Heterostructured material systems devoid of ferroic components are presumed not to display ordering associated with ferroelectricity. In heterostructures composed of transition metal oxides, however, ...the disruption introduced by an interface can affect the balance of the competing interactions among electronic spins, charges and orbitals. This has led to the emergence of properties absent in the original building blocks of a heterostructure, including metallicity, magnetism and superconductivity. Here we report the discovery of ferroelectricity in artificial tri-layer superlattices consisting solely of non-ferroelectric NdMnO(3)/SrMnO(3)/LaMnO(3) layers. Ferroelectricity was observed below 40 K exhibiting strong tunability by superlattice periodicity. Furthermore, magnetoelectric coupling resulted in 150% magnetic modulation of the polarization. Density functional calculations indicate that broken space inversion symmetry and mixed valency, because of cationic asymmetry and interfacial polar discontinuity, respectively, give rise to the observed behaviour. Our results demonstrate the engineering of asymmetric layered structures with emergent ferroelectric and magnetic field tunable functions distinct from that of normal devices, for which the components are typically ferroelectrics.
Recently a metallic state was discovered at the interface between insulating oxides, most notably LaAlO 3 and SrTiO 3 . Properties of this two-dimensional electron gas (2DEG) have attracted ...significant interest due to its potential applications in nanoelectronics. Control over this carrier density and mobility of the 2DEG is essential for applications of these unique systems, and may be achieved by epitaxial strain. However, despite the rich nature of strain effects on oxide materials properties, such as ferroelectricity, magnetism, and superconductivity, the relationship between the strain and electrical properties of the 2DEG at the LaAlO 3 /SrTiO 3 heterointerface remains largely unexplored. Here, we use different lattice constant single-crystal substrates to produce LaAlO 3 /SrTiO 3 interfaces with controlled levels of biaxial epitaxial strain. We have found that tensile-strained SrTiO 3 destroys the conducting 2DEG, while compressively strained SrTiO 3 retains the 2DEG, but with a carrier concentration reduced in comparison to the unstrained LaAlO 3 /SrTiO 3 interface. We have also found that the critical LaAlO 3 overlayer thickness for 2DEG formation increases with SrTiO 3 compressive strain. Our first-principles calculations suggest that a strain-induced electric polarization in the SrTiO 3 layer is responsible for this behavior. The polarization is directed away from the interface and hence creates a negative polarization charge opposing that of the polar LaAlO 3 layer. This behavior both increases the critical thickness of the LaAlO 3 layer, and reduces carrier concentration above the critical thickness, in agreement with our experimental results. Our findings suggest that epitaxial strain can be used to tailor 2DEGs properties of the LaAlO 3 /SrTiO 3 heterointerface.
Ballistic anisotropic magnetoresistance VELEV, J; SABIRIANOV, R. F; JASWAL, S. S ...
Physical review letters,
04/2005, Letnik:
94, Številka:
12
Journal Article
Recenzirano
Odprti dostop
Electronic transport in ferromagnetic ballistic conductors is predicted to exhibit ballistic anisotropic magnetoresistance-a change in the ballistic conductance with the direction of magnetization. ...This phenomenon originates from the effect of the spin-orbit interaction on the electronic band structure which leads to a change in the number of bands crossing the Fermi energy when the magnetization direction changes. We illustrate the significance of this phenomenon by performing ab initio calculations of the ballistic conductance in ferromagnetic Ni and Fe nanowires which display a sizable ballistic anisotropic magnetoresistance when magnetization changes direction from parallel to perpendicular to the wire axis.
Localized impurity or defect states in the insulating barrier layer separating two ferromagnetic films affect dramatically the interlayer exchange coupling (IEC), making it significantly stronger ...compared to perfect barriers. We demonstrate that the impurity-assisted IEC becomes antiferromagnetic if the energy of the impurity states matches the Fermi energy and that the coupling strength decreases with temperature. These results explain available experimental data on the IEC across tunnel barriers.
In the past few years the phenomenon of spin-dependent tunneling (SDT) in magnetic tunnel junctions (MTJs) has aroused enormous interest and has developed into a vigorous field of research. The large ...tunneling magnetoresistance (TMR) observed in MTJs garnered much attention due to possible application in random access memories and magnetic field sensors. This led to a number of fundamental questions regarding the phenomenon of SDT. One such question is the role of interfaces in MTJs and their effect on the spin polarization of the tunneling current and TMR. In this paper we consider different models which suggest that the spin polarization is primarily determined by the electronic and atomic structure of the ferromagnet/insulator interfaces rather than by their bulk properties. First, we consider a simple tight-binding model which demonstrates that the existence of interface states and their contribution to the tunneling current depend on the degree of hybridization between the orbitals on metal and insulator atoms. The decisive role of the interfaces is further supported by studies of spin-dependent tunneling within realistic first-principles models of Co/vacuum/Al, Co/Al
2O
3/Co, Fe/MgO/Fe, and Co/SrTiO
3/Co MTJs. We find that variations in the atomic potentials and bonding strength near the interfaces have a profound effect resulting in the formation of interface resonant states, which dramatically affect the spin polarization and TMR. The strong sensitivity of the tunneling spin polarization and TMR to the interface atomic and electronic structure dramatically expands the possibilities for engineering optimal MTJ properties for device applications.
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