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Yan Li and Riccardo Mazzarello*

Magnetic Contrast in Phase-Change Materials Doped with Fe Impurities

TION

Chalcogenide phase-change materials (PCMs) are capable of

undergoing fast and reversible transitions between the amor-phous and crystalline phase upon heating.[1] The two phases have markedly different optical and electronic properties, which are currently exploited in rewritable optical media (CD, DVD, Blu-Ray Disc) and new concepts for electronic Non-Volatile Memories (NVMs).[1,2] Ge2Sb2Te5 (GST) is the most widely studied PCM, due to its commercial use in optical media and its applications in NVMs. Recent experimental works[3–7] and ab initio studies based on density functional theory (DFT)[8–12] have shed light on the structural and electronic properties of the relevant phases of GST and the related binary GeTe com-pound, though the mechanisms responsible for the fast crystal-lization are not fully understood.

In a recent work,[13] the first magnetic phase-change material was synthesized by a laser synthesis method. The authors of said paper showed that GST doped with Fe impurities is ferro-magnetic and exhibits phase-change features: starting from the stable, hexagonal doped phase, a transition to the amorphous state was induced by a short but intense laser pulse; a fast, reverse transition to the crystalline state was then obtained by applying a longer, less intense laser pulse. Furthermore, both phases were shown to be ferromagnetic, but with different sat-uration magnetization (smaller in the amorphous phase). This behavior was observed for Fe doping below 19% atom: above this value, Fe precipitates and forms magnetic clusters inside the host. These findings open up the possibility of exploiting the phase-change behavior for fast magnetic switching in e.g. spintronic devices. Very recently, similar results have been obtained for Fe-doped GeTe.[14]

Fe-doped GST can be classified as a diluted magnetic semi-conductor (DMS): DMSs have gained immense attention in the recent past,[15–18] as they hold the promise of exploiting the electron spin in semiconducting devices, thereby improving the existing semiconductor technology. In order for these applica-tions to be possible, the Curie temperature Tc of the employed DMSs should exceed room temperature. For most DMSs, how-ever, Tc is well below room temperature and Fe-doped GST is no exception: the estimated Tc for the hexagonal phase provided in Ref. [13] is 173 K. DFT studies based on the coherent potential approximation (CPA) or the supercell method have significantly

© 2012 WILEY-VCH Verlag GmAdv. Mater. 2012, 24, 1429–1433

Dr. Y. Li, Prof. R. Mazzarello Institute for Theoretical Solid State Physics and JARA- Fundamentals of Future Information TechnologyRWTH Aachen UniversityD-52056 Aachen, GermanyE-mail: mazzarello@physik.rwth-aachen.de

DOI: 10.1002/adma.201104746

contributed to the understanding of the electronic structure and magnetism of several classes of crystalline DMSs, in particular magnetically doped III–V and II–VI semiconductors.[16–18] They have provided insight into the correlations between the local environment and the localized magnetic moments and into the nature of the exchange interactions between the magnetic moments.

Future exploitation of the magnetic contrast of magneti-cally doped PCMs in phase-change devices requires an under-standing of a) the physical mechanisms favoring the ferromag-netic phase, b) the microscopic origin of the contrast and c) the effect of the magnetic impurities on the phase-change proper-ties (e.g. the crystallization speed) of the host material.

In this Communication, we aim at shedding light on these crucial aspects: for this purpose, we carried out an ab initio study of the structural, electronic and zero-temperature mag-netic properties of hexagonal GST (h-GST), amorphous GST (a-GST) and cubic GST (c-GST) doped with Fe impurities. Since the CPA cannot describe the short-range order and local environment of the impurities and, furthermore, it cannot be straightforwardly applied to amorphous alloys, the supercell method was used: large models of GST containing 199–216 atoms and 7% Fe concentration were employed to describe all the three phases. We considered the ferromagnetic state only, for the experimental results of Ref. [13] seem to rule out the possibility that the ground state configuration of this DMS is a spin glass.

