RAPID COMMUNICATIONSPHYSICAL REVIEW B, VOLUME 63, 140506共R兲

Effect of spin substitution on stripe order in La1.875Ba0.125Cu1Ày My O4 „MÄZn or Ni…
O. Anegawa, Y. Okajima, S. Tanda, and K. Yamaya
Department of Applied Physics, Hokkaido University, Sapporo 060-8628, Japan
共Received 2 October 2000; published 20 March 2001兲
The destruction of stripe order by Zn (S⫽0) and the formation of a new phase by Ni (S⫽1) are demonstrated directly from the substitution dependence of the electronic coefficient of specific heat ␥ inLa1.875Ba0.125Cu1⫺y M y O4 (M ⫽Zn or Ni兲. In the Zn-substitution case, ␥ , which is strongly suppressed by stripe
phase formation, increases to a value that corresponds to the state where there is no stripe order. In theNi-substitution case, ␥ saturates at nearly half the value of the Zn-substitution case. The difference can beexplained by a difference in the spin interactions between the substituent atom and the nearest neighbor Cu
atoms, indicating that spin interaction plays an important role in the formation of charge stripes.
DOI: 10.1103/PhysRevB.63.140506

PACS number共s兲: 74.72.Dn, 75.40.Cx, 71.27.⫹a, 71.45.Lr

Many insulating cuprate oxides with strong two dimensionality exhibit high temperature superconductivity by thedoping of holes. The transition temperature T c’s have asingle maximum at a certain hole concentration. However, inLa2⫺x Bax CuO4 and rare earth substituted La2⫺x Srx CuO4 ,superconductivity is suppressed near hole concentrations of1/8 and T c exhibits two maxima.1,2 In the same concentration
range, these compounds also exhibit a structural transition
from a low temperature orthorhombic 共LTO兲 phase to a lowtemperature tetragonal 共LTT兲 phase.3,4 Nuclear quadrupole
resonance 共NQR兲 and muon-spin-relaxation 共␮SR兲 experiments have revealed the existence of some magnetic ordering below the structural transition temperature.5,6 The structural transition, the strong suppression of superconductivity,and magnetic ordering have been observed in bothLa2⫺x Bax CuO4 andLa1.6⫺x Nd0.4Srx CuO4 at
x⬃1/8.
Tranquada et al. detected several satellite peaks inelastic
neutrondiffractionforsingle-crystal
La1.6⫺x Nd0.4Srx CuO4 (x⬃1/8).7,8 The position of these satellite peaks indicates that the periods of charge and spin
modulation are equal to four and eight times the lattice constant, respectively. Stripe order was proposed in order to explain the state in which the densities of charge and spin are
not uniform in real space. In this stripe phase, there are periodically arranged lines where holes are segregated, termed
charge stripes, and the regions in between, where antiferromagnetic order is formed, are termed spin stripes. InLa2⫺x Bax CuO4 (x⬃1/8), it is difficult to observe the satellite peaks, if they exist, because of difficulties in preparing asingle crystal. However, it is widely accepted that a stripe
phase will also exist in La2⫺x Bax CuO4 (x⬃1/8) because thecompound exhibits the anomalies mentioned above.
Stripe order has also been observed in nickel oxides, anda related phenomenon—static charge stripes—has been observed in manganese oxides.9,10 So, such a stripe structure
may be a common phenomenon in transition-metal oxides.
Whereas certain features of stripe phase, such as transition
temperature and the structure of charge and spin orders, havebeen studied well,7,8 it is yet to be clarified why stripe orderforms. As stripe order is the state where spins and charges
become ordered simultaneously, it is important to investigate
how charge stripes vary when spin stripes are changed andvice versa. As an approach to this issue, we focus on two
0163-1829/2001/63共14兲/140506共4兲/$20.00

