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Study of half-metallicity in LSMO

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					                                      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 [7].
tunnel magnetoresistance junction of LSMO/SrTiO3/LSMO                            We modeled cubic LSMO with a rigid band model and
shows magnetoresistance in excess of 1500% [1]. 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 [4]. 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 [5]. 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 [6].                    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 [8]. 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                                          [1] M. Bowen, private communication.
                                                                                              [2] W.E.Picket, D.J. Singh, JMMM 172, 237 (1997).
                                                                                              [3] E.A. Livesay et al., J. Phys. Cond. Matt. 11, L2711 (1999).
                                                 100
                                                                                  (b)         [4] B. Nadgorny et al., Phys.Rev. B 63, 184433 (2001).
                                                                                              [5] Y. Tokura, Y. Tamioka, JMMM 200, 1 (1999).
                                                   0                                          [6] 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.
                                                                                              [7] R. Tyer et al., cond-mat/0303602.
                                                   0
                                                                                              [8] 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.

				
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Description: Study of half-metallicity in LSMO