The Fe-doped crystalline phases of GST were investigated using PWSCF, a plane-wave code based on periodic boundary conditions, included in the Quantum Espresso package;[19] as regards the amorphous phase, the first-principles molecular dynamics scheme developed by Kühne et al.,[20] implemented in the CP2K suite of programs[21] (also based on periodic boundary conditions), was employed to generate a model of Fe-doped a-GST by quenching from the melt within the Γ-point approxi-mation. This model was further investigated using PWSCF, so as to assess the convergence of the magnetic properties with respect to k-point sampling. Gradient-corrected Perdew-Burke-Ernzerhof (PBE) exchange-correlation functionals[22] were employed in all the simulations. Ultrasoft[23] and Goedecker[24] pseudopotentials were used in PWSCF and CP2K simulations, respectively.

We started our investigation from the hexagonal phase, in the stacking sequence proposed in Ref. [25], at the experimental lattice constant (see Figure 1a): the first step was the analysis of the energetics of Fe impurities sitting at various substitutional and interstitial sites. For this purpose, we used smaller super-cells containing 108 atoms and 1 Fe impurity and a 2 × 2 × 2 Monkhorst-Pack (MP) mesh.[26] At first, we considered Fe impu-rities substituting Ge, Sb and the three non-equivalent Te atoms shown in Figure 1a. The formation energies of these defects

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Figure 1. a) Substitutional sites for Fe in h-GST. b) DOS of h-GST containing a Fe impurity at the Ge and Sb substitutional site, respectively. The dashed and solid line indicate the total DOS and the DOS projected onto 3d states of Fe, respectively.

were computed with respect to h-GST (using an equivalent MP mesh), the ferromagnetic bcc phase of Fe and the crystalline phases of Ge, Sb and Te. The most favorable site is Sb, followed by Ge (see Table 1). In these two configurations, the system has almost integer magnetic moment of 5 and 4 μB, respectively: these findings can be understood within a simple ionic model, where Fe donates 3 and 2 electrons to make bonds with Te atoms when it substitutes at Sb and Ge sites, respectively. Bond

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Table 1. Formation energy, total magnetic moment and local moment of thcentered on Fe) for a) a Fe impurity substituting Ge, Sb and the three nonin the Sb/Ge sublattice and the Te sublattice of c-GST. In the cubic case,average formation energy and moments were computed. The average localcontaining 15 Fe atoms are also presented.

Site E [eV]

Ge 0.76

Sb 0.71

Te 1 3.19

Te 2 2.89

Te 3 2.14

Ge/Sb 0.51 ± 0.08

Te 1.08 ± 0.41

- -

lengths of Fe with the six nearest-neighbor Te atoms range from 2.75 to 2.97 Å at the Sb site and from 2.82 to 2.87 Å at the Ge site. As regards interstitial Fe defects, they turn out to be significantly more costly than substitutional Ge and Sb con-figurations (having formation energies in excess of 1.6 eV). Then we considered a big model of Fe-doped h-GST containing 216 atoms and a 7% concentration of Fe (amounting to 15 Fe atoms): since, as shown above, it is energetically more favorable

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e Fe atom (calculated by integrating the magnetization density in a sphere -equivalent Te atoms (shown in Figure 1a) in h-GST and b) a Fe impurity

for each type of defect, six different configurations were chosen and the moment and total magnetic moment per Fe atom for our model of a-GST

Total moment [μB]

Local moment [μB]

