Magnetic Properties and Band Structuresin Superlattice Phase-change Memory
Junji Tominaga*1, Alexander V. Kolobov 1 and Paul Fons 1
1Green Nanoelectronics Center, Nanoelectronics Research Institute, National Institute ofAdvanced Industrial Science & Technology (AIST), Tsukuba Central 4, 1-1-1 Higashi,
Tsukuba, 305-8562, Japan*j-tominaga@aist.go.jp

ABSTRACT
Supelattice phase-change memory films composing of a crystalline-multilayered [(GeTe)x/Sb2Te3)y]z (x, y,and z are integer) structure emerge unusual magnetic properties, such as more than 2000%
magnetoresistance at room temperature. In this paper, we discuss the characteristics from the point of view of
topological insulators.
Key words: superlattice, iPCM, topological insulator

1. Introduction
Chalcogenides, especially, Ge-Sb-Te ternary alloys, have long been used as a recoding material inrewritable optical discs. The recording and erasing mechanism is simple and based on 1st phase transition
between amorphous and crystal because the alloys have a large change in optical constant (refractive index),
which reflects two reflection levels (high and low) corresponding to 1 and 0 digit. On the other hand, a large
difference in electrical resistance has been applied to the nonvolatile electrical phase-change memory(PCM). All these physical properties in optics and electronics are emerged from the bonding states (covalentor metallic) of Ge atoms. Recently, it has been reported that the amorphous state have a shorter bond lengthwith the nearest Te atoms exist in the amorphous while three longer and three shorter bond lengths areincluded in the crystals [1]. That is, energetically optimization of the generation and elimination of thelonger bonds is a key factor to suppress the switching energy in PCM. Interfacial phase-change memory(iPCM) using superlattice composing of GeTe and Sb2Te3 sub-layer stacks provides an ideal solution for theobjective [2, 3]. In iPCM, the exchange of the bonding between covalent and metallic states can be
performed without moving the atomic positions in the Sb2Te3 sub-layers. An energy loss accompanied withthe phase-transition is greatly suppressed as a result [4].
A unique property of magnetoresistance induced by electrical field was recently discovered in iPCM
in addition to the low power switching [5]. The phenomenon is highly unusual because no magnetic
sensitivity has been observed in the Ge-Sb-Te alloys at room temperature until now. Since E*PCOS2011 andPCOS2011, our group has focused on topological insulator to explain the interesting phenomenon coming
out from the superlattice structures. In this PCOS 2012, we discuss the unique property on the point of viewof topological insulator.
2. Topological Insulator
Topological insulator (TI) is an electrical insulator in a bulk state [6]. In normal insulators, there is arigid band gap between the top of the valence bands and the bottom of the conduction bands, while in TIs
there is a band gap which is made of the inversion between the top valence band and the bottom conduction
band. Therefore, the property (topological phase) of a band gap is different. The interface between a normal
insulator and vacuum for example can be connected smoothly. On the other hand, to connect a TI to vacuum,
there must exist a point somewhere to recover the inversion bands of the TI to the normal insulating bandstate of vacuum. To realize this connection, there is no way except for a route through a gapless (Dirac point)
or a conductive edge. As a result, the surface of TI must be conductive. The surface band structure is a
specially unique because the dispersion is approximately linear near the Dirac point, where an electron has
no mass. It means high mobility without scattering. These features result in the conservation of time
reversal symmetry in TI. At the surface, spin-up and spin-down electrons flow oppositely as spin current tokeep the conservation. How such a special band structure is realized? This is performed using heavy
elements with a strong spin-orbit coupling (SOC). Zn Ga, Ge, As, Se, In, Sn, Sb, Te, Hg, Tl, Pb, Bi, and Po
are candidates for generating such a bang gap. Among them, Bi2Se3, Bi2Te3, Sb2Te3 are suitable crystals andwere already identified to be a TI experimentally [7]. It has been predicted theoretically that one unique
crystal structure of Ge-Sb-Te may become a TI, but which is called the Kooi’s sequence [8, 9].
Unfortunately, no one has observed the TI property until now. On the other hand, the superlattices made ofGeTe/Sb2Te3 in iPCM has been more promising to observe the TI properties because both GeTe and Sb2Te3
crystalline sub-layers have a same growth orientation towards <111> with a highly spatial-inversion
symmetry. In addition, a GeTe is a normal insulator [10, 11].

Figure 1 Electronic band structure of Sb2Te3 with a vacuum slab (left) and the structure of the model inP3m-1 (right). In the band structure, grey shadow regions are of the bulk without the vacuum slab. The bulk
Sb2Te3 has a ~0.5 eV band gap, while at the surface band with the vacuum slab the gap closes to become aTI.

TominagaWIEN atom 0size 0.20

Sb2Te3

2.0SbEnergy (eV)

1.0

TeEF

0.0

Ge(GeTe)X2-1.0

-2.0

K.1

M

K.2

Γ

K.3

KSb2Te3

Figure 2 Electronic band structure of iPCM [(GeTe)2(Sb2Te3)2] and the structure of the model in P3m-1(right). In the band structure, grey shadow regions are of the balk Sb2Te3. The band structure without SOC
has a 50 meV band gap, while the gap closes to become a TI when SOC is included.

3. Confirmation of TI property in iPCM
As mentioned, the band structure near the Dirac point in TI has a time reversal symmetry E(-k, spinup) = E(+k, spin-down). A external magnetic field normal to the surface may break the symmetry, and thedegeneration of the band is lifted, resulting in a Rashba-type band splitting. If the splitting energy is verylarge against thermal perturbation, TI may show a magnetic property. As we reported, iPCM showed a large
magnetoresistance under an external magnetic field of 0.1 T, while a PCM with the same composition GeSb-Te alloy did not. This is one of the strong evidences that iPCM is a TI [5].

Figure 3 I-V comparison between PCM and iPCM under a magnetic field. Red and black curves are theoriginal scan and the scan after removing the field, respectively, and blue curves are scans under 0.1 T inplane magnetic field [5].
ACKNOWLEDGEMENT
A part of the work was supported by FIRST program initiated by the Council for Science and Technology
Policy (CSTP).
References:[1] Kolobov et al., Nature Mater, 3, 703 (2004).
[2] J. Tominaga et al. in Proceedings of the European Symposium on Phase Change and Ovonic Science (E\PCOS), Milan, Italy, 2010, pp. 54-59.
[3] R. Simpson, P. Fons, A. V. Kolobov, and J. Tominaga, Nature Nano. 6, 501 (2011).
[4] J. Tominaga et al., Phys. Status Solidi B, 1932 (2012).
[5] J. Tominaga et al., Appl. Phys. Lett. 99, 152105 (2011).
[6] H. Zhang, C. Liu, X. Qi, X. Dai, Z. Fang, and S. Zhang, Nature Phys. 5, 438 (2009).
[7] Y. Xia et al. Nature Phys. 5, 398 (2009).
[8] J. Kim, J. Kim, and S. Jhi, Phys. Rev. B82, 201312 (2010).
[9] B. Sa et al. Phys. Rev. B84, 085130 (2011).
[10] B. Sa et al. Phys. Rev. Lett. 109, 096802 (2012).
[11] J. Kim et al. Phys. Rev. Lett. 109, 146601 (2012).