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Cite this: Nanoscale, 2014, 6, 14667
Received 21st August 2014,
Accepted 17th October 2014
DOI: 10.1039/c4nr04826a

Real space probe of short-range interaction
between Cr in a ferromagnetic semiconductor
ZnCrTeKen Kanazawa,* Taku Nishimura, Shoji Yoshida, Hidemi Shigekawa and Shinji Kuroda

www.rsc.org/nanoscale

The short-range interaction between Cr atoms was directly examined by scanning tunneling microscopy measurements on a
Zn0.95Cr0.05Te ﬁlm. Our measurements revealed that a Cr atom
formed a localized state within the bandgap of ZnTe and this state
was broadened for a pair of Cr atoms within a distance of ∼1 nm.

In the past few decades, so-called “spintronics”, which utilizes
both the charge and spin characteristics of carriers, has
attracted considerable attention. Diluted magnetic semiconductors (DMSs), in which doped magnetic atoms partially substitute the atomic sites in a host semiconductor, have beenenergetically studied because of their high potential for advancing future semiconductor-based spintronic devices.1 For practical applications, the development of ferromagnetic DMSs
with a higher Curie temperature (Tc) than room temperature(RT) is a key factor. Although the realization of ferromagnetism
with a Tc higher than RT has been claimed for various DMSs,
contradictory results have been reported despite the fact thatthe same DMS materials were analyzed. A possible reason forthis is that the growth of DMSs is very sensitive to the environmental conditions during growth; a small diﬀerence in thegrowth conditions induces the formation of a compositional
fluctuation or a diﬀerent structural phase, which may cause
unexpected ferromagnetism of the synthesized crystals. In
order to determine the intrinsic magnetic properties of the
DMSs studied, it is essential to examine crystals confirmed to
be of the pure diluted phase in which the magnetic impurities
are distributed randomly on the substitutional sites of the host
crystal. In addition, a microscopic analysis of the electronic
states of magnetic impurities, such as spatial distribution andlocal density of states, is indispensable for understanding theinteraction between magnetic impurities and the mechanismof ferromagnetism.
Cr-doped ZnTe is one of the most promising materials for
future semiconductor-spintronic applications. Intrinsic RT

Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573,
Japan. E-mail: kanazawa@ims.tsukuba.ac.jp; Fax: +81 29 853 5102;
Tel: +81 29 853 5102

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ferromagnetism has been observed for Zn1−xCrxTe (x ≈ 0.2).2
Furthermore, the magnetic properties of (Zn,Cr)Te can becontrolled by co-doping carrier dopants.3,4 In particular, bydoping iodine atoms as donors, even when the Cr composition
is much smaller (x ≈ 0.05), a high Tc (∼RT) was observed,
which was considered to be due to the formation of local Crrich regions in which Cr atoms substitute the Zn site at a highconcentration.3 This result is interpreted as a result of theshort-range character of the ferromagnetic interaction between
Cr atoms, which is considered to act eﬀectively between Cr
atoms located within a short distance in the Cr-rich regions.
This is diﬀerent from the case of (Ga,Mn)As, whose ferromagnetic mechanism has been considered to be carriermediated interaction.5 Therefore, a microscopic analysis of thecharacteristics of the Cr–Cr interaction in the ZnTe crystal isessential. A double-exchange or superexchange interaction wastheoretically proposed as a possible mechanism to explain themagnetism in (Zn,Cr)Te,6,7 which, however, has not yet beenclarified experimentally. Here, we present the results obtainedin a scanning tunneling microscopy/spectroscopy (STM/STS)
study on (Zn,Cr)Te. The short-range interaction between Cr
atoms, which is considered to characterize the magnetism in(Zn,Cr)Te, was directly examined in real space for the firsttime.
Fig. 1(a) shows a typical STM topographic image of a
(Zn0.95, Cr0.05)Te/ZnTe buﬀer/p-ZnTe cross-sectional surface(see the Methods section). The interface between the substrate
and the MBE layer was clearly distinguished. Fig. 1(b) and (c)are magnified STM images of the (Zn,Cr)Te and p-ZnTe substrate regions, respectively. In addition to the rows of Te atoms
along the [110] direction, several line defects along the [110]direction can be seen, which correspond to the monoatomicwide rows of vacancies generally observed on a cleaved ZnTe(110) surface.8 From the asymmetry in the brightness on bothsides of the line defects, the distinction of the in-plane crystallographic orientations between [110] and [−110] was determined, as shown in Fig. 1(a).9 In addition to such intrinsic
atomic structures on the cleaved ZnTe surface, the STM image
in Fig. 1(b) shows a number of atomic-scale bright structures

