Magnetic properties of phase-change films hidden in the structures
J. Tominaga,1,2 P. J. Fons,1,2 M. Hase,3 and A. V. Kolobov1,2
1Functional Nano-phase-change Team, 2Green Nanoelectronics Center

National Institute of Advanced Industrial Science and Technology, Tsukuba Central 4, 1-1-1Higashi, Tsukuba, 305-8562, Japan
3 Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japanj-tominaga@aist.go.jp
Abstract
Since the discovery of phase-change switching in chalcogenides, phase-change memory (PCM) has
been irrelevant and insensitive to magnetic field. Actually, in the development of digital versatile
disc (DVD) using a red laser, optical PCM had fought with magneto-optical memories. However,the fundamental concept is removed completely by the emergence of interfacial phase-change
memory (iPCM), which is designed to reduce entropic energy loss as small as possible using a
growth-oriented superlattice structure. We report that iPCM show a giant magneto-resistivity >2,000% at more than 400K under a magnetic field of 0.1T, and surprisingly can discriminate theexternal magnetic field direction optically. These magnetic properties of iPCM may open a newera to the PCM world associated with spintronics.
I. Introduction
Phase-change memory (PCM) is considered to be the leading candidate for nonvolatile memory(NVM). PCM has several advantages over other type NVMs such as magnetoresistive (MRAM),
metal oxide resistive (RRAM) and FLASH because of its scalability, understandable and hence
predictable switching mechanism, and large resistance differences between set and reset states.
Although PCM operation typically has required large power for switching, especially for the resetprocess, this drawback has been mitigated significantly by replacing the typically used monolithic
GeSbTe film by an “interfacial phase-change memory, iPCM” structure [1]. Due to a significant
reduction in entropic losses associated with both set and reset states, the required input energy has
been reduced by around 90% compared to conventional PCM devices. The energy saving propertyof iPCM devices is a consequence of the one dimensional motion of Ge atoms normal to the GeTe/
Sb2Te3 interfaces. High quality interfaces and crystallographic alignment along the growth
direction of each GeTe and Sb2Te3 sub-layer is required.
Recently, topological insulators, (TI) have attracted much attention in graphene as well as
several crystals including heavier elements [2]. Very recently, the bulk crystalline phase of
Ge2Sb2Te5 has been predicated to be a TI using first-principles computer simulations [3] as well as
the Sb2Te3 trigonal phase [4]. If both predictions of TI behavior are true, iPCM films and devices
may become some of the most suitable materials to confirm the existence of topological insulating
properties of phase-change materials.
In this paper, we report on the results of first-principles computer simulations of iPCM structures
including a spin-orbit coupling (SOC), that confirm iPCM is a suitable candidate to exhibit
topological insulating behavior and related properties.
II. First principle simulation including spin-orbit coupling
To investigate the topological characteristics of iPCM structures theoretically, inclusion of spinorbit coupling (SOC) is required. SOC is a relativistic effect and is an additional term to the total
hamiltonian Ĥ because inner electrons in a heavy element move faster than outer shell electrons
and the velocity ve is proportional to atomic number Z: ve ∝ Z/n, where n is the principal quantum
number. When relativistic effects are important, it is necessary to solve the Dirac equation to solve
for the electron band structure. The total hamiltonian Ĥ including the SOC term thus becomes [5],Ĥ = Ĥ0 + Ĥso = -(ħ2/2m)∇2 + (ħ/4m2c2) σ·(∇V(r) × p)

(1).

Where, σ is the spin operator. If eq.(1) is invariant under spatial inversion, the SOC energy Ĥsobecomes zero because of symmetry. However, if spatial inversion symmetry is broken (forexample, V(r) has a gradient), Ĥso assumes a non-zero value, resulting in spin-splitting because the∇V(r) × p term operates on spin-electrons in the same manner as a magnetic field. If ∇V(r) =(0, 0,Ez) is applied normal to a 2-dimensional surface and electrons move on the surface with amomentum p (or k), the spin directions are confined to the plane and quantized. As a result, theenergy of free electrons in the plane splits,E(k) = (ħ2/2m)k2 ± (ħ/4m2c2) Ez k = Enon-so(k) ± αRk(2).

Where, αR is called Rashba parameter, which provides an estimate of the strength of the SOC.
GST225Ge4-01 atom 0

GST225Ge4-01 atom 0 size 0.20

size 0.20
1.0

EF

0.0

Energy (eV)Energy (eV)

1.0

EF

0.0

without SOC

-1.0

K.1

"

K.2!

K.3!

with SOC

K.4

"-1.0

K.1

"

K.2!

K.3

!

