VOLUME 80, NUMBER 20

PHYSICAL REVIEW LETTERS

18 MAY 1998

Phase Transition between Dimerized-Antiferromagnetic and Uniform-Antiferromagnetic
Phases in the Impurity-Doped Spin-Peierls Cuprate CuGeO 3
T. Masuda,* A. Fujioka, Y. Uchiyama, I. Tsukada, and K. Uchinokura
Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan(Received 9 January 1998)
We report a first-order phase transition between dimerized-antiferromagnetic and uniformantiferromagnetic phases in impurity-doped spin-Peierls (SP) cuprate Cu12x Mgx GeO3 . As Mg
concentration increases, linear reduction of the SP transition temperature (TSP ) and linear increase of
the Néel temperature (TN ) are observed for x up to xc ⯝ 0.023. At xc the SP transition suddenly
disappears and TN jumps discontinuously. The peak of the susceptibility at xc around TN is not as sharp
as those at other concentrations, which indicates the separation of low and high concentration phases.
These results indicate the existence of a first-order phase transition between dimerized-antiferromagnetic
and uniform-antiferromagnetic long-range orders. [S0031-9007(98)06165-1]
PACS numbers: 75.10.Jm, 75.30.Kz, 75.50.Ee

Since Hase, Terasaki, and Uchinokura discovered thefirst inorganic spin-Peierls (SP) material CuGeO3 in 1993
[1], this material has attracted much attention. Soon
after that, the substitution effect of nonmagnetic impurity(Zn21 ) for Cu21 was studied by Hase et al. [2], and a newmagnetic phase was discovered below the spin-Peierls
transition temperature (TSP ), which turned out to haveantiferromagnetic long-range order (AF-LRO) [3,4]. The
neutron scattering experiments were studied on Si- [5] andZn-doped [6,7] CuGeO3 , and both dimerization superlattice peak and AF magnetic peak were observed.
Fukuyama et al. explained the coexistence of the dimerization and the AF-LRO in CuGe12x Six O3 using phase
Hamiltonian [8]. According to their theory, both dimerization and 具S z 典 of spins on Cu21 ions have spatiallyinhomogeneous distribution. Recent mSR study on Znand Si-doped CuGeO3 indicated the spatial inhomogeneityof 具S z 典 of spins on Cu21 ions in AF-LRO phase [9], which
supports the theory of Fukuyama et al.
Transition temperature vs impurity concentration (T-x)
phase diagrams have been reported on Zn- and Si-doped
CuGeO3 [6,10–12]. In both cases Néel temperature (TN )increases gradually, reaches its maximum, and decreases
moderately. The TSP decreases linearly as x increases.
However, in the case of Zn-doped CuGeO3 , TSP wasreported to have a plateau in highly doped region [6],while in the case of Si-doped CuGeO3 the corresponding
plateau was not observed [11]. The T-x phase diagram iscontroversial in the relatively highly doped region, and thestudy on the substitution by other species of impurities isneeded.
In this paper we study the T -x phase diagram in
Cu12x Mgx GeO3 in detail and report (a) the clear disappearance of TSP , the corresponding jump of TN , and(b) the existence of different AF-LRO’s with and without
the lattice dimerization.
All single crystals were grown by a floating-zone
method. A typical dimension of the grown crystals is