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Fig. 1 STM/STS measurements of (Zn,Cr)Te/p-ZnTe. (a) Cross-sectional
STM topographic image of the (Zn,Cr)Te/p-ZnTe multilayered structure(Vs = +5.0 V, I = 20 pA). (b) and (c) Magniﬁed STM images of the (Zn,Cr)
Te layer (Vs = −2.0 V, I = 40 pA) and the p-ZnTe substrate (Vs = −1.5 V,
I = 60 pA), respectively. (d) Schematic view of the (Zn,Cr)Te unit cell
including a substitutional Cr atom. (e) dI/dV–V spectra averaged overthe (Zn,Cr)Te and ZnTe areas shown in (a).

in the terrace area, which are attributed to doped Cr atoms inthe surface region. To investigate the eﬀect of Cr atoms, STS
measurement was carried out. As shown in Fig. 1(e), the dI/
dV–V curves averaged over the (Zn,Cr)Te and ZnTe areas inFig. 1(a) are diﬀerent from each other. The dI/dV obtained overthe (Zn,Cr)Te area shows the existence of in-gap states. Froman early theoretical study of the electronic configuration of(Zn,Cr)Te,6 these states are considered to be the localizedstates of Cr 3d electrons.
Fig. 2(a) and (b) show a high-resolution STM image of the(Zn,Cr)Te layer and its magnification, respectively. The crosssections along the two lines in Fig. 2(b) are shown in Fig. 2(c).
The in-plane dimension of the Cr image is about 1 nm, considerably smaller than the size of Mn and Fe atoms observedin a GaAs matrix beneath the surface at a similar negative bias
voltage,10 i.e., the acceptor state images of ionized A− andneutral A0 states for Mn and Fe, respectively. In general, ashallow acceptor or donor impurity in a host semiconductor isimaged as a relatively large structure because of its disperse
electronic charge distribution. In contrast, since the Cr2+ statein (Zn,Cr)Te is deep (∼1.3 eV from the valence band top), it
remains in the form of Cr2+ without generating holes in thehost ZnTe even under a tip-induced band bending at a bias
voltage of ∼−2.0 V. Therefore, the observed small size of the Cr
images is also consistent with a deep energy position within
the band gap of Cr atoms substituting the Zn site of the host
crystal. The Cr images were classified into two types withtwo diﬀerent heights, as indicated by H (high) and L (low) in
Fig. 2(b). To clarify the two types of structures, we carried out
a simple density functional theory (DFT) calculation by the
spin-polarized Perdew–Burke–Ernzerhof generalized gradientapproximation (PBE-GGA) using the ABINIT code11 withplane-wave-based norm-conserving pseudopotentials (see theMethods section).

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NanoscaleFig. 2 STM observation of Cr atoms on a (Zn,Cr)Te surface. (a) High
resolution STM image of Cr dopant atoms on a (Zn,Cr)Te(110) surface (Vs= −2.0 V, I = 10 pA). (b) Magniﬁed image of the area indicated by thedotted square in (a). H and L indicate the two types of images with two
diﬀerent heights. (c) Cross sections obtained along the two dashed linesin (b). (d) and (e) Results of the DFT calculations for the Cr atoms locatedin the ﬁrst and second (110) atomic layers, respectively. Spatial distributions are the LDOS integrated over EF − 0.1 eV < E < EF (EF: Fermi
energy). The side-view images below represent cross-sectional drawings
along the dashed lines in the respective top-view images.

