Quantum Beam Science Research

5-8 Unmasking the Ferromagnetism in Magnetically-Doped Topological Insulators
ᴪ Toward Ultra-Low Power-Consumption Spintronic Devices ᴪ252p→3d core absorption
L3Intensity

L2

T = 5 K H = 0.1 T

15

L2

Photon energy

100Intensity

M

Cr0.05(Sb0.9Bi0.1)1.95Te3
20

Difference＝XMCD

Intensity (a.u.)

Circ. pol. light
Photon spin

L3Sample
j=3/2 j=1/2

Te 3d5/2
䚐5pCr 2p3/2
䚐3dCr 2p1/2
䚐3d5(a)00.20Spin-orbit coupling

Fig.5-22 XMCD schematics in core-level absorptionThe XMCD is defined as the difference in absorption intensity
between left- and right-polarized light.

−0.2
−0.4
−0.6Crx(Sb1−yBiy)2−xTe3

Te 3d5/2
䚐5px= 0.05x= 0.15−0.8(b)
570

1.5

580

585

590

595

Photon energy (eV)Cr0.05(Sb0.7Bi0.3)1.95Te3I+I−Fig.5-23 Experimental spectra of Crx(Sb1−yBiy)2−xTe3 at theCr 2p→3d and Te 3d→5p edges
XAS (a) and XMCD (b) spectra measured at 5 K and 0.1 T.

Sb 3d3/2䚐5p

1.0Intensity (a.u.)

5750.5(c)Sb 3d5/2䚐5p00.005

Cr0.05(Sb0.7Bi0.3)1.95Te3

T=5K
H = 0.1 T0(Sb0.5Bi0.5)2Te3 [no Cr]−0.005525

530

535(d)
540545

Fig.5-24 Experimental spectra of Crx(Sb1−yBiy)2−xTe3 at the
Sb 3d→5p edges
XAS (c) and XMCD (d) spectra measured at 5 K and 0.1 T.

Photon energy (eV)Recently, topological insulators (TIs), characterized asbulk insulators with conducting states on their surface, haveattracted attention globally owing to the quantum-anomalousHall effect (QAHE) discovered in ferromagnetic Cr-doped(Sb,Bi)2Te3 TI at very low temperatures. The QAHE enables
electric currents to flow with little energy consumption. To
realize QAHE at room temperature, unmasking the underlying
physics behind the ferromagnetism is required. We have
performed an X-ray magnetic-circular dichroism (XMCD)
experiment at the BL23SU of SPring-8 and revealed the originof ferromagnetism by detecting not only the magnetic momentof Cr 3d electrons but also those of “nonmagnetic” Sb and Te
5p electrons. The XMCD spectrum is deﬁned as the difference
between the core-absorption spectra (XAS) of the left- andright-handed circularly polarized light, as schematically shownin Fig.5-22.
Fig.5-23(a) shows the Cr 2p → 3d XAS spectra ofCrx(Sb1−yBiy)2−xTe3 (x = 0.05, y = 0.1, TC = 15 K) measured
at a magnetic field of 0.1 Tesla and a temperature of 5 K.
Fig.5-23(b) shows the XMCD spectra for two different Cr
contents (x = 0.05, 0.15). We ﬁnd that the signals are negativeand positive at the 2p3/2→3d and 2p1/2→3d edges, respectively,

which signifies that the Cr 3d electrons are responsible forferromagnetism. As indicated by an arrow in Fig.5-23(b),a negative signal at the Te 3d5/2→5p edge below the Cr 2p3/2
edge is observed. This tells us that the magnetic moments of
the Te 5p and Cr 3d electrons are coupled in an antiparallelmanner.
Fig.5-24 shows the Sb 3d→5p XAS (c) and the associated
XMCD (d) spectra for Cr0.05(Sb0.7Bi0.3)1.95Te3. Here, we findpositive and negative signals at the 3d5/2→5p and 3d3/2→5pedges, respectively. No XMCD spectrum is found for thesample without Cr doping [(Sb0.5Bi0.5)2Te3]. The result tells
us that the Sb 5p moment is aligned parallel to the Cr 3d
magnetic moment. Therefore, we confirm that the Cr 3d
magnetic moments are mediated by the Te and Sb 5p holes,
which work as a glue for the largely separated Cr spins in thecrystal and should be a key to the ferromagnetism of magnetic
TI Crx(Sb1−yBiy)2−xTe3.
Our results will open the way to the future material designof QAHE systems and may provide a guideline for practicalapplication of next-generation ultra-low power-consumption
devices using TIs.

ReferenceYe, M., Saitoh, Y. et al., Carrier-Mediated Ferromagnetism in the Magnetic Topological Insulator Cr-Doped (Sb,Bi)2Te3, Nature Communications, vol.6,
2015, p.8913-1-8913-7.

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JAEA R&D Review 2016-17