about 4–5 mm in diameter and about 4–8 cm in length.
The true concentration of impurity x was determined byinductively coupled plasma atomic emission spectroscopy(ICP-AES). We use Ar gas as a plasma source andperform quantitative analysis by the calibration curve
method. The x for Mg is over 80% of nominal concentration xnom for 0 # xnom , 0.1. This is in contrast to thatin Zn-doped CuGeO3 [7], where the ratio is below 80%.
This means that Mg is more easily doped to the Cu site
and is expected to be a more adequate impurity than Zn forthe study of the substitution effect of nonmagnetic ions.
This is one of the reasons why we have reinvestigated the
T -x phase diagram in detail in Mg-doped CuGeO3 . The
absence of impurity phase or structure change with x was
confirmed by x-ray diffraction after pulverization of thesingle crystals at room temperature.
Measurements of dc magnetic susceptibility were performed with commercial SQUID magnetometer (x-MAG,
Conductus Co., Ltd.) for 34 samples (0 # x , 0.089).
The susceptibility changes anisotropically at low temperatures as shown in Fig. 1. We can see that (a) Mg
can be doped [12,13], (b) Mg-doping induces AF-LRO asin Zn- [4], Ni- [10,14], Mn- [3], and Co-doped CuGeO3[15], and (c) the magnetic easy axis is along the c axis
below TN , which is the same as in the case of Zn-doped
CuGeO3 [4]. Both TN and TSP were determined from thecrossing points of linear functions fitted to the susceptibility in applied field parallel to the c axis [xc 共T兲] aboveand below the transitions.
Fisher theorized that the magnetic heat capacity of a“simple” antiferromagnet is proportional to ≠共xk T 兲兾≠T
and TN is best determined by the maximum in ≠共xk T 兲兾≠T
(xk is the susceptibility along the easy axis, which corresponds to xc in this case) [16]. The maximum in xk ,therefore, occurs at a temperature slightly higher than TN .
Experimental verification of this suggestion has been reported in several antiferromagnets [16]. We analyzed
some of the data by this method and get, e.g., TN 苷 4.3 K

4566© 1998 The American Physical Society

0031-9007兾98兾80(20)兾4566(4)$15.00

VOLUME 80, NUMBER 20

PHYSICAL REVIEW LETTERS

FIG. 1. Temperature dependence of susceptibility on thesample of x 苷 0.041 in the field applied parallel to the threeprincipal axes. Néel transition at 4.4 K is observed.

for the sample with x 苷 0.035 (note that the temperature step was 0.1 K). This value is closer to the TN 苷
4.2 K, determined from our heat capacity measurement,
than to the value 4.5 K, determined from the maximum inxc . In the present paper, however, we determine TN by
the method described previously because Fisher’s method
can be applied to a simple antiferromagnet, to which the
low-concentration antiferromagnetic phase in Mg-doped
CuGeO3 does not belong and because in the present study
the change of TN with x is more important than the absolute value of TN .
Figure 2 shows Mg concentration dependence of TSP
and TN : T -x phase diagram. TN increases from 3.4 to
4.2 K abruptly at x ⯝ 0.023 and reaches its maximum.
We define this critical concentration as xc . TN has aplateau at xc , x & 0.04 and decreases smoothly at x *0.04. The Néel transition was not observed in the sample
of x 苷 0.089 above 1.9 K. On the other hand, TSP
reduces linearly from 14.2 K of pure CuGeO3 and suddenly
disappears at xc around 10 K and is not observed at x . xc .
Figure 3(a) shows xc 共T 兲 of Mg-doped CuGeO3 [x 苷
0.019, 0.023共⯝ xc 兲, 0.028, and 0.082]. Figures 3(b)–3(e)show the same data as in Fig. 3(a) near TN . Below andeven above xc sharp transitions are observed in Figs. 3(b),3(d), and 3(e). The measurements were done in thesteps of 0.1 K, and the broadening of the peaks was notobserved. Therefore the errors of TN are less than 0.05 K
at these x’s. At xc , however, the broadening of the peaks
is observed as shown in Fig. 3(c). This behavior indicatesthe existence of two transition temperatures T1 and T2 ,which is caused by a phase separation into low and highconcentration phases. It is noted that a phase separation
always appears in the case of a first-order phase transition.

18 MAY 1998

FIG. 2. The T -x phase diagram of Cu12x Mgx GeO3 . Circles
and squares indicate TSP and TN , respectively. At x 苷
0.023 jump of TN and sudden disappearance of TSP are
observed. Filled triangles represent T1 (upward triangle) andT2 (downward one) at xc , which are determined as shown in
Fig. 3(c). SP and P mean spin-Peierls and paramagnetic states.
The meanings of D-AF and U-AF are explained in the text.