Fig. 2(d) and (e) show the DFT results of local density ofstates (LDOS) integrated over EF − 0.1 eV < E < EF (EF: Fermi
energy) for the Cr atoms located in the first and second (110)atomic layers, respectively. A large LDOS area, which is brightin the STM image,12 exists near the topmost Te atoms neighboring the Cr for both structures. The anisotropic shape indicates that the Cr-related states, which contributed to the STM
image, were hybridized with the states of the nearby Te atoms.
The spatial distribution of the LDOS formed by the Cr in thesecond layer is more localized and higher than that formed by
the Cr in the first layer, and these two states are considered tocorrespond to the H and L states observed by STM, respectively. The Cr atoms in deeper sites were not observed unlike
the case of the Mn atoms in a GaAs matrix. This is considered
to be due to the small spatial extent of the Cr states discussed
above.
Next, to investigate the Cr-related electronic states in moredetail, we performed STM/STS analyses at a single atomic
level. Fig. 3(a) shows an STM image of a (Zn,Cr)Te region.
Fig. 3(b)–(d) respectively show magnified images of the three
regions indicated by A, B and C in Fig. 3(a), with circular lowpass filtering, whose radius is insignificantly smaller than the
distance between the central point and the spot generated
from [110] atomic rows, carried out in order to enhance thecontrast of local structures and make the position of themclear. The Cr images labeled #1, 2, 4, 5 and 6 were classified
into the two types indicated in Fig. 2, H and L, by measuring
their heights and positions with respect to the surface latticeof the topmost Te atoms indicated by the dashed lines, andthe schematic structures illustrating their occupying sites in(Zn,Cr)Te are also shown. The additional two images, L in A
and H in C, were determined in a similar way.

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Fig. 3 STM/STS analyses of Cr–Cr interaction. (a) STM image of the (Zn,Cr)Te area (Vs = −2.0 V, I = 20 pA). (b), (c) and (d) Left: magniﬁed images ofthe square areas indicated by A, B and C in (a), respectively. Low-pass ﬁltering was carried out to enhance the local electronic structures. Right:
schematic structures of the atomic conﬁgurations of the STM images on the left. (e) dI/dV–V spectra measured above the Cr atoms labeled #1 to #7
and on the area of ZnTe without Cr in the bottom-left square shown in (a). The inset is a magniﬁcation of the dI/dV–V curves in the square region.

The image labeled #3 was brighter (about 0.3 Å higher) andmore elongated along the [110] direction than the Cr atoms inthe H state. In order to analyze the atomic arrangement, we
performed DFT calculations for the <110> Cr–Cr pair locatedin the first (110) atomic layers. Fig. 4(a) shows the magnification of the STM image labeled #3 in Fig. 3(d), in which thelattice mesh and Zn, Te and Cr atoms are shown together. The
results of the calculations obtained for Vs = −0.1 V (EF − 0.1 eV< E < EF) and Vs = +0.5 V (EF < E < EF + 0.5 eV) are shown in
Fig. 4(c) and (d), respectively. Due to the eﬀect of overlap of
impurity states of the two Cr atoms, the topmost Te atom,which is located at the center between the Cr atoms, has
diﬀerent states from those of the other topmost Te atoms
neighboring Cr. When we compare the calculated distributionof LDOS for these atoms in the occupied state (c) and theunoccupied state (d), the overall features are not diﬀerent fromeach other, but the protrusion of 0.35 Å at the position of thecentral Te atom relative to the other neighboring atoms, whichshould be in a similar state to the “L”, in (d) suggests that theobserved STM image for Cr #3 would be closer to the distribution of the unoccupied state (d). These results indicate that

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Fig. 4 STM image and DFT calculations for the structure labeled 3 inFig. 3. (a) Magniﬁcation of the low-pass ﬁltered image of label 3 shownin Fig. 3(d) and schematic of its structural model. Results of DFT calculations of local density of states (LDOS) integrated over (b) EF − 0.1 eV <E < EF and (c) EF < E < EF + 0.5 eV. (d) Typical LM-STS spectra obtainedon (Zn,Cr)Te measured with a semiconductor laser (λ = 400 nm, lightintensity = 5 mW). For example, the yellow arrow indicates the magnitude of the surface photo voltage at Vs = −2.0 V.