points in La2⫺x Bax CuO4 ; partial substitution of Zn or Ni
atoms for Cu atoms, and measurement of the electronic specific heat coefficient. The spin moment of the Cu sites in Zn
or Ni substitution is changed from S⫽1/2 to S⫽0 or S⫽1,respectively. Such spin substitutions will have a direct effecton spin stripes,11 leading to a variation of the charge stripes.
The electronic specific heat coefficient ␥ is proportional tothe electronic density of states and is sensitive to changes inelectronic structure. In fact, a significant suppression of ␥
has been observed in a narrow range of hole concentrations
around x⫽1/8 in La2⫺x Bax CuO4 where the charge stripes
are formed.12 Therefore, the measurement of ␥ will be suitable for investigating the variation of charge stripes due tochanges of spin stripes.
In this paper, we report the experimental results of thechange in ␥ when Zn and Ni atoms are partially substituted
into the Cu site in La2⫺x Bax CuO4 with x⫽1/8. The spin
moment on the substituent Cu site is changed from S⫽1/2 toS⫽0 or S⫽1. The results reveal the destruction of stripe
order by Zn (S⫽0) and the formation of a new phase by Ni(S⫽1). The different behaviors can be explained by differences in the spin interaction between the substituted atomand the nearest neighbor Cu atoms, indicating that spin interaction plays an important role in the formation of charge
stripes.
Polycrystalline samples of La1.875Ba0.125Cu1⫺y M y O4(M ⫽Zn,Ni) were prepared from La2 O3 , BaCO3 , CuO, ZnO,and NiO powders of 5N purity by solid state reaction. The
primary materials, with the desired stoichiometry, weremixed well and ground in an agate mortar. The powder washeated in a furnace with an initial temperature increase of
200 °C/h, and reacted at 950 °C in air for 30 h. The material
was quenched by dropping it into liquid nitrogen. The reacted material was then reground in an agate mortar. The
ground material was pressed into cylindrical pellets, whichwere then reacted at 1050 °C in air for 10 h and quenched.
Finally, the pellets were annealed at 500 °C in an O2 atmosphere. Each sample was confirmed to be single phase by
x-ray diffraction analysis. The specific heat of the samples
was measured by the thermal relaxation method in a temperature region of 1.5 to 7 K. The typical weight of samples
used in specific heat measurements was about 30 mg.
The temperature dependence of the specific heat of theLa1.875Ba0.125Cu1⫺y M y O4 (M ⫽Zn or Ni兲 systems is shown

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FIG. 2. Dependence of ␥ on y for La1.875Ba0.125Cu1⫺y Zny O4 .
Open circle is the value extrapolated from above y⫽y Zn
0 , solidlines are guides to the eye.
FIG. 1. Temperature dependence of the specific heat of
La1.875Ba0.125Cu1⫺y M y O4 . 共a兲 M ⫽Zn 共b兲 M ⫽Ni. Horizontal axis is
the square of temperature T 2 , vertical axis is the specific heat divided by temperature C/T, solid lines represent C⫽ ␥ T⫹ ␤ T 3⫹ ␣ T 5.

in Fig. 1, where C/T is plotted as a function of T 2 . For all
samples examined, C/T increases linearly with increasing T 2below T 2 ⫽25 K2 (T⫽5 K兲, and exhibits an upward curvature with increasing temperature above T⫽5 K. The moststriking feature of this relationship is that the value of C/Tshifts to larger values with increasing y. However, the rate ofshift decreases at larger y. In the Zn substitution case, therate becomes small above y⫽0.06, whereas in the Ni substitution case, the rate becomes small above y⫽0.02. The temperature dependence of specific heat at low temperatures isusually represented by the fitting function C⫽ ␥ T⫹ ␤ T 3 ,where the first term is the contribution of electrons and thesecond term is that of phonons. The specific heat of phonons
follows a T 3 law when the temperature is lower than onefiftieth of the Debye temperature ␪ D , which is estimated by␪ D3 ⬃12␲ 4 Nk B /(5 ␤ ), where N is the total number of all atoms. The ␪ D of the present samples is estimated to be about
240 K. The T 3 law is an adequate approximation below 5 K,
however, it does not hold above 5 K, as shown in Fig. 1.
Above 5 K, a function C⫽ ␥ T⫹ ␤ T 3 ⫹ ␣ T 5 was found to fitthe data well. The results are indicated by the solid lines inFig. 1. A T 5 term is needed in the fitting function, representing the contribution from anharmonic lattice vibration or low
lying optical phonons. The value of ␥ is obtained from thisfitting function. The slopes of C/T with T 2 are nearly identical for all samples regardless of the nature of the substitution 共Zn or Ni兲, indicating that the lattice contribution to C
remains almost unchanged.
Figure 2 shows values of ␥ plotted as a function of y for
the Zn substitution case. The ␥ value for the unsubstituted
sample is 3.6 (mJ/mol K2 ), which is in agreement with thevalue reported by Okajima et al.12 From the figure it can beseen that ␥ increases rapidly with increasing Zn content up
to y⫽0.06. Above y⫽0.06, the rate of increase drops by afactor of 10. Thus, there is a characteristic concentration ofZn; y Zn
0 ⫽0.06, which suggests that the electronic state in theregion above y Zn
0 is different from that at y⫽0 where stripe
order is formed. By extrapolating the y dependence of ␥above y Zn
0 to y⫽0, we can estimate ␥ 0 which is expected
when the state above y Zn
0 is realized at y⫽0. The value of ␥ 0obtained is 11 (mJ/mol K2 ), which is about three times larger
than that of the unsubstituted sample in which a stripe phase
is formed.
The estimated value of ␥ 0 from Fig. 2 is shown in
Fig. 3 together with the ␥ dependence on x ofLa2⫺x Bax CuO4 measured by Okajima et al.12,13 Okajima
et al. observed a large suppression of ␥ near x⫽0.125 inLa2⫺x Bax CuO4 ,12,13 whereas Momono et al. investigated indetail ␥ as a function of x in La2⫺x Srx CuO4 for which stripe

