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Spintronics using interfacial phase change memory, iPCM

1

Junji Tominaga1,2, Jan Richter1, Yuta Saito2, Alexander Kolobov2, Paul Fons2 and Shuichi Murakami3
Green Nanoelectronics Center, Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and2

Technology (AIST), Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan3

Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japanj-tominaga@aist.go.jp

Key words: interfacial phase change memory (iPCM), superlattice PCM, spintronics, topological insulator

1. INTRODUCTIONNobody including the present authors had believed so far that spintoronics was related to phase change memory(PCM). Interfacial phase change memory (iPCM) however may open a new era and become one of the powerful tools
for spintronics. Because of its unique characteristics excluding magnetic elements, it is expected that completelydifferent magnetic devices will be realized using phase change materials besides Fe- or Co-based magnetic device
and memory.
The present PCM and the materials whatever works optically or electrically are usually non-magnetic because
Ge, Sb and Te are non-magnetic. It was already confirmed [1]. However, the invention of iPCM and the discovery
of a room-temperature giant magnetoresistance broke the common sense [2-4]. IPCM is a kind of PCMs, whichswitches between one crystalline phase and another crystalline phase, while a typical PCM is operated between
amorphous and crystalline phases. In order to realize the crystal-crystal phase transition in iPCM, a Ge-Sb-Te alloy
film is built up using crystalline sub-layers of GeTe and Sb2Te3, which share a crystalline growth axis: [111] inrhombohedral GeTe and [111] of A-7-type rhombohedral Sb2Te3. The switching mechanism and the device
performance were already reported [5]. Of course, iPCM does not include any magnetic element. Why does iPCM
show the giant magnetoresistance? Since the discovery, we have engaged in making clear the mechanism.
2. MODELS
IPCM is a crystalline superlattice built up from heterocrystalline layers of GeTe and Sb2Te3:[(GeTe)l(Sb2Te3)m]n block, where l, m and n are integers. For example, a [(GeTe)2(Sb2Te3)1]n block has the same
composition as Ge2Sb2Te5. The former has the atomic layer structure, while the latter has a NaCl-type fcc latticewith a random occupation of Ge or Sb at cation sites. Sb2Te3 has a feature to grow the crystal phase on a Si wafer
orientating the [111] axis normal to the surface at a high temperature [8]. On the other hand, fcc-GeTe has a feature togrow the crystal phase orientating the [111] axis normal to the Sb2Te3 (111) surface at 200~250oC [6]. Therefore, ahighly anisotropic film is formed by controlling the formation temperature.
Figure 1 shows the film structure of atypical GeSbTe superlattice [(GeTe)2(Sb2Te3)1] n with a=4.22 A and c=18.71 A.

It is recently noticed that the Sb2Te3

sub-layer (quintuple layer QL) is a 3D-topological insulator. On the other hand, the GeTe layer is known to be aferroelectric material [7]. The models shown in Fig. 1 are two of the atomic sequence orders in the GeTe sub-layer.
Although the other sequence besides the models, Ge-Te-Te-Ge (Petrov) is possible, we found that the Petrov sequence
is unstable at the film deposition temperature (~250oC) than the other two sequences using ab-inito molecular
dynamics simulations.

It is easily understood that phase transition between the two models in Figure 1 occurs by Ge-

flip-flop transition through the Te plane in the GeTe sub-layers. In addition, the transition process beaks the spatial
inversion symmetry of the Te-Ge-Ge-Te sequence.

Figure 1 Basic cell models of iPCM with [(GeTe)2(Sb2Te3)1]n.

Two Ge atoms are installed in the GeTe layer,

and the Sb2Te3 layer is composed of a Te-Sb-Te-Sb-Te stacking sequence (QL).
RESET) and Ge-Te-Ge-Te sequence (right: SET).

Te-Ge-Ge-Te sequence (left:It is noticed that Te-Ge-Ge-Te sub-layer is dielectric due to holding the spatial inversion symmetry (RESET state)while the Ge-Te-Ge-Te sub-layer is ferroelectric due to the break of the inversion symmetry (SET state). Each sublayers are weakly bonded by van der Waals force.

Due to the feature, Sb2Te3 QLs may play a role in a 3D-

topological insulator independently in some extent. As increasing the QL blocks (l ≧ 1) in the superlattice[(GeTe)2(Sb2Te3)m]n, the surface bands are generated at the top and bottom surfaces play independently. On theother hand, when the GeTe sub-layer is thicker or thinner than a critical thickness, the iPCMs have a band gap, due tothe hybridization of the bottom edgeless band and the top edgeless band in the adjacent Sb2Te3 sub-layers [8]. In theformer case, the iPCM becomes a 3D-TI, while the latter becomes a bulk insulator. But, the top and bottom surfacesof the film hold the 2D-TI features if the number of the QL is ≧ 1.

It is noticed that a Dirac cone is formed in thebulk band structure when the hybridization is balanced. The structure of [(GeTe)l(Sb2Te3)m]n iPCM films may take thecritical condition, depending on each block thickness.

3. MAGNETIZATION OF IPCM
Figure 2 shows the bulk band structure of [(GeTe)2(Sb2Te3)2] iPCM film. It is found that band gap is negligibly
small and pz-orbitals in Ge , Sb and Te invade into the conduction bottom band. A material with such a special
condition is called Dirac semimetal.

In the model (left) in Figure 1, the band structures of the spin-up and spin-down

are degenerated, while the degeneration is lifted into spit bands corresponding to each spin-state in the model (right).
When an external electrical field is applied, there may generate a difference in between the spin densities of the state.
This is called Rashba effect. Recently, it was reported that GeTe has a large Rashba effect [7]. If the iPCMs are
Dirac semimetals, a band gap may easily open when a voltage is applied to an iPCM device, resulting in a non-ohmic
resistance change.

In addition, in an external magnetic field, the spin-density of the state may be no longer equal in

between spin-up and spin-down electrons, resulting in an electrical-field-induced magnetization.

Figure 2

Bulk band structure of [(GeTe)2(Sb2Te3)2] iPCM (left).

Te-pz contributions (the size of the circlers on the lines).
valence top to the conduction bottom.

Three zoom images are of Ge-pz, Sb-pz andThe pz orbitals in Ge, Sb and Te invade from the4. SUMMARYElectrical-field-induced magnetic properties in iPCMs may be generated by the break of the Dirac cone in the bulk
state due to the combination between the ferroelectric & antiferroelectric GeTe layers and the topological insulating
Sb2Te3 layers. IPCM is the first engineering material to probe topological insulating properties.

Acknowledgement
A part of the work was supported by FIRST program initiated by the Council for Science and Technology Policy(CSTP).

We wish to thank Ms. Reiko Kondou for her assistance in the fabrication of iPCMs.

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