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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.


The switching mechanism of PC-RAM and iPCM non-volatile memories. Whereas the former utilizes an amorphous-to-crystalline phase transition, the latter is based on a crystal-to-crystal phase transition. The cell designs can be similar in both cases. The PC and iPCM layer thicknesses are typically about 40 nm, and the bottom electrode (heater) diameter is less than 100 nm. Tc, Tm, and Tt are the crystallization temperature, melting temperature, and transition temperature, respectively. V and i are the applied voltage and current. Due to the presence of a melting-free mechanism, the switching energy is reduced by 95% in the iPCM device compared with that of the PC-RAM device using a similarly designed cell platform [9, 10, 16].
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Figure 1: The switching mechanism of PC-RAM and iPCM non-volatile memories. Whereas the former utilizes an amorphous-to-crystalline phase transition, the latter is based on a crystal-to-crystal phase transition. The cell designs can be similar in both cases. The PC and iPCM layer thicknesses are typically about 40 nm, and the bottom electrode (heater) diameter is less than 100 nm. Tc, Tm, and Tt are the crystallization temperature, melting temperature, and transition temperature, respectively. V and i are the applied voltage and current. Due to the presence of a melting-free mechanism, the switching energy is reduced by 95% in the iPCM device compared with that of the PC-RAM device using a similarly designed cell platform [9, 10, 16].

Mentions: PC-RAM, recently commercialized by the world’s largest memory makers, Samsung and Micron, is based on a phase transition between the amorphous and crystalline states in ternary GST alloys, making use of the large property contrast between the two phases. The contrast is associated with the contrasting bonding nature between the constituent atoms in the structures. When a voltage exceeding a certain value is applied to a device in the high-resistivity amorphous (RESET) state, it switches to the low-resistivity crystalline (SET) state. During the SET pulse, the GST is heated above the crystallization temperature, whereas the RESET pulse melts the material, reverting it to the amorphous phase with large entropic losses [14–16]. Pulses with typical durations of ∼100 ns and ∼500 ns are used for the RESET and SET processes, respectively. In contrast, in iPCM, which has the structure of a short-period [(GeTe)l(Sb2Te3)m]nsuperlattice (where l, m, and n are integers), where GeTe and Sb2Te3 share a common growth axis, the [111] direction of the rhombohedral GeTe layer and the [111] axis of the rhombohedral (A7) Sb2Te3 layer, being parallel to each other and normal to the substrate surface, is crystalline in both the SET and RESET states [9]. Importantly, even sputtered iPCM devices have a strong preferred [111] growth direction and exhibit high-quality interfaces [17]. The switching mechanisms of PC-RAM and iPCM are summarized in figure 1.


Giant multiferroic effects in topological GeTe-Sb 2 Te 3 superlattices
The switching mechanism of PC-RAM and iPCM non-volatile memories. Whereas the former utilizes an amorphous-to-crystalline phase transition, the latter is based on a crystal-to-crystal phase transition. The cell designs can be similar in both cases. The PC and iPCM layer thicknesses are typically about 40 nm, and the bottom electrode (heater) diameter is less than 100 nm. Tc, Tm, and Tt are the crystallization temperature, melting temperature, and transition temperature, respectively. V and i are the applied voltage and current. Due to the presence of a melting-free mechanism, the switching energy is reduced by 95% in the iPCM device compared with that of the PC-RAM device using a similarly designed cell platform [9, 10, 16].
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 1: The switching mechanism of PC-RAM and iPCM non-volatile memories. Whereas the former utilizes an amorphous-to-crystalline phase transition, the latter is based on a crystal-to-crystal phase transition. The cell designs can be similar in both cases. The PC and iPCM layer thicknesses are typically about 40 nm, and the bottom electrode (heater) diameter is less than 100 nm. Tc, Tm, and Tt are the crystallization temperature, melting temperature, and transition temperature, respectively. V and i are the applied voltage and current. Due to the presence of a melting-free mechanism, the switching energy is reduced by 95% in the iPCM device compared with that of the PC-RAM device using a similarly designed cell platform [9, 10, 16].
Mentions: PC-RAM, recently commercialized by the world’s largest memory makers, Samsung and Micron, is based on a phase transition between the amorphous and crystalline states in ternary GST alloys, making use of the large property contrast between the two phases. The contrast is associated with the contrasting bonding nature between the constituent atoms in the structures. When a voltage exceeding a certain value is applied to a device in the high-resistivity amorphous (RESET) state, it switches to the low-resistivity crystalline (SET) state. During the SET pulse, the GST is heated above the crystallization temperature, whereas the RESET pulse melts the material, reverting it to the amorphous phase with large entropic losses [14–16]. Pulses with typical durations of ∼100 ns and ∼500 ns are used for the RESET and SET processes, respectively. In contrast, in iPCM, which has the structure of a short-period [(GeTe)l(Sb2Te3)m]nsuperlattice (where l, m, and n are integers), where GeTe and Sb2Te3 share a common growth axis, the [111] direction of the rhombohedral GeTe layer and the [111] axis of the rhombohedral (A7) Sb2Te3 layer, being parallel to each other and normal to the substrate surface, is crystalline in both the SET and RESET states [9]. Importantly, even sputtered iPCM devices have a strong preferred [111] growth direction and exhibit high-quality interfaces [17]. The switching mechanisms of PC-RAM and iPCM are summarized in figure 1.

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.