Here we analyzed the data by fitting three linear functionsof T and determined crossing points as T1 and T2 . Theseare 3.43 and 3.98 K at x 苷 0.023 [Fig. 3(c)].
According to the susceptibility data we can explain
Fig. 2 as follows. First, the jump of TN at x 苷 xc
indicates that AF-LRO at x , xc and x . xc belong toessentially different phases, and there is a distinct phasetransition between them.
Second, the disappearance of the SP transition at xc
implies that the lattice dimerization is absent; i.e., thelattice is uniform in the region of x . xc . Therefore it
is inferred that at T , TN the lattice remains uniform.
We define this phase as the uniform-antiferromagnetic
phase (U-AF phase). The U-AF phase is supposed to
be classical; there is no spatial inhomogeneity of 具S z 典
of the spins on Cu21 ions. In the sample of x 苷 0.041
the absence of dimerization was confirmed by neutron
diffraction measurement down to 1.3 K [17]. On the other
hand, in the region of x , xc the lattice is dimerized
below TSP . It is expected that the lattice is dimerized
below TN , which was also confirmed by neutron scattering
measurement on the sample of x 苷 0.017 [17]. There
should be spatial inhomogeneity of Cu spins as is claimed
so far in Si-doped CuGeO3 [8]. Here we define this phaseas dimerized-antiferromagnetic phase (D-AF phase).
Last, the broad peak of xc 共T 兲 in the sample of x ⯝ xc
indicates the transition from D-AF to U-AF phases is thefirst order as x is varied. The displacement of Cu21 ion,d, from a uniform lattice changes abruptly from finite
value to zero at x 苷 xc . As briefly mentioned previously
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VOLUME 80, NUMBER 20

(a)

8.2%
2.8%
2.3%
1.9%

2.0

-5

Susceptibility (10 emu/g )2.5

PHYSICAL REVIEW LETTERS

1.5

1.0

0.5

0.0
0

5

10

1.0

20

2.3%

(c)

1.0

(b)

1.9%
0.8

0.8

2.65 K-5

Susceptibility (10 emu/g )15

Temperature (K)

0.6
2.0
1.0

2.5

3.0

3.5

0.6
3.0

3.43 K 3.98 K
3.5

4.0

4.5

2.6

(d)

2.8%

(e)

2.4
0.8

4.27 K

2.2

2.12 K
0.6
3.5

4.0

4.5

2.0
5.0

2.0

2.5

8.2%

3.0

Temperature (K)FIG. 3. (a) xc 共T 兲 of Cu12x Mgx GeO3 with x 苷 0.019, 0.023(⯝ xc ), 0.028, and 0.082. (b) – (e) xc 共T 兲 near TN . While below
and above xc the peaks are sharp as shown in ( b), (d), and (e),at x ⯝ xc the peak is broad as shown in (c). We determined
the transition temperatures T1 and T2 at x 苷 xc as crossing
points of fitted three linear functions of T. T1 苷 3.43 K andT2 苷 3.98 K in (c).

the absence of the structure change with x was confirmedby x-ray diffraction at room temperature.
Once we know the presence of the first-order phasetransition in Cu12x Mgx GeO3 , it becomes important toreview T -x phase diagrams of Cu12x Mx GeO3 (M 苷
impurity). In the case of Zn-doped CuGeO3 , the absenceof TN between 3.0 and 4.2 K at x ⬃ 0.017 was observed(see Fig. 2 of Ref. [18]). This suggests that the first-orderphase transition also exists in this system. However, thejump of TN and the corresponding disappearance of the
SP transition have not been clearly confirmed so far. This,
we think, is because the distribution of Zn in the sample isnot so uniform as that of Mg, and the phase boundary was
disturbed by this effect. In the case of Ni-doped CuGeO3 ,sudden disappearance of TSP and abrupt increase of TN4568

