87, 067210 (2001). 100 (d) 0 100 -1.0 -0.5 E (Ry) 0.0 0.5 Fig.1 Local DOS for a supercell La4Sr2Mn6O18 (right). The configuration is (a) one MnO2 layer of Mn+4 sandwiched between the SrO layers (marked as black balls) and Mn+3 in all other MnO2 planes. Here (b) refers to SrO plane, (c) to MnO2 plane and (d) to LaO plane. The left- hand-side picture shows the structure.