Meng-Qiu Cai

Magnetoelectric effect and critical thickness for ferroelectricity in Co/BaTiO3/Co multiferroic tunnel junctions

Based on first-principles calculations, the magnetoelectric effect and the critical thickness for ferroelectricity in Co/BaTiO3/Co multiferroic tunnel junctions were investigated. The calculated results show that the ferroelectricity of multiferroic tunnel junctions can be maintained until a critical thickness of 1.6 nm. The magnetic moments of interface atoms are sensitive to the ferroelectric displacements, which leads to a sizable magnetoeletric effect in multiferroic tunnel junctions. The multiferroic tunnel junctions may opens an avenue for the development of novel electronic devices.

Uniaxial strain-modulated conductivity in manganite superlattice (LaMnO3/SrMnO3)

We have investigated the magnetic ordering and electrical conductivity transitions of (LaMnO3)2/(SrMnO3)2 superlattices grown on SrTiO3 substrate based on density-functional theory. It is found that the uniaxial tensile strain along the z axis of about 1.4% induced a magnetic transition from antiferromagnetic to ferromagnetic ordering. At the interface the orbital order changes from a combination of x2−y2 and 3z2−r2 to x2−y2 as strain becomes more compressive; as a result the electrical transport is transformed from three-dimensions to two-dimensions at the high uniaxial compressive strain. Our results suggest that the out-of-plane electrical conductivity can be modulated and controlled by uniaxial strain.

Phase diagram of ferroelectric nanowires and its mechanical force controllability

A phenomenological thermodynamic model of ferroelectric nanowires is developed to investigate the size-temperature phase diagrams. Considering existence of the surface tension, size, and external applied stress effects, the approximated expressions of the transition temperatures at which the paraelectric phase loses its stability with respect to the appearance of the spontaneous polarization are derived. At the same time, the size-temperature phase diagrams as functions of the ferroelectric nanowire radius are obtained, and also show its external mechanical stress controllability.

First-principles study of the critical thickness in asymmetric ferroelectric tunnel junctions

The absent critical thickness of fully relaxed asymmetric ferroelectric tunnel junctions is investigated by first-principles calculations. The results show that PbTiO3 thin film between Pt and SrRuO3 electrodes can still retain a significant and stable polarization down to thicknesses as small as 0.8 nm, quite unlike the case of symmetric ferroelectric tunnel junctions. We trace this surprising result to the generation of a large electric field by the charge transfer between the electrodes caused by their different electronic environments, which acts against the depolarization field and enhances the ferroelectricity, leading to the reduction, or even complete elimination, for the critical thickness.

Anisotropy of the electrical transport properties in a Ni2MnGa single crystal: Experiment and theory

Electrical transport properties in ferromagnetic shape memory Ni–Mn–Ga single crystal have been investigated both in experiment and theory by analyzing electrical resistivity along different crystallographic directions during heating. The experimental results show a clear first-order martensitic transformation and a large anisotropic resistivity (AR) of 23.7% at the tetragonal martensitic phase. The theoretical conductivity (σ=1/ρ), estimated using first-principles calculations combined with classical Boltzman transport theory, proves essential crystallographic anisotropic resistivity (AR=31%) in the martensitic phase and agrees well with experimental results. The AR in the martensitic phase is reveled to mainly originate from the splitting of the minority-spin Niu20023d and Gau20024p states near the Fermi level and hence reconstruction of the minority-spin Fermi surface upon martensitic transformation.