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

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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.


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Temperature dependence of (a) the resistivity, (b) Hall mobility, and (c) carrier concentration of the [(GeTe)2(Sb2Te3)1]2 structure (red) and the control alloy with the same composition ratio (blue). The Hall properties of (a)–(c) were measured under a 5 kOe magnetic field.
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Figure 5: Temperature dependence of (a) the resistivity, (b) Hall mobility, and (c) carrier concentration of the [(GeTe)2(Sb2Te3)1]2 structure (red) and the control alloy with the same composition ratio (blue). The Hall properties of (a)–(c) were measured under a 5 kOe magnetic field.

Mentions: We further studied transport properties using Hall effect measurements on iPCM films. In this case the electric field is applied in plane, whereas the magnetic field (5 kOe) is applied normal to the plane. If the iPCM RESET phase does have a Dirac cone, two-dimensional metallic transport should be observed. Figure 5 shows the results obtained for an iPCM [(GeTe)2(Sb2Te3)1]2 film fabricated at 520 K with a control film of GST alloy fabricated with the same film thickness and at the same temperature. After the initial drop, the resistivity (ρxx) of the RESET phase increased to 7.8 × 10−4Ω cm at ∼420 K, at which point the SET phase of the iPCM film was established. Therefore, when the electric field is applied in plane, the RESET phase of the iPCM film behaves like a metal at temperatures between 350 K and 420 K. It should be noted that in the control GST alloy film with the same average composition, the resistivity monotonically decreased as expected for a semiconductor. At the same time, the Hall mobility steeply increased, reaching a value of ∼180 cm2 V-s−1 at ∼350 K, and then monotonically decreased, whereas the carrier concentration of 5.7 × 1019 cm−3 was essentially unchanged until the phase transition (apart from the initial drop). The results from the iPCM film are crucially different from the behavior of the alloy of the same average composition of our control sample and other results reported in the literature [15]. We have also checked the magnetization of the films using SQUID at 2 K and 300 K (see supplemental figure S2). As expected, the magnetization of the RESET phase was negligible (within the experimental errors) because of the spin-band degeneracy. These results support the existence of a metallic or gapless state with high mobility in the RESET phase. The change in the Hall mobility and carrier concentration with temperature is interpreted as arising from internal stresses which generate optimal conditions for the formation of a proper TI or Dirac semimetal phase, reflecting the two-dimensional nature [19, 20].


Giant multiferroic effects in topological GeTe-Sb 2 Te 3 superlattices
Temperature dependence of (a) the resistivity, (b) Hall mobility, and (c) carrier concentration of the [(GeTe)2(Sb2Te3)1]2 structure (red) and the control alloy with the same composition ratio (blue). The Hall properties of (a)–(c) were measured under a 5 kOe magnetic field.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 5: Temperature dependence of (a) the resistivity, (b) Hall mobility, and (c) carrier concentration of the [(GeTe)2(Sb2Te3)1]2 structure (red) and the control alloy with the same composition ratio (blue). The Hall properties of (a)–(c) were measured under a 5 kOe magnetic field.
Mentions: We further studied transport properties using Hall effect measurements on iPCM films. In this case the electric field is applied in plane, whereas the magnetic field (5 kOe) is applied normal to the plane. If the iPCM RESET phase does have a Dirac cone, two-dimensional metallic transport should be observed. Figure 5 shows the results obtained for an iPCM [(GeTe)2(Sb2Te3)1]2 film fabricated at 520 K with a control film of GST alloy fabricated with the same film thickness and at the same temperature. After the initial drop, the resistivity (ρxx) of the RESET phase increased to 7.8 × 10−4Ω cm at ∼420 K, at which point the SET phase of the iPCM film was established. Therefore, when the electric field is applied in plane, the RESET phase of the iPCM film behaves like a metal at temperatures between 350 K and 420 K. It should be noted that in the control GST alloy film with the same average composition, the resistivity monotonically decreased as expected for a semiconductor. At the same time, the Hall mobility steeply increased, reaching a value of ∼180 cm2 V-s−1 at ∼350 K, and then monotonically decreased, whereas the carrier concentration of 5.7 × 1019 cm−3 was essentially unchanged until the phase transition (apart from the initial drop). The results from the iPCM film are crucially different from the behavior of the alloy of the same average composition of our control sample and other results reported in the literature [15]. We have also checked the magnetization of the films using SQUID at 2 K and 300 K (see supplemental figure S2). As expected, the magnetization of the RESET phase was negligible (within the experimental errors) because of the spin-band degeneracy. These results support the existence of a metallic or gapless state with high mobility in the RESET phase. The change in the Hall mobility and carrier concentration with temperature is interpreted as arising from internal stresses which generate optimal conditions for the formation of a proper TI or Dirac semimetal phase, reflecting the two-dimensional nature [19, 20].

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.


Related in: MedlinePlus