Transport and magnetic properties of Cr-, Fe-, Cu-doped topologicalinsulators

Y. H. Choi,1 N. H. Jo,1 K. J. Lee,1 J. B. Yoon,2 C. Y. You,2 and M. H. Jung1,a)

1Department of Physics, Sogang University, Seoul 121-742, South Korea2Department of Physics, Inha University, Incheon 402-751, South Korea

(Presented 15 November 2010; received 24 September 2010; accepted 9 November 2010; published

online 23 March 2011)

We report a new class of three-dimensional topological insulators of transition metal doped Bi2Se3,

TrxBi2Se3, and Bi2-xTrxSe3 (Tr ¼ Cr, Fe, Cu) with x ¼ 0.15, for the intercalation and substitution cases.

With varying the doping atoms, a great variety of properties are observed in a single crystal form. The

Cu-intercalated crystal shows superconducting behavior below 3 K, where the diamagnetic signal is

found. The increase of carrier density at low temperature is likely to be responsible for the

superconductivity. The magnetically doped case of the Fe-substituted Bi2Se3 exhibits dominance of

ferromagnetic interactions, whereas the Cr-substituted Bi2Se3 favors antiferromagnetic interactions.

From these results, we learn that the peculiar transport and magnetic properties of topological insulator

Bi2Se3 are easily tunable by chemical doping elements, which change the Fermi energy and manipulate

the charge carriers. VC 2011 American Institute of Physics. [doi:10.1063/1.3549553]

I. INTRODUCTION

One of the big issues in condensed matter physics is the

discovery of topological insulators. Topological insulators

are new materials that have a bulk bandgap, but have con-

ducting surface state which is protected by time-reversal

symmetry. Quantum Hall effect was led to the discovery of

topological order for the past years.1 The quantum spin Hall

insulator states were reported in 2-dimensional (2D) topolog-

ical insulators such as graphene2 and HgTe quantum wells.3

Recently, the 3D topological insulator properties have been

observed in BiSb, Sb, Bi2Se3, Bi2Te3, and the conducting

surface states have been confirmed by angle-resolved photo-

emission spectroscopy (ARPES).4–7 Because of the peculiar

topological order parameter, the properties are easily tunable

by controlling the Fermi level as well as manipulating the

charge carriers through chemical doping. For example, it has

been reported that Bi2Se3 can be converted into supercon-

ductor by Cu intercalation.8 Furthermore, the ARPES experi-

ments have shown that the Fe substitution of Bi2Se3 results

in opening a gap at the surface state.9 Theoretically, the 3D

topological insulators of Bi2Te3, Bi2Se3, and Sb2Te3 has

been suggested to become magnetically ordered insulators

by doping with transition metal elements such as Cr and

Fe.10 Nevertheless, there has been no transport and magnetic

information enough to underline the peculiar tunable states

by doping. Thus, we report the transport and magnetic prop-

erties of Bi2Se3 with different doped atoms such as Cr, Fe,

and Cu for both substitution and intercalation cases.

II. EXPERIMENTAL

The single crystals of TrxBi2Se3 and Bi2-xTrxSe3 (Tr

¼ Cr, Fe, Cu) were grown by melting a stoichiometric

mixture. The mixture of high-purity Bi, Se, and Tr elements

was sealed in evacuated quartz ampoules. The ampoule

was heated up to 850 oC for 12 hs and was kept at that tem-

perature for 1 h. Then, it was slowly cooled to 620 oC for 46

hs and was quenched in cold water. The obtained crystals

were easily cleaved along the plane with shiny flat surface.

X-ray diffraction (XRD) measurements were carried out by

using Bruker D8 diffractometer with Cu K a radiation. The

samples were characterized by using electron probe micro-

analyzer (EPMA) and inductively coupled plasma (ICP)

spectrometer. The transport properties were measured

by using Quantum Design physical property measurement

system (PPMS). The superconducting quantum interference

device-vibrating sample magnetometer (SQUID-VSM) from

Quantum Design was used to measure the magnetic

properties.