K.4

"

Figure 1 Band structures with (right) and without (left) SOC for the [(GeTe)2(Sb2Te3)1] iPCM
structure with tetrahedrally coordinated Ge atoms. Due to SOC, the spin degeneracy of each bandis lifted.
Two density functional codes, CASTEP and WIEN2K were used to calculate the band structuresof iPCM superlattice structures built from the unit block [(GeTe)2(Sb2Te3)1]. WIEN2K is an allelectron code that includes relativistic wave-functions and includes spin-orbit coupling. InCASTEP calculations, the local spin density approximation (LSDA) was used, while in WIEN2K alinearized augmented plane waves (LAPW) was used in conjunction with the GGA for theexchange-correlation term. First, two models with tetrahedrally coordinated Ge atoms andtrigonally coordinated Ge atoms were relaxed at 0 K under spin-polarized conditions using theCASTEP in order for the more computationally complex all-electron to convergence faster, andthen the relaxed structures were moved to WIEN2K. Figure 1 shows two band structures of[(GeTe)2(Sb2Te3)1] with and without SOC. Without SOC, each energy band is spin-degenerate,
while with SOC the degeneracy is lifted. In the vicinity of k=0 (theΓpoint), the band symmetry ispreserved, however, the spin polarization is exchanged. It should be noticed that time reversal
symmetry: E(k, ↑) = E(-k, ↓) is invariant, while spatial inversion symmetry is not preserved; E(k, ↑)≠ E(-k, ↑). At the Γpoint, two valleys appear symmetrically for k<0 and in k>0 with a maximum
energy difference around 0.2 eV. (see the red and blue lines at the bottom of the conduction bands).
Each band has a different spin polarization, which is also depicted in figure 1. This is a very
important criteria to be a candidate as a TI with a large Rashba effect. Up and down spin electrons
lifting of the spin degeneracy of the bands are all aligned within the interfaces of the iPCM <111>
structure, and application of an electrical field normal to the surface may induce a large Rashba
effect similar to magnetic field of SOC. The iPCM structure can have magnetic sensitivity, which iscompletely absent in the identical composition alloy of Ge2Sb2Te5!
In our simulations, it wasconfirmed that other iPCM structures with plural blocks of Sb2Te3, [(GeTe)2(Sb2Te3)n] show similar
TI feature to the iPCM with [(GeTe)2(Sb2Te3)1].
III. Experimental verification of IT property and the Rashba effect in iPCM structures
As explained above, an electrical field applied normal to the <111> surface of an iPCM structure
makes electrons on the surface or interfaces spin-polarize with Rashba effect because the external
electrical field plays the role of generating an internal magnetic field via the σ·(∇V(r) × p). In
addition, it is hard for the spin-currents to penetrate normal to the iPCM film. Interestingly, by theRashba effect as shown in eqs. (1) and (2), except for k=0, spin electrons feel a different energy
depending upon the spin state around the Γpoint. On the other hand, applying an external
magnetic field in the plane make the spins states tighten up because the magnetic field plays asimilar role to the σ·(∇V(r) × p) in eq.(1). Therefore, by illuminating circular polarized electromagnetic light onto the iPCM film, one of the spin currents will be more induced than the otherspin current. An optical response, such as reflectivity change, will be observed. The expectedbehavior, however, is clearly different from magneto-optical Kerr effect which shows a reduction
in strength with wavelength. Figure 2 shows optical reflectivity under ~0.2 T magnetic field,
which was applied in plane along the edge of an iPCM structure [(GeTe)2(Sb2Te3)2]12 on a Si wafer.
It is found that the iPCM structure can be to identify the polarity of the magnetic field.

(Sb2Te3)5nm/[(Ge1Te1)2(Sb2Te3)2]12~0.5%

Figure 2. Optical reflectivity change induced by application of an external magnetic field (anaverage of 0.2 T) applied in plane along the edge of an iPCM structure [(GeTe)2(Sb2Te3)2]12 on a Siwafer. The different magnetic polarity shows different reflectivity. This means that the iPCM film
can be used to identify the magnetic field direction. Circular polarized light was used. It is
confirmed that this was not a magneto-optical effect (Kerr or Faraday) by use of a polarizer.
IV. Conclusion
Magneto-reflection of an iPCM structure of GeSbTe was first observed under magnetic field.
The film was sensitive to application of an external magnetic field, and the magnetic field direction
could be identified. Due to the first principle computer simulations including SOC effect, iPCM
structures are TI with a large Rashba effect.
Acknowledgement: This research is granted by JSPS through FIRST Program initiated by CSTP. We
wish to thank Elpida Memory for helpful discussion on the device measurement and analysis.

References:[1] R. Simpson, P. Fons, A. Kolobov, and J. Tominaga, Nature Nanotechnology. 6, 501 (2011).
[2] J. E. Moore, Nature 464, 194 (2010).
[3] J. Kim, J. Kim, and S. Jhi, Phys. Rev. B82, 201312 (2010).
[4] H. Zhang, C. Liu, X. Qi, X. Dai, Z. Fang, and S. Zhang, Nature Phys. 5, 438 (2009).
[5] W. M. Al-Sawai, Ph. D. dissertation, Northeastern Univ. 2010.