M. Avignon

Effect of cationic disorder on the spin polarization in FeMo double perovskites

Ordered Sr2FeMoO6 is expected to have complete spin polarization, however all samples usually present some degree of Fe/Mo disorder which reduces the tunneling magnetoresistance in granular samples. It can be inferred that disorder is detrimental to the half-metallicity. We present an electronic approach of this disorder using a model based on a correlated electron picture with localized Fe-spins and conduction electrons interacting with the local spins via a double-exchange–type mechanism. This succeeds in stabilizing a ferromagnetic ground state in the absence of disorder. Disorder is treated within the dynamical mean-field approach which is equivalent to the coherent potential approximation. We shall show how electron disorder affects the density of states and the ground-state properties with a particular emphasis on the spin polarization.

Formation of spin-polarons in the ferromagnetic Kondo lattice model away from half-filling

Even though realistic one-dimensional experiments in the field of half-metallic semiconductors are not at hand yet, we are interested in the underlying fundamental physics. In this regard we study a one-dimensional ferromagnetic Kondo lattice model, a model in which a conduction band is coupled ferromagnetically to a background of localized d moments with coupling constant J(H), and investigate the T = 0 phase diagram as a function of the antiferromagnetic interaction J between the localized moments and the band-filling n, since it has been observed that doping of the compounds has led to formation of magnetic domains. We explore the spin-polaron formation by looking at the nearest-neighbour correlation functions in the spin and charge regimes for which we use the density matrix renormalization group method, which is a highly efficient method to investigate quasi-one-dimensional strongly correlated systems.

The effect of Coulomb interaction at ferromagnetic-paramagnetic metallic perovskite junctions.

We study the effect of Coulomb interactions in transition metal oxide junctions. In this paper we analyze charge transfer at the interface of a three layer ferromagnetic-paramagnetic-ferromagnetic metallic oxide system. We choose a charge model considering a few atomic planes within each layer and obtain results for the magnetic coupling between the ferromagnetic layers. For large numbers of planes in the paramagnetic spacer we find that the coupling oscillates with the same period as in Ruderman-Kittel-Kasuya-Yoshida (RKKY) theory but the amplitude is sensitive to the Coulomb energy. At small spacer thickness however, large differences may appear as a function of the number of electrons per atom in the ferromagnetic and paramagnetic materials, the dielectric constant at each component, and the charge defects at the interface plane, emphasizing the effects of charge transfer.

Magneto-elastic phase transitions in one-dimensional systems.

The magneto-elastic phase diagram in one-dimensional systems relating to the interplay between magnetism and lattice distortion is studied in a double-exchange and super-exchange model considering classical localized spins and the limit of large Hunds coupling. At low super-exchange interaction energy, a phase transition occurs between electron-full ferromagnetic distorted and electron-empty antiferromagnetic undistorted phases via phase separation. In this case, all electrons and lattice distortions are found within the ferromagnetic domain. For higher super-exchange interaction energy, phase separations consisting of two-xa0or three-site distorted independent magnetic polarons separated by electron-empty undistorted antiferromagnetic links are obtained. In this regime, each polaron contains an electron, leading to a Wigner crystallization. The lattice distortion and charge distribution inside the polarons are also calculated.

Theoretical calculations of valence states in Fe-Mo compounds

The half-metallic ferromagnetic double perovskite compound Sr2FeMoO6 is considered as an important material for spintronic applications. It appears to be fundamental to understand the role of electronic parameters controlling the half-metallic ground state. Fe-Mo double perovskites usually present some degree of Fe/Mo disorder which generally increases with doping. In this work, we study the valence states of Fe-Mo cations in the off-stoichiometric system Sr2Fe1+xMo1−xO6 (−1 ≤ x ≤ 1/3) with disorder. Our results for Fe and Mo valence states are obtained using the Green functions and the renormalization perturbation expansion method. The model is based on a correlated electron picture with localized Fe-spins and conduction Mo-electrons interacting with the local spins via a double-exchange-type mechanism.

Theoretical study of the double perovskite Sr2FeMO6Sr2FeMO6(M=Mo,W)(M=Mo,W)

We study the double perovskite Sr2FeMO6Sr2FeMO6 using a tight-binding model with the renormalized perturbation expansion technique. An analysis of the antiferromagnetic (AF) phase shows that the electronic energy remains larger than in the ferromagnetic (F) state but the difference decreases with increasing charge transfer energy. Therefore, with increasing Fe–M charge transfer energy, the transition to the AF state can be easily driven by the superexchange interaction. The F–AF transition in the system Sr2FeMoxW1-xO6Sr2FeMoxW1-xO6 vs. doping is also presented.

Theoretical study of the double perovskite

We study the double perovskite Sr2FeMO6Sr2FeMO6 using a tight-binding model with the renormalized perturbation expansion technique. An analysis of the antiferromagnetic (AF) phase shows that the electronic energy remains larger than in the ferromagnetic (F) state but the difference decreases with increasing charge transfer energy. Therefore, with increasing Fe–M charge transfer energy, the transition to the AF state can be easily driven by the superexchange interaction. The F–AF transition in the system Sr2FeMoxW1-xO6Sr2FeMoxW1-xO6 vs. doping is also presented.

Theoretical study of the double perovskite Sr2FeMO6(M=Mo,W)

We study the double perovskite Sr2FeMO6Sr2FeMO6 using a tight-binding model with the renormalized perturbation expansion technique. An analysis of the antiferromagnetic (AF) phase shows that the electronic energy remains larger than in the ferromagnetic (F) state but the difference decreases with increasing charge transfer energy. Therefore, with increasing Fe–M charge transfer energy, the transition to the AF state can be easily driven by the superexchange interaction. The F–AF transition in the system Sr2FeMoxW1-xO6Sr2FeMoxW1-xO6 vs. doping is also presented.