III. RESULTS AND DISCUSSION

Figure 1 shows the XRD data for all single crystals. It

is clear that the samples are single phased with the rhombo-

hedral structure of Bi2Se3 (R3m space group).11 Most of

XRD peaks correspond to (0 0 L) reflections, indicating the

cleaved surface is the ab plane. No significant difference

between intercalated and substituted samples (TrxBi2Se3 and

Bi2-xTrxSe3, respectively) is observed, in contrast to the pre-

vious report on Cu-intercalated Bi2Se3 where the c-axis lat-

tice is increased.8 A splitting in the peaks around 60 and 70

degree occurs due to the difference in wavelengths in the

x-ray source of Cu K a1 and Cu K a2 radiations used to mea-

sure the diffraction patterns. Furthermore, an additional peak

around 40 degree for Cr-substituted and Fe-intercalated

Bi2Se3 might be detected because the single crystals are

mounted with a slight tilt angle or a slight misalignment of

the crystallinity. The stoichiometric ratio was nominally

checked by the EPMA and ICP techniques. We could check

a)Author to whom correspondence should be addressed. Electronic mail:

mhjung@sogang.ac.kr.

0021-8979/2011/109(7)/07E312/3/$30.00 VC 2011 American Institute of Physics109, 07E312-1

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the stoichiometric composition of roughly Bi: Se ¼ 2:3 and

the corresponding doping level. A small deviation from the

stoichiometric ratio might stem from the composition gradi-

ent inside the quartz ampoule.9

We measured the temperature dependence of electrical

resistivity, which shows typical metallic behavior for all sin-

gle crystals of TrxBi2Se3 and Bi2-xTrxSe3 (Tr ¼ Cr, Fe, Cu)

with x ¼ 0.15. In Fig. 2, the typical resistivity curve for the

Cu-substituted Bi2Se3 is displayed. It is known that the as-

grown crystals of Bi2Se3 display metallic behavior because

the Fermi energy lies in the conduction band due to the gen-

eration of electrons donated by Se vacancies.12,13 By chemi-

cal doping, we can drive the Fermi level inside the energy

gap, leading to nonmetallic behavior. For example, nonme-

tallic resistivity profile was observed in Bi2-xCaxSe3 with a

narrow doping window 0.002< x< 0.0025,12 and p-type

conducting profile was observed with higher doping level.13

However, all resistivity curves of TrxBi2Se3 and Bi2-xTrxSe3

(Tr¼Cr, Fe, Cu) for x¼ 0.15 show metallic behavior, which

is expected for Se vacancies. This result implies that there is

no significant change of the Fermi energy by doping for Cr,

Fe, and Cu.

The charge carrier type was found by the Hall measure-

ment. The sign of linear slope of Hall voltage versus mag-

netic field is negative, giving rise to the n-type carriers for

all the crystals. The estimated carrier density of the as-grown

Bi2Se3 is approximately 5.8 � 1019/cm3, independent of tem-

perature. For the Cu doped cases, the Cu-substituted BiSe3

shows a temperature independence of carrier density �2.7

� 1018/cm3 between 50 and 300 K, while the Cu-intercalated

Bi2Se3 shows a strong temperature dependence of carrier

density �1.7 � 1018/cm3 at 300 K and 2.2 � 1020/cm3 at

50 K. The increment of carrier density at low temperature in

the Cu-intercalated Bi2Se3 could be responsible for the

superconducting transition with TC ¼ 3 K. In the inset of

Fig. 2, a diamagnetic signal below TC is verified for the

superconductivity. The origin of nonzero resistivity below

TC is the small amount of superconducting volume fraction,

as described in Ref. 8. Note that at 300 K the carrier density

for the Cu-intercalated Bi2Se3 is lower than that for the

Cu-substituted Bi2Se3. This tendency is common for the Cr

and Fe doped cases, where the carrier density is independent

of temperature. For example, the carrier density of the Cr-

intercalated Bi2Se3 is 1 order of magnitude lower than that

of the Cr-substituted Bi2Se3. On the other hand, the mobility

found in the intercalated samples is much higher. Consider-

ing the temperature independence of carrier density, it is

obvious that the mobility is increased as the temperature is

lowered because of the metallic resistivity behavior. For

example, the mobility of the Cr-substituted Bi2Se3 is about

2000 cm2/Vs at 50 K, which is 1 order of magnitude is

higher than that at 300 K.

