We develop an understanding of the anomalous metal state of the parent compounds of recently discovered iron-based superconductors starting from a strong-coupling viewpoint, including orbital degrees of freedom. On the basis of an intermediate-spin (S =1) state for the Fe2+ ions, we derive a Kugel-Khomskii spin-orbital Hamiltonian for the active t2g orbitals. It turns out to be a highly complex model with frustrated spin and orbital interactions. We compute its classical phase diagrams and provide an understanding for the stability of the various phases by investigating its spin-only and orbital-only limits. The experimentally observed spinstripe state is found to be stable over a wide regime of physical parameters and can be accompanied by three different types of orbital orders. Of these the orbital-ferro and orbital-stripe orders are particularly interesting since they break the in-plane lattice symmetry-a robust feature of the undoped compounds. We compute the magnetic excitation spectra for the effective spin Hamiltonian, observing a strong reduction in the ordered moment, and point out that the proposed orbital ordering pattern can be measured in resonant x-ray diffraction.

We show that charge ordered rare-earth nickelates of the type RNiO3 (R = Ho, Lu, Pr and Nd) are multiferroic with very large magnetically-induced ferroelectric (FE) polarizations. This we determine from first principles electronic structure calculations. The emerging FE polarization is directly tied to the long-standing puzzle of which kind of magnetic ordering is present in this class of materials: its direction and size indicate the type of ground-state spin configuration that is realized. Vice versa, the small energy differences between the different magnetic orderings suggest that a chosen magnetic ordering can be stabilized by cooling the system in the presence of an electric field.

We put forward double perovskites of the R2NiMnO6 family (with R a rare-earth atom) as a new class of multiferroics on the basis of ab initio density functional calculations. We show that changing R from La to Y drives the ground-state from ferromagnetic to antiferromagnetic with ↑↑↓↓ spin patterns. This E * -type ordering breaks inversion symmetry and generates a ferroelectric polarization of few µC/cm 2 . By analyzing a model Hamiltonian we understand the microscopic origin of this transition and show that an external electric field can be used to tune the transition, thus allowing electrical control of the magnetization.PACS numbers: 71.45. Gm, 71.10.Ca, Materials with simultaneous magnetic and ferroelectric ordering -multiferroics -are attracting enormous scientific interest due to their potential for applications in memory and data storage devices [1,2]. Multiferroics with magnetically induced ferroelectric order are particularly interesting due to their strong magneto-electric coupling, which is required for an electric (magnetic) control of magnetic (electric) order parameter: a very desirable property from a technological point of view [3]. Therefore, the search for new multiferroics with the ferroelectric order driven by the magnetic order is currently a very active and important field of research.In this letter, we show, by using first-principles density functional theory (DFT) calculations and a model Hamiltonian analysis, that the magnetic order in the double perovskite compounds R 2 NiMnO 6 (RNMO) changes from ferromagnetic for R=La and Sm to the E * -type for R=Y. The E * -type magnetic structure consists of ↑-↑-↓-↓ spin chains along the cubic perovskite-like directions (see Fig. 3), or equivalently, zig-zag ferromagnetic (FM) spin-chains antiferromagentically coupled in-plane to the neighboring zig-zag chain, with an out-of-plane FM coupling [4]. E * -type magnetism breaks inversion symmetry and thus allows a ferroelectric polarization to occur. We will show that indeed Y 2 NiMnO 6 is multiferroic with a electric polarization of few µC/cm 2 . We will show that vice versa the magnetic transition from ferro to E * -type can be tuned by an external electric field, thus allowing electric control of the ferromagnetic order parameter.The double perovskite La 2 NiMnO 6 (LNMO) is a ferromagnetic insulator with a Curie temperature close to room temperature (T c ∼ 280 K). The structure of LNMO changes from rhombohedral (R3) at high temperature, to monoclinic with P2 1 /n symmetry group at low temperature [5,6]. DFT has been successful in understanding the ferromagnetic (FM) insulating behaviour and the dielectric anomalies observed experimentally in LNMO [7,8]. We work in the framework of density-functional theory, using the Vienna ab initio simulation package [9], in which the Kohn-Sham equations are computed using the projector augmented wave method (PAW) and solved describing electronic exchange and correlation in the generalized gradient approximation (PBE) [10,11].The spin resolved density of states (D...