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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.


(a) Magnetoresistance under different magnetic field strengths 0.8, 1.0, and 1.2 kOe and (b) the associated resistance hysteresis in a typical iPCM [(GeTe)2/(Sb2Te3)4]8 device (inset) at room temperature. Magnetoresistance hysteresis occurs after the electric field is turned off with a magnetic field still present. The black circles indicate the initial resistance without an applied magnetic field, the blue circles indicate resistance values with a magnetic field, the red circles indicate the first R–V curve after removal of the magnetic field, and finally the black triangles correspond to the second R–V cycle after the magnetic field was turned off. In each process, the initial and terminated states were both RESET.
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Figure 3: (a) Magnetoresistance under different magnetic field strengths 0.8, 1.0, and 1.2 kOe and (b) the associated resistance hysteresis in a typical iPCM [(GeTe)2/(Sb2Te3)4]8 device (inset) at room temperature. Magnetoresistance hysteresis occurs after the electric field is turned off with a magnetic field still present. The black circles indicate the initial resistance without an applied magnetic field, the blue circles indicate resistance values with a magnetic field, the red circles indicate the first R–V curve after removal of the magnetic field, and finally the black triangles correspond to the second R–V cycle after the magnetic field was turned off. In each process, the initial and terminated states were both RESET.

Mentions: Figure 3(a) shows R–V curves of an iPCM [(GeTe)2/(Sb2Te3)4]8 device when external magnetic fields of different intensities are applied normal to the interfaces. Without an external magnetic field, the transition to the SET phase occurs at the same threshold voltage as in the alloy of the same average composition (cf figure 6 of [18]) with an important difference: the resistance of iPCM increases by an order of magnitude for voltages between 0 V and 0.8 V (the threshold voltage), whereas the resistance of the composite amorphous phase in PC-RAM remains constant (or gradually decreases, which has been explained by the Frenkel–Poole mechanism [15]). Upon increasing the magnetic field to 1.2 kOe, the resistance in the RESET phase increases by two orders of magnitude between 0 V and 2.5 V and then suddenly drops to the low-resistance SET phase. The minimum magnetic field required for the magnetoelectric effect was ∼0.8 kOe, and lower fields had a negligible effect (not shown). It should also be noted that the resistance curves in the range between 0 and 0.5 V almost completely overlap regardless of the magnitude of the magnetic field. Therefore, it can be surmised that the unusual resistance increase in this voltage range is due to an intrinsic magnetic field induced by the applied electric field, which is equivalent to an external magnetic field of ∼0.8 kOe, after which the external magnetic field starts to have an effect on the threshold voltage. This implies that the α of ΔM = tαE in the RESET phase has diagonal elements since both the induced magnetization and the applied electric field have the same direction normal to the surface.


Giant multiferroic effects in topological GeTe-Sb 2 Te 3 superlattices
(a) Magnetoresistance under different magnetic field strengths 0.8, 1.0, and 1.2 kOe and (b) the associated resistance hysteresis in a typical iPCM [(GeTe)2/(Sb2Te3)4]8 device (inset) at room temperature. Magnetoresistance hysteresis occurs after the electric field is turned off with a magnetic field still present. The black circles indicate the initial resistance without an applied magnetic field, the blue circles indicate resistance values with a magnetic field, the red circles indicate the first R–V curve after removal of the magnetic field, and finally the black triangles correspond to the second R–V cycle after the magnetic field was turned off. In each process, the initial and terminated states were both RESET.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 3: (a) Magnetoresistance under different magnetic field strengths 0.8, 1.0, and 1.2 kOe and (b) the associated resistance hysteresis in a typical iPCM [(GeTe)2/(Sb2Te3)4]8 device (inset) at room temperature. Magnetoresistance hysteresis occurs after the electric field is turned off with a magnetic field still present. The black circles indicate the initial resistance without an applied magnetic field, the blue circles indicate resistance values with a magnetic field, the red circles indicate the first R–V curve after removal of the magnetic field, and finally the black triangles correspond to the second R–V cycle after the magnetic field was turned off. In each process, the initial and terminated states were both RESET.
Mentions: Figure 3(a) shows R–V curves of an iPCM [(GeTe)2/(Sb2Te3)4]8 device when external magnetic fields of different intensities are applied normal to the interfaces. Without an external magnetic field, the transition to the SET phase occurs at the same threshold voltage as in the alloy of the same average composition (cf figure 6 of [18]) with an important difference: the resistance of iPCM increases by an order of magnitude for voltages between 0 V and 0.8 V (the threshold voltage), whereas the resistance of the composite amorphous phase in PC-RAM remains constant (or gradually decreases, which has been explained by the Frenkel–Poole mechanism [15]). Upon increasing the magnetic field to 1.2 kOe, the resistance in the RESET phase increases by two orders of magnitude between 0 V and 2.5 V and then suddenly drops to the low-resistance SET phase. The minimum magnetic field required for the magnetoelectric effect was ∼0.8 kOe, and lower fields had a negligible effect (not shown). It should also be noted that the resistance curves in the range between 0 and 0.5 V almost completely overlap regardless of the magnitude of the magnetic field. Therefore, it can be surmised that the unusual resistance increase in this voltage range is due to an intrinsic magnetic field induced by the applied electric field, which is equivalent to an external magnetic field of ∼0.8 kOe, after which the external magnetic field starts to have an effect on the threshold voltage. This implies that the α of ΔM = tαE in the RESET phase has diagonal elements since both the induced magnetization and the applied electric field have the same direction normal to the surface.

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.