Limits...
Giant multiferroic effects in topological GeTe-Sb 2 Te 3 superlattices

View Article: PubMed Central - PubMed

ABSTRACT

Multiferroics, materials in which both magnetic and electric fields can induce each other, resulting in a magnetoelectric response, have been attracting increasing attention, although the induced magnetic susceptibility and dielectric constant are usually small and have typically been reported for low temperatures. The magnetoelectric response usually depends on d-electrons of transition metals. Here we report that in [(GeTe)2(Sb2Te3)l]m superlattice films (where l and m are integers) with topological phase transition, strong magnetoelectric response may be induced at temperatures above room temperature when the external fields are applied normal to the film surface. By ab initio computer simulations, it is revealed that the multiferroic properties are induced due to the breaking of spatial inversion symmetry when the p-electrons of Ge atoms change their bonding geometry from octahedral to tetrahedral. Finally, we demonstrate the existence in such structures of spin memory, which paves the way for a future hybrid device combining nonvolatile phase-change memory and magnetic spin memory.

No MeSH data available.


The band structure for the iPCM-RESET phase under an external electric field (0.51 V nm−1) at 0 K (upper left), the band structure details around the Γ point (lower left), and the corresponding spin-polarized density of states for Ge (upper right) and Te (lower right) atoms.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
getmorefigures.php?uid=PMC5036496&req=5

Figure 4: The band structure for the iPCM-RESET phase under an external electric field (0.51 V nm−1) at 0 K (upper left), the band structure details around the Γ point (lower left), and the corresponding spin-polarized density of states for Ge (upper right) and Te (lower right) atoms.

Mentions: When an external electric field of 0.001 electrostatic units (a.u.) (corresponding to 0.51 V nm−1) is applied normal to the iPCM film in a simulation including SOC, the Dirac cone is broken and a gap opens as shown in figure 4 (left). Because the conductivity of Sb2Te3 is higher than that of GeTe, the voltage drop is mainly across the GeTe block (∼0.9 nm). The total voltage applied to the structure with eight repeat units (in the experiment we used eight repeats; hence we also consider eight repeats in the simulation) becomes 3.7 V. This value is four times larger than that obtained experimentally (0.85 eV), but it should be kept in mind that even though the simulations were performed at 0 K, in the experiment the device contained a TiN heater rod, and an increase in temperature in the presence of a current is likely to facilitate the breaking of spatial inversion symmetry. In the foregoing simulation, the maximum energy difference, ΔEtvb, between the spin-up and spin-down bands of 0.07 eV was obtained at k ∼ 0.06 Å−1 (the M-point corresponds to k = 0.76 Å−1). In a different simulation (not shown) we found that a displacement of two Ge atoms from the stable position by 0.1 Å and 0.2 Å along the c-axis caused a splitting ΔEtvb between the spin-up and spin-down bands of 0.09 eV and 0.24 eV, respectively, whereas the band gap, ΔEgap, simultaneously expanded to 0.15 eV and 0.35 eV. These results support the assumption that the superlattice becomes more insulating through the breaking of spatial inversion. In the presence of the Rashba effect, the spin density of states (SDOS) for two opposite spin orientations is no longer degenerate and originates from Ge p-electrons (figure 4 (right)).


Giant multiferroic effects in topological GeTe-Sb 2 Te 3 superlattices
The band structure for the iPCM-RESET phase under an external electric field (0.51 V nm−1) at 0 K (upper left), the band structure details around the Γ point (lower left), and the corresponding spin-polarized density of states for Ge (upper right) and Te (lower right) atoms.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC5036496&req=5

Figure 4: The band structure for the iPCM-RESET phase under an external electric field (0.51 V nm−1) at 0 K (upper left), the band structure details around the Γ point (lower left), and the corresponding spin-polarized density of states for Ge (upper right) and Te (lower right) atoms.
Mentions: When an external electric field of 0.001 electrostatic units (a.u.) (corresponding to 0.51 V nm−1) is applied normal to the iPCM film in a simulation including SOC, the Dirac cone is broken and a gap opens as shown in figure 4 (left). Because the conductivity of Sb2Te3 is higher than that of GeTe, the voltage drop is mainly across the GeTe block (∼0.9 nm). The total voltage applied to the structure with eight repeat units (in the experiment we used eight repeats; hence we also consider eight repeats in the simulation) becomes 3.7 V. This value is four times larger than that obtained experimentally (0.85 eV), but it should be kept in mind that even though the simulations were performed at 0 K, in the experiment the device contained a TiN heater rod, and an increase in temperature in the presence of a current is likely to facilitate the breaking of spatial inversion symmetry. In the foregoing simulation, the maximum energy difference, ΔEtvb, between the spin-up and spin-down bands of 0.07 eV was obtained at k ∼ 0.06 Å−1 (the M-point corresponds to k = 0.76 Å−1). In a different simulation (not shown) we found that a displacement of two Ge atoms from the stable position by 0.1 Å and 0.2 Å along the c-axis caused a splitting ΔEtvb between the spin-up and spin-down bands of 0.09 eV and 0.24 eV, respectively, whereas the band gap, ΔEgap, simultaneously expanded to 0.15 eV and 0.35 eV. These results support the assumption that the superlattice becomes more insulating through the breaking of spatial inversion. In the presence of the Rashba effect, the spin density of states (SDOS) for two opposite spin orientations is no longer degenerate and originates from Ge p-electrons (figure 4 (right)).

View Article: PubMed Central - PubMed

ABSTRACT

Multiferroics, materials in which both magnetic and electric fields can induce each other, resulting in a magnetoelectric response, have been attracting increasing attention, although the induced magnetic susceptibility and dielectric constant are usually small and have typically been reported for low temperatures. The magnetoelectric response usually depends on d-electrons of transition metals. Here we report that in [(GeTe)2(Sb2Te3)l]m superlattice films (where l and m are integers) with topological phase transition, strong magnetoelectric response may be induced at temperatures above room temperature when the external fields are applied normal to the film surface. By ab initio computer simulations, it is revealed that the multiferroic properties are induced due to the breaking of spatial inversion symmetry when the p-electrons of Ge atoms change their bonding geometry from octahedral to tetrahedral. Finally, we demonstrate the existence in such structures of spin memory, which paves the way for a future hybrid device combining nonvolatile phase-change memory and magnetic spin memory.

No MeSH data available.