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Table 1 Summary of the conﬁgurations of Cr atoms. The values of Vs atwhich dI/dV–V curves in Fig. 3 rise from zero value are also listed

Cr–Cr pair

Direction

Distance (nm)

3
4-L
5-L
4–5
1–2
6-H

<110>
<123>
<013>
<111>
<123>
<111>

0.43
1.14
0.96
1.06
1.14
2.11

Vs at which dI/dV
rise in Fig. 3(e) (V)
−0.7
−0.8
−0.9
−0.9

despite the fact that the STM image is obtained at a negative
bias voltage, unoccupied states are included. This is considered to be due to the influence of the tip induced band
bending (TIBB). In order to estimate the eﬀect of the TIBB
actually, we performed light-modulated scanning tunnelingspectroscopy (LM-STS)13 with a semiconductor laser, whose
wavelength and light-intensity are 400 nm and 5 mW, respectively. The LM-STS result is shown in Fig. 4(d). With the
chopped laser illumination, the two virtual I–V curves, whichcorrespond to those under dark (Ioﬀ: blue) and illuminated(Ion: red) conditions, can be simultaneously obtained. The
spectrum of the surface photo voltage (SPV), which provides
information about the polarity and magnitude of the surface
band bending under the dark conditions, is obtained by calculating the lateral shift of the two I–V curves with respect to thebias voltage for the I–V curve under the dark conditions (green
circles). The SPV value of about −0.4 V at Vs = 0 V is almost
consistent with the above discussion, in which the STM image
may reflect the unoccupied surface states in the range of EF < E< EF + 0.5 eV despite the observation at negative Vs. For therespective Cr pairs shown in Fig. 3, the distances and the directional relations between the two Cr atoms are summarized inTable 1.
To obtain a deeper understanding of the interaction
between Cr atoms, analysis of the electronic state of Cr 3d electrons on the atomic scale is essential. In fact, as shown in
Fig. 1(e), the localized state of Cr 3d electrons appeared as ingap states in the STS spectrum. To examine the interactions
between Cr atoms, we carried out dI/dV–V measurements.
Fig. 3(e) shows the dI/dV–V spectra measured above the Cratoms labeled #1 to #7 and on the area of ZnTe without Cr in
the bottom-left square shown in Fig. 3(a). The inset of Fig. 3(e)is a magnification of the dI/dV–V curves in the range of bias
voltage −1.0 V < Vs < −0.8 V, in which the dI/dV–V curves risefrom zero value. The spectrum obtained over the area of ZnTe
without Cr is similar to that obtained for the p-type substrateshown in Fig. 1(e). The dI/dV spectra measured above the Cratoms in the atomic scale clearly exhibited a larger DOS thanthat obtained over the area of ZnTe without Cr, similarly to theresults shown in Fig. 1(e). However, the spectra were divided
into two types with diﬀerent behaviors in the range of bias
voltage −1.4 V < Vs < −0.8 V; the dI/dV–V curves measured
above Cr #3, 4, 5, 7 rise at a relatively smaller bias voltage ofaround −0.8 V and have larger values in the negative bias
region −1.4 V < Vs < −0.8 V than those measured above Cr #1,