HEXAGONAL

3.89 3.19

4.70 3.43

2.05 1.74

2.38 2.39

1.59 2.36

CUBIC

4.08 ± 0.14 3.18 ± 0.09

1.78 ± 1.00 1.84 ± 0.81

AMORPHOUS

2.76 2.33 ± 0.82

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for Fe to substitute at Ge and Sb sites, Fe atoms were placed randomly at these sites only. This model was fully relaxed using PWSCF and a 2 × 2 × 2 MP mesh. In this model, the impurity states are broadened and form impurity bands; however, due to the low concentration of Fe, its electronic structure (Figure 2a) can be qualitatively understood in terms of the structure of the single Fe impurities discussed above (Figure 1b). Inspection of the total density of states (DOS) and the DOS projected onto Fe 3d levels suggests that the magnetic properties of this system might stem from the interplay of two exchange mechanisms: on the one hand, the small gap of GST and the relatively large con-centration of carriers implies that carrier-mediated p-d exchange plays an important role, as already recognized in Ref. [13]; this mechanism should be enhanced by non-stoichiometric Ge/Sb vacancies (not considered in our model), which are invariably present in crystalline GST and turn it into a p-type semicon-ductor.[27] On the other hand, the large DOS of the minority spin band at the Fermi energy suggests that double exchange could be relevant too.[17]

To generate an amorphous model of Fe-doped GST, in the absence of experimental data on the density of this phase, we started from our model of h-GST and scaled the coordinates

© 2012 WILEY-VCH Verlag GmAdv. Mater. 2012, 24, 1429–1433

Figure 2. a) DOS of h-GST, a-GST and c-GST doped with Fe impurities (Fe ction. The dashed and solid line indicate the total DOS and the DOS projectedmoments of Fe atoms for each phase.

and cell parameters so as to obtain the experimental value of the volume of pure a-GST.[28] The system was heated and equil-ibrated for 10 ps at 2500 K, then it was quenched to 1000 K in 20 ps and equilibrated at this temperature for 50 ps. The amorphous phase was obtained by quenching from 1000 K to 300 K in 100 ps. After 30 ps of equilibration at 300 K, the system was quenched to 0 K and further relaxed using PWSCF.

The resulting structural properties are similar to those reported in Ref. [8] and [9] for pure a-GST (see Figure 3). All Sb and Te atoms and 68% of Ge atoms are in a defective octahe-dral-like geometry with octahedral bonding angles but a lower than six coordination. The remaining Ge atoms are tetrahedrally coordinated. The average coordination numbers of Ge, Sb, Te and Fe, defined using the first minimum in the partial pair cor-relation functions as cutoff distance, are 4.1, 3.5, 2.8 and 5.9 Å, respectively. The distribution of primitive rings is reported in Figure 3b: it has a maximum at four-membered rings, typical of the rocksalt phase. Moreover, the majority of these rings (75%) have ABAB alternation (A = Ge,Sb; B = Te). Due to the presence of Fe, however, a relatively large number of three-membered rings is present, as compared to pure a-GST:[8,9] all but two of these rings contain at least a Fe atom.

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oncentration 7%). All the DOS were calculated within the PBE approxima- onto the 3d states of Fe, respectively. b) Distribution of the local magnetic

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Figure 3. a) g(r) of the model of Fe-doped a-GST (shown in the inset) generated by quenching from the melt. b) Statistics of irreducible n-fold ring configurations of Fe-doped a-GST. c) Distribution of the coordination numbers of different species.

Thereby, for low Fe concentrations, the peculiar structural properties of a-GST (in particular, the large number of four-membered rings and vacancies), which are believed to be the primary cause for fast crystallization,[9,10] are not dramatically affected by the presence of Fe. These results are in line with experiments, which show that the decrease in crystallization rate of a-GST upon moderate Fe doping is small.[13] Of course, the presence of Fe will affect the structural properties of GST much more significantly at larger impurity concentrations close to the solubility limit.

In the amorphous phase, Fe atoms have different local bonding geometries and also different number and type of nearest neighbors: in particular, bond lengths of Fe with neigh-boring atoms vary considerably and, on the average, 25% of Fe neighbors are not Te atoms, in contrast to Fe-doped h-GST. As a result, the local magnetic moments of Fe, which depend sen-sitively on these properties, a) vary considerably from Fe atom to Fe atom and b) on the average, they are smaller than the values found in h-GST for the substitutional Ge and Sb sites (see Figure 2b). This behavior is ultimately due to the fact that the local environment in a-GST is radically different from that of the crystalline phase, in contrast with other systems, such as group IV semiconductors, where the local tetrahedral bonding is preserved in the amorphous phase. The reduction of the local moments should be responsible for the decrease in the saturation magnetization upon amorphization observed experimentally.[13]

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Therefore, the observed magnetic contrast between the two phases provides indirect evidence for a drastic change in local order during amorphization.