FIG. 3. Dependence of ␥ on x for La1.875Ba0.125CuO4 . Closed
circles are values measured by Y. Okajima et al. 共Ref. 12 and 13兲,open circle is ␥ 0 , solid lines are guides to the eye.

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PHYSICAL REVIEW B 63 140506共R兲

order was not formed, and found that ␥ increases monotonically with increasing x.14 It is suggested that if the stripe
phase is not formed at x⫽0.125 in La2⫺x Bax CuO4 , then ␥would increase monotonically with increasing x similar tothe La2⫺x Srx CuO4 case, which is indicated by a broken linein Fig. 3. The value of ␥ 0 estimated from Fig. 2 coincides
well with the broken line in Fig. 3, and hence we can conclude that stripe order is not formed in the region above y Zn
0 .
,stripeorder
maybedestroyed
gradually
beBelow y⫽y Zn0cause ␥ increases continuously with y.
The destruction process of the stripe phase by Zn substitution can be understood as follows: The stripe phase consists of charge and spin stripes. In the charge stripes, there isno spin order and holes exist, whereas in spin stripes, there isantiferromagnetic order and no holes. When a Zn atom issubstituted into the spin stripe, spin interactions between theZn atom and the nearest neighbor Cu atoms cease because
the Zn atom has no spin. As a result, holes in neighboring
charge stripes may transfer to the Cu sites around the Zn
atom and, at the same time, the spins in the site area fluctuate. As the Zn content increases, holes may exist uniformly
over the entire CuO2 plane, leading to the full destruction ofspin stripes. Therefore, the ␥ value that is strongly suppressed by the formation of the charge stripe will return to its
original value as a result of Zn substitution. The existence ofholes over the entire CuO2 plane does not result in metallicconduction, rather, the temperature dependence of the resistivity of La1.875Ba0.125CuO4 reveals that the material after Zn
substitution exhibits insulating behavior due to the localization of holes.15 The value of ␥ obtained in this study may not
be significantly influenced by the localization effects.16
The effect of Zn doping has been extensively examined inLa2⫺x Srx CuO4 system. The La2⫺x Srx CuO4 system shows
inelastic incommensurate magnetic peaks, which are observed in neutron scattering experiments.17 Around x⫽1/8, asmall amount of Zn doping shifts the spectral weight of spin
fluctuations from the inelastic to the quasi-elastic region.18
This fact shows that static magnetic ordering appears in Zndoped La2⫺x Srx CuO4 around x⫽1/8. It has been consideredthat dynamical stripe correlations exist in the La2⫺x Srx CuO4
system and they are pinned by a small amount of Zn doping.
This idea was first proposed by Tranquada et al., although
they pinned the dynamical stripe correlations by the structural deformations in the low-temperature-tetragonal structure of La1.6⫺x Nd0.4Srx CuO4 .7 Many experimental evidencesof the former pinning have been accumulated for the last fiveyears.18,19 It should be noted that, in all these works, Zn isdoped to the samples without the static stripe order, and theZn content is smaller than or equal to 0.02. In the presentwork, on the other hand, Zn is doped to the samples with thestatic stripe order, and the Zn content is up to 0.125, which issix times larger. Since the stripe order is already pinned by
the lattice deformations in La2⫺x Bax CuO4 with x⫽1/8, thepinning effect by Zn doping is never observed. Alternatively,
the destruction process by Zn is observed, as in the presentwork.
Figure 4 shows values of ␥ plotted as a function of y for
the Ni substitution case. ␥ increases linearly as Ni content yFIG. 4. Dependence of ␥ on y for La1.875Ba0.125Cu1⫺y Niy O4 .
Solid lines are guides to the eye.