We have also studied the magnetic properties of

TrxBi2Se3 and Bi2-xTrxSe3 (Tr ¼ Cr, Fe, Cu) with x ¼ 0.15.

Representative magnetic data for the Cr- and Fe-substituted

Bi2Se3 crystals are displayed in Fig. 3. The magnetic suscep-

tibility of the as-grown Bi2Se3 is small (�8� 10-6 emu/mol),

which is independent of temperature. For the Cr-substituted

Bi2Se3, the high-temperature data satisfy the Curie-Weiss

law given by v ¼ v0 þ C/(T�hP) where v0 is the temperature

independent susceptibility, C the Curie constant, and hP the

paramagnetic Curie temperature. From this fit, we could

obtain the effective magnetic moment leff ¼ 0.95 lB/f.u. and

the paramagnetic Curie temperature hP ¼ �229.8 K. Assum-

ing Cr is divalent with 4.9 lB, from the obtained leff value

the substituted content of Cr ion is estimated to be about

19%, which is close to the Cr starting composition x ¼ 0.15.

The negative large value of hP implies a strong antiferromag-

netic exchange in the Cr-substituted Bi2Se3. The linear de-

pendence of the magnetization versus field curve at low

temperature is likely to result from the antiferromagnetism,

although we cannot rule out another possible origin of para-

magnetism. On the other hand, the magnetic susceptibility of

the Fe-substituted Bi2Se3 yields the effective magnetic

moment leff ¼ 0.69 lB/f.u. and the paramagnetic Curie tem-

perature hP ¼ þ5.2 K. The estimated composition of Fe ion

is approximately 13% from the assumption of divalent Fe

ion and 18% from trivalent Fe ion. The positive hP value

implies a ferromagnetic exchange, which is manifested in

FIG. 2. Temperature dependence of electrical resistivity for the Cu-substituted

Bi2Se3. The inset represents the magnetic susceptibility of the Cu-intercalated

Bi2Se3.

FIG. 1. X-ray diffraction patterns of Bi2Se3 with different doping elements

of Cr, Fe, and Cu for both intercalation and substitution cases,

i.e., TrxBi2Se3 and Bi2-xTrxSe3 (Tr ¼ Cr, Fe, Cu) with x ¼ 0.15.

07E312-2 Choi et al. J. Appl. Phys. 109, 07E312 (2011)

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the magnetization curve showing ferromagnetic behavior at

low temperature. This ferromagnetism is similar to previous

reports on highly Fe-doped Bi2Se3,9 but there are no anoma-

lies at ferrimagnetic transition temperatures of Fe-Se alloy.14

Thus, we suggest that this ferromagnetism is attributed to the

intrinsic Fe doping effect on Bi2Se3, rather than any other

ferromagnetic inclusion. For the Cr- and Fe-intercalated

cases that are not shown here, we obtained much larger val-

ues of the effective magnetic moment than the expected val-

ues. This may result from the randomly distributed Cr and

Fe inclusion. Further studies are needed to address the com-

plicated and anomalous magnetic properties affected by

chemical doping into Bi2Se3.

IV. CONCLUSION

The chemical doping effects of Bi2Se3 including substi-

tution and intercalation were studied by changing the doping

elements of Cr, Fe, and Cu. As reported earlier, we found

superconducting behavior in the Cu-intercalated crystal and

nonsuperconducting behavior in the Cu-substituted crystal.

