We report a first-principles study of ͑LaAlO 3 ͒ m / ͑SrTiO 3 ͒ n heterostructures using density-functional theory at the LDA+ U level. Our results support the original explanation of Ohtomo and Hwang ͓Nature ͑London͒ 427, 423 ͑2004͔͒ that the charge at the n-type interface may be due to electrostatic doping. The internal electric field in the LaAlO 3 layer is calculated to be 0.24 V / Å. Though it is not sufficient to cause the dielectric breakdown in a wide band-gap La aluminate, it causes charge transfer into the adjacent narrower gap SrTiO 3 layer. The quasi-two-dimensional nature of the charge distribution is caused by a combination of the crystal-field effect, pseudo-Jahn-Teller distortion, and interface chemistry. Our theoretical estimate suggests that the interfacial carrier density of about 2 ϫ 10 13 cm −2 can be easily achieved.

Ferromagnet-ferroelectric-metal superlattices are proposed to realize the large room-temperature magnetoelectric effect. Spin dependent electron screening is the fundamental mechanism at the microscopic level. We also predict an electric control of magnetization in this structure. The naturally broken inversion symmetry in our tri-component structure introduces a magnetoelectric coupling energy of P M 2 . Such a magnetoelectric coupling effect is general in ferromagnet-ferroelectric heterostructures, independent of particular chemical or physical bonding, and will play an important role in the field of multiferroics.Ferroelectricity and ferromagnetism are important in many technological applications and the quest for multiferroic materials, where these two phenomena are intimately coupled, is of significant technological and fundamental interest [1,2,3,4,5,6]. In general, ferroelectricity and ferromagnetism tend to be mutually exclusive or interact weakly with each other when they coexist in a single-phase material [2]. Increasing the spin-orbit interaction of the electrons [7] or strategically designing for magnetic and electric phase control [6,8] may enhance the magnetoelectric (ME) coupling effect in a single phase multiferroic material. Practical applications of the ME effect, however, remain hindered by the small electric polarization and low Curie temperature [3,4,5].Artificial composites of ferroic materials may enable the room-temperature ME effect, since both large and robust electric and magnetic polarizations can persist to room temperature. Two types of ME coupling at a ferromagnet(FM)-dielectric interface have been reported, one employing the mechanical interaction [9,10] or chemical bonding [11] and the other mediation by carriers (screening charges) [12,13,14,15]. The role of electrostatic screening in ferroelectric capacitors has been studied by macroscopic models [16]. Recently, ab-initio studies of nanoscale ferroelectric(FE) capacitors [17,18] and FE tunnel junctions [19,20,21] have further confirmed that electrostatic screening is the fundamental mechanism at the FE/normal metal (NM) interface. In this letter we propose a strategy of achieving robust ME coupling in a tri-component FM/FE/NM superlattice. The additional magnetization, caused by spin-dependent screening [12,13], will accumulate at each FM/FE interface. Due to the broken inversion symmetry between the FM/FE and NM/FE interfaces, there would be a net additional magnetization in each FM/FE/NM unit cell, unlike the symmetric structures discussed in the previous work. The addition of magnetization in this superlattice will result in a large global magnetization.The tri-component superlattice is illustrated in Fig.1 (a). When the FE layer is polarized, surface charges are created. These bound charges are compensated by the... screening charge in both FM and NM electrodes. In the FM metal, the screening charges are spin polarized due to the ferromagnetic exchange interaction. The spin dependence of screening leads to additional magne...

Topotactic phase transformation enables structural transition without losing the crystalline symmetry of the parental phase and provides an effective platform for elucidating the redox reaction and oxygen diffusion within transition metal oxides. In addition, it enables tuning of the emergent physical properties of complex oxides, through strong interaction between the lattice and electronic degrees of freedom. In this communication, the electronic structure evolution of SrFeO epitaxial thin films is identified in real-time, during the progress of reversible topotactic phase transformation. Using real-time optical spectroscopy, the phase transition between the two structurally distinct phases (i.e., brownmillerite and perovskite) is quantitatively monitored, and a pressure-temperature phase diagram of the topotactic transformation is constructed for the first time. The transformation at relatively low temperatures is attributed to a markedly small difference in Gibbs free energy compared to the known similar class of materials to date. This study highlights the phase stability and reversibility of SrFeO thin films, which is highly relevant for energy and environmental applications exploiting the redox reactions.

Using first-principles density functional theory, we investigate the interfacial magnetoelectric coupling in a tri-component superlattice composed of a ferromagnetic metal (FM), ferroelectric (FE), and normal metal (NM). Using Fe/FE/Pt as a model system, we show that a net and cumulative interfacial magnetization is induced in the FM metal near the FM/FE interface. A carefully analysis of the magnetic moments in Fe reveals that the interfacial magnetization is a consequence of a complex interplay of interfacial charge transfer, chemical bonding, and spin dependent electrostatic screening. The last effect is linear in the FE polarization, is switchable upon its reversal, and yields a substantial interfacial magnetoelectric coupling.

Transition-metal oxides (TMOs) with brownmillerite (BM) structures possess one-dimensional oxygen vacancy channels (OVCs), which play a key role in realizing high ionic conduction at low temperatures. The controllability of the vacancy channel orientation, thus, possesses a great potential for practical applications and would provide a better visualization of the diffusion pathways of ions in TMOs. In this study, the orientations of the OVCs in BM-SrFeO are stabilized along two crystallographic directions of the epitaxial thin films. The distinctively orientated phases are found to be highly stable and exhibit a considerable difference in their electronic structures and optical properties, which could be understood in terms of orbital anisotropy. The control of the OVC orientation further leads to modifications in the hydrogenation of the BM-SrFeO thin films. The results demonstrate a strong correlation between crystallographic orientations, electronic structures, and ionic motion in the BM structure.