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2, 6. The bias voltage at which the dI/dV curve starts to risefrom zero for the respective Cr atoms is also listed in Table 1.
This diﬀerence in the dI/dV–V curves shows a clear correlation
with the distance to the neighboring Cr atom shown in
Table 1; the value of dI/dV in this range is large for a Cr atomwith its neighbor within 1.1 nm, while the value is small for a
Cr atom with its neighbor beyond 1.1 nm. Therefore, the larger
values of dI/dV of Cr #3, 4, 5, 7 are interpreted to be a result of
broadening of the Cr localized state due to the interaction with
the neighboring Cr atom. The observation of this broadening
only for Cr pairs with a short distance less than 1 nm indicatesthe short-range character of the interaction between Cr.
The spectra measured above Cr #1, 2, 6, which are considered to be unaﬀected by the interaction, are very similar toeach other, regardless of whether the Cr atom was located in
the first or second layer from the surface. On the other hand, aslight diﬀerence can be seen among the spectra measured
above Cr #3, 4, 5, 7. The spectra of Cr #3, 7 rise at slightly
smaller bias voltages than those of Cr #4, 5, as shown in
Fig. 3(e). Since the distance between Cr atoms in Cr #3, which
consists of the nearest-neighbor pair, is much shorter than Cr#4, 5, the diﬀerence in the spectra is considered to reflect thestrength of the interaction; the Cr localized state is much more
broadened due to a stronger interaction between Cr atoms
within a shorter distance. The spectrum of Cr #7 is almost
similar to that of Cr #3, in spite of the isolated appearance inthe observed STM image. A possible explanation for this apparent contradictory observation is that there is another Cr atomin the nearest neighbor site underneath the observed Cr atom.
To examine the Cr–Cr distance dependence of the spectrain more detail, we calculated the DOS for various atomic
arrangements of Cr–Cr pairs in the host ZnTe crystal using
GGA-PBE. As a host semiconductor matrix, we used a 2 × 2 × 2
supercell of the ZnTe crystal structure. One (0) or two (0 and
one of 1–5) of the 32 Zn sites in the supercell shown in
Fig. 5(a) were replaced by one or two Cr atoms, and the energy
distribution of the spin-dependent DOS was calculated foreach structure.
Fig. 5(b) and (c) respectively show the results obtained for
undoped ZnTe and ZnTe doped with a single Cr. Spin-polarized states were formed in the energy gap region of the host
ZnTe by the doped Cr atom. The calculated DOS was basicallysimilar to the earlier theoretical studies,6,7 though the in-gap
states were located at a shallower energy level relative to thehost valance band than the experimental results of earlier
photo-EPR and optical absorption measurements.14
According to an earlier study,6 the observed in-gap statesconsist mainly of ta states, which originate from Cr 3d-electrons. The five energy levels are degenerate in a free atomic
state; however, when a Cr atom substitutes the Zn site to forma tetrahedral coordination with the neighboring Te atoms,
these energy levels are split into triply-degenerate dε anddoubly-degenerate dγ states by the crystal field. Moreover,because the electronic orbitals of the dε states are strongly
hybridized with the valence electrons of Te atoms, bonding
states tb and anti-bonding states ta are formed in the valenceThis journal is © The Royal Society of Chemistry 2014

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Fig. 5 DFT calculations of DOS for ZnTe and (Zn,Cr)Te structures. (a) Schematic structure of the 2 × 2 × 2 supercell used for calculations. One (0)Zn or two (0 and one of 1–5) Zn atoms of the 32 Zn sites were replaced by one/two Cr atoms in the calculations. (b) DOS obtained for undoped
ZnTe. (c) DOS obtained for the ZnTe doped with a single Cr atom at the site labeled 0. Up and down indicate spin directions. (d) DOS calculated forvarious Cr–Cr conﬁgurations indicated by diﬀerent colored lines. (e) DOS calculated for the Cr–Cr structures with ferromagnetic and antiferromagnetic spin orientations.

band and in the ZnTe band gap, respectively. Since the hybridization between the dγ states and the host valence states isweak, they form non-bonding states, e, at a lower energy position than that of ta. The Cr 3d-electrons and Te p-electrons
occupy the exchange-split hybridized states until the ta levelwith the spin-up state, at which the Fermi level EF is located.
Therefore, the electronic states near the Fermi energy are
dominated by ta impurity states. The Vs = 0 in a STS spectrum
generally corresponds to the EF of the sample surface.
However, in our STS measurement, dI/dV–V curves start todeviate from zero at Vs ∼ −0.9 V for the isolated Cr atom
(Fig. 3(e)), not at Vs = 0. In addition, the result of calculationsin Fig. 5(d) indicates that the DOS starts to rise from zero at a
position ∼0.5 eV above EF. These results, therefore, suggest adiﬀerence of ∼1 eV between the value of Vs in the observed
STS spectra and the value of EF in the calculated DOS. A possible reason for the diﬀerence is the influence of band bendingof ∼1 eV at Vs ∼ −0.9 V. This is in good agreement with theresult of the LM-STS measurements as shown in Fig. 4(d) andthe fact that the STM image of the label #3 includes the unoccupied states despite the negative bias voltage used inmeasurements as pointed out before. The band bending is
mainly caused by the diﬀerence of work functions between
W-tip and (Zn,Cr)Te.15

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When a pair of Cr atoms is located within a short distance,
the half-filled ta states interact with each other due to the ferromagnetic coupling (double exchange mechanism) and theelectronic states are considered to be broadened. Fig. 5(d)shows the calculated DOS for Cr pairs with diﬀerent arrangements, where the configurational dependence of the spectra
can be clearly shown. In particular, a nearest-neighbor Cr pairin the [110] direction exhibited the electronic state with thebroadest energy dispersion and the strongest exchange ferromagnetic interaction estimated from the total energy diﬀerence between antiferromagnetic (AFM) and ferromagnetic(FM) states (EFM–EAFM), as shown in Table 2. These results arealso in good agreement with the experimentally obtained