The carrier density for the Cu-intercalated Bi2Se3 at low

temperature is 2 orders of magnitude larger than others. This

increase of carrier density is closely related with the appear-

ance of superconductivity. Even though the un-doped Bi2Se3

shows a temperature independence of magnetic susceptibil-

ity, the Fe and Cr doped crystals tend to be ferromagnetic

and antiferromagneic, respectively. The different magnetic

interactions depending on the doping elements are of great

interests because it may spawn a new class of studies on

topologically magnetic insulators in the field of spintronics.

ACKNOWLEDGMENTS

This work was supported by Basic Science Research

Program through the National Research Foundation of Korea

(NRF) funded by the Ministry of Education, Science and

Technology (KRF-2010-0005427) and by IT R&D program

of MKE/KEIT (2009-F-004-01, STT-MRAM).

1D. J. Thouless, M. Kohmoto, M. P. Nightingale, and M. den Nijs, Phys.

Rev. Lett. 49, 405 (1982).2C. L. Kane and E. J. Mele, Phys. Rev. Lett. 95, 226801 (2005).3M. Konig, S. Wiedmann, C. Brune, A. Roth, H. Buhmann, L. W. Molen-

kamp, X. L. Qi, and S. C. Zhang, Science 318, 766 (2007).4D. Hsieh, D. Qian, L. Wray, Y. Xia, Y. S. Hor, R. J. Cava, and M. Z.

Hasan, Nature 452, 970 (2008).5D. Hsieh, Y. Xia, L. Wray, D. Qian, A. Pal, J. H. Dil, J. Osterwalder,

F. Meier, G. Bihlmayer, C. L. Kane, Y. S. Hor, R. J. Cava, and M. Z.

Hasan, Science 323, 919 (2009).6Y. Xia, D. Qian, D. Hsieh, L. Wray, A. Pal, H. Lin, A. Bansil, D. Grauer,

Y. S. Hor, R. J. Cava, and M. Z. Hasan, Nature Phys. 5, 398 (2009).7Y. L. Chen, J. G. Analytis, J. H. Chu, Z. K. Liu, S. K. Mo, X. L. Qi,

H. J. Zhang, D. H. Lu, X. Dai, Z. Fang, S. C. Zhang, I. R. Fisher, Z. Hus-

sain, and Z.-X. Shen, Science 325, 178 (2009).8Y. S. Hor, A. J. Williams, J. G. Checkelsky, P. Roushan, J. Seo, Q. Xu,

H. W. Zandbergen, A. Yazdani, N. P. Ong, and R. J. Cava, Phys. Rev.

Lett. 104, 057001 (2010).9Y. L. Chen, J. H. Chu, J. G. Analytis, Z. K. Liu, K. Igarashi, H. H. Kuo,

X. L. Qi, S. K. Mo, R. G. Moore, D. H. Lu, M. Hashimoto, T. Sasagawa,

S. C. Zhang, I. R. Fisher, Z. Hussain, and Z. X. Shen, Science 329, 659

(2010).10R. Yu, W. Zhang, H.-J. Zhang, S.-C. Zhang, X. Dai, and Z. Fang1, Science

329, 61 (2010).11H. Lind and S. Linden, Solid State Sci. 5, 47 (2003).12J. G. Checkelsky, Y. S. Hor, M. H. Liu, D. X. Qu, R. J. Cava, and N. P.

Ong, Phys. Rev. Lett. 103, 246601 (2009).13Y. S. Hor, A. Richardella, P. Roushan, Y. Xia, J. G. Checkelsky, A. Yaz-

dani, M. Z. Hasan, N. P. Ong, and R. J. Cava, Phys. Rev. B 79, 195208

(2009).14C. R. Lin, Yu-Jhan Siao, Shin-Zong Lu, and Chie Gau, IEEE Trans. Mag.

45, 4275 (2009).

FIG. 3. Temperature dependence of magnetic susceptibility for the Cr- and

Fe-substituted Bi2Se3 measured in a field of 20 kOe and 30 kOe, respec-

tively. The insets represent the corresponding magnetization vs. field curves

measured at 20 K, 30 K, and 300 K.

07E312-3 Choi et al. J. Appl. Phys. 109, 07E312 (2011)

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