All-manganite heterostructures

In the context of strongly correlated oxide heterostructures, we have studied the electronic reconstruction in multilayers in which all components are manganites of different properties. Manganites are specially interesting because they present a rich phase diagram due to the close competition of different interactions. Our general model includes the kinetic energy (through the double exchange interaction that results of considering an infinite Hund's coupling), the antiferromagnetic superexchange, the cooperative Jahn-Teller interaction, and the long range Coulomb interaction between the charges in the system (within a Hartree approximation). The model is solved self-consistently.
  1. Magnetoresistance in an all-manganite spin-valve.
    J. Salafranca, M.J. Calderón, and L. Brey.
    Phys. Rev. B 77, 014441 (2008). arXiv:0709.3720.
    2. Manganites present a great variety of phases as a function of composition and doping. Those in the ferromagnetic phase are good candidates for building heterostructures with large tunneling magnetoresistance due to their high degree of spin polarization (some manganites are half metals). We concentrate here in investigating theoretically an all-manganite trilayer with a thin antiferromagnetic and insulating film sandwiched between two ferromagnetic and metallic layers, and study the nanoscale magnetic structure produced at the interfaces, and the conductance through the system. An example of such a heterostructure is La2/3Sr1/3MnO3 / Pr2/3Ca1/3MnO3 / La2/3Sr1/3MnO3. The LSMO layers are in the ferromagnetic, metallic double exchange phase while PCMO is antiferromagnetic of the CE type and insulating in the bulk (see Figure). The Hamiltonian, which involves double exchange, a Hubbard term to avoid double occupancy, electron-electron interaction, and an antiferromagnetic superexchange term that effectively includes the lattice contribution to the energy, is solved self-consistently for different possible spin configurations at the interfaces.
    trilayer
  2. Electron gas at the interface between two antiferromagnetic insulating manganites.
    M.J. Calderón, J. Salafranca, and L. Brey.
    Phys. Rev. B 77, 024415 (2008). arXiv:0804.3454.
    3. We study theoretically the magnetic and electric properties of the interface between two antiferromagnetic and insulating manganites: La0.5Ca0.5MnO3, a strong correlated insulator, and CaMnO3, a band insulator. We find that a ferromagnetic and metallic electron gas is formed at the interface between the two layers. We confirm the metallic character of the interface by calculating the in-plane conductance. The possibility of increasing the electron-gas density by selective doping is also discussed.

  3. All-Manganite Tunnel Junctions with Interface-Induced Barrier Magnetism.
    Z. Sefrioui, C. Visani, M.J. Calderón, K. March, C. Carrétéro, M. Walls, A. Rivera-Calzada, C. León, R. López Antón, T. R. Charlton, D. Imhoff, L. Brey, M. Bibes, J.Santamaría, and A. Barthélémy. Advanced Materials 22, 5029 (2010).
    The recent discovery of unexpected phases at oxide interfaces provides new insights into the physics of strongly correlated electron systems. The possibility of tailoring the electronic structure of such interfaces has triggered a great technological drive to functionalize them into devices. Here, we report the creation of an insulating phase with a signifi cant interfaceinduced magnetic moment and its use as an active barrier in magnetic tunnel junctions. This phase appears at the interface between two manganese perovskite oxides, one being ferromagnetic-metallic (FM-M) and the other antiferromagneticinsulating (AFM-I), and mimics a spin-fi lter behavior. Our results show that uncompensated moments at engineered magnetic interfaces may be a promising route to generate spin-polarized currents, circumventing the issues of spin-fi lter (SF) materials scarcity and offering new opportunities for antiferromagnets – by far the most numerous high temperature magnetic materials – in spintronics.

  4. Effect of strain on the orbital and magnetic ordering of manganite thin films and their interface with an insulator.
    A. Baena, L. Brey, and M.J. Calderón. arXiv:1009.4548.
    We study the effect of uniform uniaxial strain on the ground state electronic configuration of a thin film manganite. Our model Hamiltonian includes the double-exchange, the Jahn-Teller electron-lattice coupling, and the antiferromagnetic superexchange. The strain arises due to the lattice mismatch between an insulating substrate and a manganite which produces a tetragonal distortion. This is included in the model via a modification of the hopping amplitude and the introduction of an energy splitting between the Mn eg levels. We analyze the bulk properties of half-doped manganites and the electronic reconstruction at the interface between a ferromagnetic and metallic manganite and the insulating substrate. The strain drives an orbital selection modifying the electronic properties and the magnetic ordering of manganites and their interfaces.