We also performed some PBE calculations including a Hub-bard U correction[29] applied to the Fe d states (using values of U up to 5 eV) in order to check the robustness of our results with respect to these corrections: these calculations also yielded high spin states in h-GST and smaller magnetic moments in a-GST. The projected DOS of Fe-doped a-GST (without U cor-rections) are shown in Figure 2a: the minority spin band is rather broad, which reflects the broad distribution of local mag-netic moments. Since a-GST has a lower carrier concentration than h-GST, we expect that the p-d exchange mechanism should be of lesser importance in this phase. This difference should be reflected in the finite-temperature properties of the two phases and probably result in a lower Tc for a-GST.

We considered ferromagnetic models only: in principle, owing to the presence of different environments in both phases (Ge and Sb sites in h-GST), ferrimagnetic configura-tions originating from the interplay of the discussed exchange mechanisms and antiferromagnetic superexchange could also occur. These configurations would yield finite, albeit smaller, magnetizations.

Finally, we have investigated the cubic, rocksalt phase of GST doped with Fe. It is well known that, upon fast laser irradia-tion, GST crystallizes into this metastable phase:[27] it is thereby

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plausible that the phase of the recrystallized sample studied in Ref. [13] is cubic, rather than hexagonal. In this phase, Te occupies one sublattice, whereas 40% Ge, 40% Sb and 20% vacancies are randomly arranged in the second one and the formation energies for substitutional Fe depend sensitively on the local environment of the defect, in particular the first and second coordination shells. Thereby, for each type of defect, we calculated the average formation energy by choosing randomly six configurations. Supercells containing 130 atoms and 1 Fe impurity were used. It turns out that Fe atoms prefer to occupy sites in the Ga/Sb/vacancy sublattice. In this configuration, the average total magnetic moment is of the order of 4.1 μB (see Table 1). Subsequently, we generated a large model of c-GST containing 199 atoms, with Fe atoms (at the same 7% con-centration) on Ge/Sb/vacancy sites only, and relaxed it using PWSCF and a 2 × 2 × 2 MP mesh.

The projected DOS and the distribution of the local mag-netic moments of the relaxed model are shown in Figure 2a. The DOS are similar to those of h-GST and suggest similar exchange mechanisms, however the average magnitude of the local moments is slightly smaller in c-GST. The decrease of the saturation magnetization of the recrystallized phase with respect to the initial phase observed experimentally[13] could thereby be related to the change in crystalline phase, though, in principle, this behavior could also be explained by assuming that, upon fast recrystallization, the system contains additional magnetic defects, such as interstitial and Te-substitutional Fe, having smaller local moments.

In conclusion, we have shown that our models of crystal-line and amorphous Fe-doped GST exhibit a magnetic contrast stemming from a reduction of the magnitude of the magnetic moments of Fe atoms in a-GST, which is related to the fact that the local geometries and chemical environments in a-GST are very different from those of the crystalline phase, at vari-ance with most other classes of materials. Since this peculiar behavior of the amorphous phase is shared by all PCMs along the pseudobinary line GeTe-Sb2Te3, we predict that the mag-netic contrast between the two phases should be a general fea-ture of Fe doped PCMs along this line. We have also shown that the presence of Fe does not affect significantly the struc-tural properties of a-GST (presence of four-membered rings and vacancies), which are believed to be responsible for the fast switching. Finally, we have suggested that different exchange mechanisms should come into play in the two phases and prob-ably result in a lower Curie temperature for a-GST, although this point deserves further investigation.

AcknowledgementsDiscussions with S. Caravati and M. Wuttig are gratefully acknowledged.

Received: December 12, 2011Published online: February 13, 2012

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