A Magnetoresistance Induced by a Nonzero Berry Phase in GeTe/Sb₂Te₃ Chalcogenide Superlattices

Abstract

The chalcogenide alloy Ge-Sb-Te (GST) has not only been used in rewritable digital versatile discs, but also in nonvolatile electrical phase change memory as a key recording material. Although GST has been believed for a long time not to show magnetic properties unless doped with magnetic impurities, it has recently been reported that superlattices (SLs) with the structure [(GeTe) L (Sb 2 Te 3 ) M ] N (where L, M, and N are usually integers) have a large magnetoresistance at room temperature for particular combinations of L and M. Here it is reported that when [(GeTe) L (Sb 2 Te 3 ) M ] N chalcogenide SL films are thermally annealed at 470 K and cooled down to room temperature under an external magnetic field accompanied by current pulse injections, a large magnetoresistance change (>2500 Ω) is induced. This study shows that the phenomenon has a strong correlation with the GeTe thickness and the periodic structure of the SL films, and that it is induced by the structural phase transition between electrically nonpolar and polar phases in the GeTe layers in the SLs. This study proposes that the relationship between the polar (ferroelectric) phase and the Berry curvature in the SLs is responsible for the magnetoresistance change.

Phase‐Change Superlattice Materials toward Low Power Consumption and High Density Data Storage: Microscopic Picture, Working Principles, and Optimization

“…By opposite, the composition and thickness of the Sb‐Te sublattice layer can vary depending on the different group and devices. For example, Simpson et al, Takaura et al, Ohyanagi and Takaura, and Saito et al reported a series of GST‐SLs with 4 nm Sb 2 Te 3 sublayers, Tominaga et al reported some GST‐SLs with 1 nm Sb 2 Te 3 sublayer, Momand et al and Casarin et al reported some GST‐SLs with 3 nm Sb 2 Te 3 sublayers, Wang et al reported a GST‐SL with 6 nm Sb 2 Te 3 sublayers. Moreover, Kalikka et al and Noé et al fabricated a series of GST‐SLs with thickness‐varied Sb 2 Te 3 sublayers (varied from 1 to 8 nm).…”

“…In addition, although the GeTe sublayer in GST‐SL was often regarded as [(GeTe) 2 ] in previous models (Figure ), its thickness in samples/devices have been described as 0.7, 0.8, 0.9, or 1 nm in literatures depending on the publications and groups. Perhaps, this is due to how the GeTe‐sublayer thickness was defined or due to difficulty in the thickness control during material growth of GST‐SL.…”

Impact of Stoichiometry on the Structure of van der Waals Layered GeTe/Sb2Te3 Superlattices Used in Interfacial Phase‐Change Memory (iPCM) Devices

“…By contrast, in a PCM cell based on a GST alloy, a high RESET current is needed to melt the material prior to amorphization. iPCMs open numerous opportunities for multilevel storage, hybrid devices combining resistive and magnetic memories, logic gate devices, and also devices for THz pulse detection . Incorporation of SLs in iPCM devices by using ULSI microelectronics technology has been demonstrated for large density memory arrays…”

“…All these models assume the existence of GeTe layers surrounded by Sb 2 Te 3 QLs in the SL. Interpretation of the magnetoresistance effects observed in SLs also rely on the existence of (GeTe) 2 blocks in the so‐called ferroelectric state. Nevertheless, high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) images of a [GeTe (1 nm)/Sb 2 Te 3 (3 nm)] 15 SL deposited by MBE reveal that Ge atoms have diffused into the adjacent Sb 2 Te 3 layers, which results in the local formation of different Ge–Sb–Te blocks containing either seven atomic planes (Ge 1 Sb 2 Te 4 ), nine atomic planes (Ge 2 Sb 2 Te 5 ), eleven atomic planes (Ge 3 Sb 2 Te 6 ), or thirteen atomic planes (Ge 4 Sb 2 Te 7 ) separated by vdW gaps .…”

All‐Optical Detection of Periodic Structure of Chalcogenide Superlattice Using Coherent Folded Acoustic Phonons

“…Note that the SL structure consists of a ferroelectric normal insulator, GeTe, and a topological insulator (TI), Sb 2 Te 3 , and for certain structures it was shown using ab initio calculations that the RESET phase becomes a Dirac semimetal, while the SET phase becomes a Weyl semimetal . As can be anticipated from the characteristics of a TI, the band gap closes to give rise to a Dirac cone for certain thickness combinations or applied stress of the GeTe and Sb 2 Te 3 layers …”