A surface magnetoelectric effect is revealed by density-functional calculations that are applied to ferromagnetic Fe(001), Ni(001), and Co(0001) films in the presence of an external electric field. The effect originates from spin-dependent screening of the electric field which leads to notable changes in the surface magnetization and the surface magnetocrystalline anisotropy. These results are of considerable interest in the area of electrically controlled magnetism and magnetoelectric phenomena.

The interplay between the electron transport in metal/ferroelectric/metal junctions with ultrathin ferroelectric barriers and the polarization state of a barrier is investigated. Using a model which takes into account screening of polarization charges in metallic electrodes and direct quantum tunneling across a ferroelectric barrier we calculate the change in the tunneling conductance associated with the polarization switching. We find the conductance change of a few orders of magnitude for metallic electrodes with significantly different screening lengths. This giant electroresistance effect is the consequence of a different potential profile seen by transport electrons for the two opposite polarization orientations. 73.40.Gk, 77.80.Fm, 85.50.Gk In recent years, ferroelectric materials have attracted significant interest because of their promising potential in various technological applications.1,2 For example, due to their spontaneous dielectric polarization that can be switched by an applied electric field, ferroelectrics can be used as binary data storage media in nonvolatile random access memories. Recent experimental and theoretical findings suggest that ferroelectricity persists down to vanishingly small sizes, which opens a possibility to further miniaturize electronic devices based on ferroelectric materials.3 In particular, it was discovered that, in organic ferroelectrics, ferroelectricity can be sustained in thin films of a few monolayer thickness. 4 In perovskite ferroelectric oxides, ferroelectricity was observed down to a nanometer scale. 5This fact is consistent with first-principle calculations that predict a nanometer critical thickness for a perovskite ferroelectric film sandwiched between two metals. 6 The existence of ferroelectricity at such a small film thickness makes it possible to use ferroelectrics as tunnel barriers in metal/ferroelectric/metal (M/FE/M) junctions. Recent experiments indicate that the electrical resistance in M/FE/M junctions with ultrathin barriers depends on the orientation of the dielectric polarization which can be switched by an applied electric field. 7 The origin of this electroresistance effect is not completely understood and to the best of our knowledge no modeling of this phenomenon has been performed.In this Letter, using a simple model for an ultrathin ferroelectric (FE) barrier separating two different metal electrodes (M 1 and M 2 ), we investigate the electroresistance effect in ferroelectric (M 1 /FE/M 2 ) tunnel junctions. We show that the reversal of the dielectric polarization in the ferroelectric produces a change in the electrostatic potential profile across the junction. This leads to the resistance change which can reach a few orders of magnitude for metal electrodes with significantly different screening lengths. We designate this phenomenon as the giant electroresistance (GER) effect.The physical mechanism which is responsible for the GER in ferroelectric tunnel junctions (FTJs) is the change of the electrostatic potential profile ϕ(z) ind...

Exploiting the valley degree of freedom to store and manipulate information provides a novel paradigm for future electronics. A monolayer transition-metal dichalcogenide (TMDC) with a broken inversion symmetry possesses two degenerate yet inequivalent valleys, which offers unique opportunities for valley control through the helicity of light. Lifting the valley degeneracy by Zeeman splitting has been demonstrated recently, which may enable valley control by a magnetic field. However, the realized valley splitting is modest (∼0.2 meV T). Here we show greatly enhanced valley spitting in monolayer WSe, utilizing the interfacial magnetic exchange field (MEF) from a ferromagnetic EuS substrate. A valley splitting of 2.5 meV is demonstrated at 1 T by magnetoreflectance measurements and corresponds to an effective exchange field of ∼12 T. Moreover, the splitting follows the magnetization of EuS, a hallmark of the MEF. Utilizing the MEF of a magnetic insulator can induce magnetic order and valley and spin polarization in TMDCs, which may enable valleytronic and quantum-computing applications.