18 MAY 1998

from 2.5 to 3.6 K at x 苷 0.020 were clearly observed[14]. This corresponds to the phase transition observed in
Cu12x Mgx GeO3 . However, the behavior is more complex
owing to the difference of the easy axis (nearly parallel tothe a axis in Ni-doped CuGeO3 ) [10,14], and the detailwill be discussed separately [14].
The plateau of TSP at relatively large x is observed by
the neutron diffraction, but only very weakly by the susceptibility measurement, in the case of Zn-doped CuGeO3[6,7]. This may also be explained by spatial variation ofZn concentration. Scattering from low concentration (i.e.,
dimerized) region can be observed by the neutron diffraction even though the volume of that region is small. On
the other hand, the susceptibility measurement detects theaverage property of a sample. From the above discussion
the first-order phase transition seems to be universal for
all dopants, at least in the case of doping to the Cu site.
The results of antiferromagnetic resonance [19] and ofangular dependence of magnetization vs magnetic field[14] on Zn-doped (4%) CuGeO3 were explained very wellusing the mean-field sublattice model [14,19]. This may
also be explained by the fact that the magnetic phaseof these samples at x . xc is perfectly classical U-AF.
Different behaviors are expected in the samples at x , xc .
While D-AF phase has AF-LRO characteristic to SP
state, U-AF phase has classical AF-LRO, which arises
because the interchain exchange interaction of CuGeO3
is not so weak [20] as that of other typical organic SP
materials [21,22]. In other words, if SP transition had
not occurred in CuGeO3 , even pure CuGeO3 would bea classical AF material. The disappearance of lattice
dimerization may induce the phase transition from D-AF
to U-AF phases through spin-lattice coupling. The energies of D-AF and U-AF phases including both spin andlattice should be calculated in the ground state and sudden
disappearance of SP transition should also be explained.
Weiden et al. also reported the T-x phase diagram of
Mg-doped CuGeO3 from susceptibility measurements [12]in which an xc ⯝ 0.04 can be inferred. This disagreementin xc is most likely due to accurate measurement of x or
sample inhomogeneity. However, Weiden et al. [12] do
not give any details in how x and TN were measured.
On the other hand, we first checked that the emission
spectra of Cu 共苷 327.396 nm兲, Ge 共苷 209.423 nm兲, andMg 共苷 279.533 nm兲 do not interfere with each other in
ICP-AES measurement. Second, we made sure that thedetection limit of the intensity of Mg spectrum is muchsmaller than the intensity of our usual samples (about
4 mg of Cu12x Mgx GeO3 for 0.001 & x) for ICP-AES
measurement. Third, we performed quantitative analysis
on a few nearest neighboring samples, and we confirmedthat the fluctuation of x is within 0.001. The detailed
composition analysis and the good choice of impurity makethe discovery of the present phase transition possible.
As for unresolved problems, the properties of the twophases should be studied close to the first-order phase

VOLUME 80, NUMBER 20

PHYSICAL REVIEW LETTERS

boundary using various kinds of physical measurements:
neutron diffraction and neutron inelastic scattering, mSR,
specific heat, and x-ray diffraction at low temperatures.
For the Zn-doped CuGeO3 , we are planning to reinvestigate the phase diagram around x ⬃ 0.017 in detail and toclarify whether the jump of TN really exists or not. Further theoretical explanation of the phase transition is alsoneeded. Another question is whether the phase transition
exists in Si-doped CuGeO3 ; in other words, whether it isunique to the doping to the Cu site or not. Detailed studies
on the T -x phase diagram of CuGe12x Mx O3 are needed.
In summary, we studied in detail the T -x phase diagramof Cu12x Mgx GeO3 and discovered a first-order phasetransition between D-AF and U-AF phases. At xc dchanges from finite value to zero and spatial distributionof 具S z 典 also changes from inhomogeneous to uniform
distributions. The transition seems to be universal for thedoping to the Cu site, and we can explain some of theunsolved problems in impurity-doped CuGeO3 by this T-x
phase diagram.
We thank G. Shirane, Y. Fujii, H. Nakao, M. Nishi,
and K. Hirota for neutron diffraction measurement on
Mg-doped CuGeO3 . This work was supported in partby Grant-in-Aid for Scientific Research (A), Grant-inAid for Scientific Research on Priority Area “Anomalous
Metallic State near the Mott Transition,” Grant-in-Aid
for COE Research “Phase Control of Spin-Charge-Photon(SCP) Coupled System” from the Ministry of Education,
Science, Sports, and Culture, and NEDO (New Energy
and Industrial Technology Development Organization)
International Joint Research Grant.

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18 MAY 1998

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