Table 2 Summary of theoretical calculations of the total energy diﬀerence between the antiferromagnetic (AFM) and ferromagnetic (FM)states (EFM − EAFM)

Cr–Cr pair

EFM − EAFM (eV)<110><110> × 2
<112>
<111>
<001>

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spectra shown in Fig. 3(e). Finally as shown in Fig. 5(e), for an
antiferromagnetic Cr–Cr pair in the [110] direction, the calculated DOS is almost the same as that of a single Cr atom.
Namely, broadening occurs only when the alignment of Cr
spins is ferromagnetic, as expected. Here, we have to refer tothe superexchange interaction as another possible mechanismof the ferromagnetism with short-range character in thissystem. Generally, the superexchange interaction should work
between two Cr atoms in the nearest neighboring arrangement
through a Te atom located between the two Cr atoms.
However, the broadening of the Cr impurity state due to theferromagnetic interaction was observed for the Cr pairs whose
interval is farther than that of the nearest neighboringarrangement. This result suggests that the double exchangeinteraction is suitable to explain the ferromagnetic
mechanism.
As a preceding STM study on the interaction between magnetic impurities in a DMS, Kitchen et al. observed ferromagnetic interaction between Mn atoms in (Ga,Mn)As.16 Theyobserved that the in-gap conductance peak in the STS spectra,
which originates from the electron tunneling into the empty
hole state of Mn2+ ions, split into the bonding and antibonding states for a Mn pair due to the interaction between
Mn. They found that the strength of the interaction, which wasevaluated from the splitting energy of the conductance peaks,
depends strongly on the distance and the directional relation
between Mn atoms; the interaction was the strongest for thenearest neighboring pair along the <110> direction, highly anisotropic, and almost disappeared for pairs beyond the sixnearest neighbors. These observed features of the Mn–Mninteraction cannot be explained by the model of the carriermediated mechanism; the interaction mediated by free holes
should be isotropic and range over a relatively long distance.
Alternatively, the p–d hopping model was proposed as a possible ferromagnetic interaction.17 On the other hand, in thepresent study on (Zn,Cr)Te, we observed the broadening of the
Cr impurity state due to the ferromagnetic interaction between
Cr and found that the interaction depends strongly on the distance between Cr atoms, which is quite similar to the interaction of Mn–Mn pairs reported in ref. 16. This similarity
suggests that the fundamental origin of ferromagnetism inthese DMSs can be based on a similar short-range interaction
directly working between 3d electrons of transition-metal
atoms hybridized with valence p electrons of the host anions.
This description may give a general understanding of ferromagnetic DMSs.
It should be pointed out that the ferromagnetic interaction
appears in a diﬀerent way between (Zn,Cr)Te and (Ga,Mn)As in
the STS measurements: the broadening of the Cr state in (Zn,
Cr)Te in contrast to the splitting of the Mn state in (Ga,Mn)As.
This diﬀerence may reflect the diﬀerence in the relative energy
position of the magnetic impurity to the valence band of thehost semiconductor and in the number of 3d electrons. In thecase of (Zn,Cr)Te, the Cr state appears as a continuum state
extending from the top of the valence band, which wasobserved as a conductance tail in the negative bias region in

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Nanoscalethe STS spectra due to the process of electron tunneling fromthe occupied state. On the other hand, in (Ga,Mn)As, the Mnhole state was observed as an independent conductance peakin the positive bias region in the STS spectra due to theprocess of electron tunneling into the empty state. However, inorder to clarify the issue, we need a further systematic study.
Furthermore, here, the range of the ferromagnetic interaction
observed in this study, which works within the distance of∼1.1 nm, has to be compared to the spatial dimension of
the ferromagnetic domain of ∼4 nm measured by
D. Soundararajan et al. with magnetic force microscopy (MFM)
for a Zn0.95Cr0.05Te film grown on a glass substrate.18 Generally, the dimension of a ferromagnetic domain should belarger than the distance within which a ferromagnetic interaction works between two magnetic dopant atoms. The magnitude relation of these two values in this (Zn,Cr)Te system isconsistent with the generality. Furthermore, since the distance
between Cr atoms in a ferromagnetic domain should besmaller than 1.1 nm, the local Cr concentration in the magnetic domain observed in the latter MFM study is larger than
the average concentration 5%.