increases for y⭐0.02. Above y⫽0.02, ␥ remains almost
constant, although the data only extend to y⫽0.06. Thus,
there is also a characteristic Ni content, y Ni
0 ⫽0.02, demonstrating that different electronic states exist above and below
y Ni0 , similar to the Zn substitution case. The slope of ␥ /ybelow y⫽0.02 is similar to that for Zn, however, ␥ at y Ni
0 is
almost a half that at y Zn.
Thesmallnessofthisvalue
indi0cates that the stripe phase is not fully destroyed by Ni substitution above y⫽y Ni
0 .
It is noted that ␥ increases with y below y⫽0.02 in the Ni
substitution case, in the same way as in the Zn substitution
case. This indicates that there are several regions in which
spins fluctuate and to which holes can transfer from thecharge stripes, as mentioned in the Zn substitution case. The
spin moment of a Ni atom is 1 (S⫽1), which means thatspin interactions between the Ni atom and the nearest neighbor Cu atoms are effectively enhanced. When a single Ni
atom is substituted into the CuO2 plane, the stripe order isformed in such a way that the center of the spin stripe isfixed on the Ni site. The stripe order is not changed markedly
and only an excess spin moment appears at the Ni site. Whenseveral Ni atoms are substituted, stripe orders form independently around each Ni site, resulting in the emergence ofseveral sections with stripe order in the CuO2 plane. Thestripe order is not always connected smoothly at the boundary between the sections. Therefore, spins fluctuate in theboundary region and holes can exist, leading to an increasein ␥ .
Above y⫽0.02, on the other hand, ␥ remains constantwith increasing Ni content. This invariance indicates that theboundary region where holes can exist ceases to grow, thatis, the section with stripe order does not divide any further.
Excess Ni atoms are not always placed at the center of thespin stripe. In the Ni content region, the spin stripe may beswollen around the Ni atom and the next spin stripe may
narrow in accommodation, resulting in the charge stripe being bent around the Ni atom in each section. A single charge
stripe may be bent several times around Ni atoms. Thus, thecharge stripe can be envisaged as a meandering river ofcharge, which characterizes the ‘‘meandering’’ stripe phase,

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O. ANEGAWA, Y. OKAJIMA, S. TANDA, AND K. YAMAYAPHYSICAL REVIEW B 63 140506共R兲

in contrast to a ‘‘straight’’ stripe phase. Recently, Kivelson
et al. theoretically proposed that the stripe phase is an electronic quantum liquid-crystal phase in which transverse fluctuations of the charge stripe occur and the meandering riversof charge move dynamically.20 The meandering stripe phasementioned here is not dynamic, however is very similar tothe liquid-crystal phase.
The partial substitution of Zn or Ni atoms for Cu atoms
changes the spin moment on the substituted Cu sites fromS⫽1/2 to S⫽0 or S⫽1, respectively. The spin interactions
between the substituted atom and the nearest neighbor Cu
atoms differs for between Zn and Ni atoms. Such differences
are brought about by the marked difference in the dependence of ␥ on the substituent, as mentioned above. Charge
stripes are completely destroyed by Zn substitution, whereas
they become ‘‘meandering’’ rivers of charge with Ni substitution. This demonstrates experimentally and directly thatspin interactions not only create antiferromagnetic order inspin stripes, but also play an important role in the formationof charge stripes. This will also be true for other systems
with stripe order, such as nickel oxides. In order to clarify
the relationship between spin interaction and the formationof stripe order, the investigation of spin-substitution effects
in nickel oxides is meaningful.
In summary, it was found that substitutions that inducechanges in spin from S⫽1/2 to either S⫽0 or S⫽1 havedifferent effects on the stripe phase of La1.875Ba0.125CuO4 .
Charge stripes are completely destroyed by substitutions resulting in a change from S⫽1/2 to S⫽0 共Zn substitution兲,
whereas the substitution producing S⫽1 共Ni substitution兲
changes charge stripes to a meandering phase. Based onthese facts, we claim that spin interactions not only create
antiferromagnetic order in spin stripes, but also play an important role in the formation of charge stripes.

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The authors thank Dr. H. Mayama for his useful discussions. Research by Y. Okajima was supported by a Grant-inAid for the Encouragement of Young Scientists from theJapan Society for the Promotion of Science 共No. 11750001兲.

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