Methods
A sample with a multi-layered structure was fabricated bymolecular beam epitaxy (MBE); a buﬀer undoped ZnTe layer(thickness ∼10 nm) and a (Zn0.95, Cr0.05)Te (∼250 nm) layer
were successively grown on a p-type ZnTe (001) substrate(P-doped, 1 × 1018 cm−3). The growth rate was fixed at ∼1 Å s−1
with the substrate temperature maintained at 573 K; underthese growth conditions, most of the Cr atoms randomly
occupy Zn sites in the host ZnTe crystal.3 During the growth
process, the surface was monitored in situ by reflection highenergy electron diﬀraction (RHEED). A clean cross-sectional(110) surface was obtained by cleaving the sample. All STM/
STS measurements were performed with an Omicron lowtemperature (LT) scanning tunneling microscope under ultrahigh-vacuum conditions (<10−8 Pa) at 77 K (Fig. 1) and 8 K(Fig. 2 and 3) using an electrochemically sharpened W tip(ϕ = 0.3 mm).
To calculate the electronic states in the surface region, weconstructed supercells with slab models consisting of fiveZnTe(110) atomic layers and a vacuum region also with a thickness of five atomic layers. The atoms in the topmost layer weredisplaced from the positions of the ideal (110) surface following the surface reconstruction reported by Meyer et al.19 On
the other hand, to model the in-plane periodicity, we used 3 ×
2 supercells for the [110] and [100] directions. Here, each Zn
atom in the slab model was replaced by a Cr atom. Since theSTM tunneling current should reflect the LDOS near the Fermi
energy (EF) of the surface region at the STM tip position, we
calculated the electronic states around EF. The plane-wave
cutoﬀ energy for all calculations was 60 Ry. We used 2 × 2 × 1
Monkhorst-Pack k-point grids to sample the Brillouin zone.
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the LDOS integrated over the energy region of EF − 0.1 eV < E <EF (EF: Fermi energy) for the structures with a Cr atom locatedin the first and second layers, respectively. The obtained LDOS
is highly concentrated near the Cr atom and spreads from the
Cr atom toward the nearest Te atoms. In addition, the LDOS
between the Cr atom and a neighboring Te atom is relatively
small. These results indicate that the electronic states in thisenergy region can be attributed mainly to the ta state.
On the other hand, to calculate the DOS of Cr–Cr pairs in
the 2 × 2 × 2 supercell of the ZnTe crystal structure as shown in
Fig. 5, we used 2 × 2 × 2 Monkhorst-Pack k-point grids.

Conclusions
In summary, we have investigated the local electronic states of
Cr atoms in a diluted magnetic semiconductor (Zn,Cr)Te bycross-sectional STM and STS measurements. We successfully
observed single Cr atoms substituting Zn-sites located in thefirst and second layers from the surface and their impurity
states at an energy position located within the band-gap of thehost ZnTe. By comparing the observed STS spectra for Cr pairs
with the result of theoretical calculations, we concluded thatthe impurity state is broadened for a pair of Cr atoms with adistance shorter than 1.1 nm due to the ferromagnetic interaction, which is in good agreement with a short-range character of the ferromagnetic mechanism based on the double
exchange interaction proposed for this system. These findings
are important for understanding the origin of magnetic interaction in DMSs. They may also provide helpful information for
the realization of high-temperature ferromagnetism using aproper bottom-up approach to control the arrangement ofmagnetic atoms, for example, atom manipulation by scanningprobe microscopy or site selective growth by MBE.

Acknowledgements
Support from the Japan Society for the Promotion of Sciencein the form of Grants-in-Aid for Scientific Research is
acknowledged.

Notes and references
1 D. Nikonov and G. Bourianoﬀ, J. Supercond. Novel Magn.,
2008, 21, 479; C. Liu, F. Yun and H. C. Morkoc, J. Mater.

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