arXiv:cond-mat/0209537v1 [cond-mat.str-el] 23 Sep 2002
Modification of the ground state in Sm-Sr manganites by oxygen isotope substitution
N. A. Babushkina and E. A. Chistotina
Institute of Molecular Physics, Russian Research Center “Kurchatov Institute”, Kurchatov Sqr. 1, Moscow, 123182 Russia
O. Yu. Gorbenko and A. R. Kaul
Department of Chemistry, Moscow State University, Vorobievy Gory, Moscow, 119899 Russia
D. I. Khomskii
Laboratory of Solid State Physics, Materials Science Center,
University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
K. I. Kugel
Institute for Theoretical and Applied Electrodynamics,
Russian Academy of Sciences, Izhorskaya Str. 13/19, Moscow, 125412 Russia
The effect of16O →18O isotope substitution on electrical resistivity and magnetic susceptibility of
Sm1−xSrxMnO3manganites is analyzed. It is shown that the oxygen isotope substitution drastically
affects the phase diagram at the crossover region between the ferromagnetic metal state and that
of antiferromagnetic insulator (0.4 < x < 0.6), and induces the metal-insulator transition at for x
= 0.475 and 0.5. The nature of antiferromagnetic insulator phase is discussed.
PACS numbers: 75.30.Vn, 64.75.+g, 64.60.Ak, 75.30.Kz, 82.20.Tr
Despite intense studies, the nature of the colossal mag-
netoresistance in manganites is still a matter of hot de-
bate. This phenomenon is usually observed when the
system is close to a borderline between ferromagnetic
metallic (FM) phase and an insulating (I) one. The type
of the latter may be different (paramagnetic or antiferro-
magnetic (AF)), but there are more and more indications
that there exists also some kind of charge ordering (CO)
– either real long-range CO (see review article  and ref-
erences therein) or, at least, short-range CO correlations
[2, 3]. When the system is close to a FM-I crossover, even
weak perturbations can induce this crossover: change of
temperature , magnetic field , pressure , irradia-
tion , etc. In La-Nd-Ca  and La-Pr-Ca  mangan-
ites, one can induce this crossover and consequently the
metal-insulator transition even by changing the oxygen
isotope content: whereas the low-temperature state of
16O samples is FM, the samples18O become CO AF in-
sulator. This was confirmed by direct neutron scattering
The question arises whether this spectacular phe-
nomenon is confined only to this particular situation of
insulating phase with the charge ordering and the CE-
type magnetic structure, i.e. whether the nature of com-
peting phases, in particular, the insulating one, is cru-
cial, or one can get similar behavior in other systems
close to the FM-I crossover. In studying this question, we
have found yet another system with the metal-insulator
transition induced by the oxygen isotope substitution:
Sm1−xSrxMnO3with x in the 0.475–0.5 range. The neu-
tron scattering results for this system at x=0.4  and
preliminary data for x=0.45 with18O  suggest that,
in contrast to (La,Pr)CaMnO3, the insulating phase here
has not the CE, but most probably A-type antiferromag-
netic structure. If so, we could conclude that the metal-
insulator transition induced by the isotope substitution
is a general property of a FM-I crossover independent of
the detailed nature of the insulating phase. Let us also
note here that X-ray, neutron, and electron diffraction
demonstrated possible existence of a short-range charge
ordering for Sm1−xSrxMnO3in the concentration range
under discussion . The results of ESR  and Raman
 measurements lead to the same conclusion.
Ceramic Sm1−xSrxMnO3 samples were prepared by
the solid-state reaction technique, the detailed procedure
is described in Ref. 16. The enrichment of the samples
by18O was performed at T = 950◦C and at pressure p
=1 bar during 200 hours using the method reported in
Refs. 9, 17. The resistivity of the samples was measured
by the conventional four-probe technique in the temper-
ature range from 4.2 to 300 K. The measurements of real
part of ac magnetic susceptibility χ′(T) were performed
in ac magnetic field with frequency 667 Hz and amplitude
of about 0.4 Oe.
The temperature dependence of electrical resistivity
ρ(T) for Sm1−xSrxMnO3samples with x = 0.425, 0.450,
0.475, 0.500, and 0.525 annealed both in16O and18O
atmosphere is presented in Fig. 1. Four16O-containing
samples, with x = 0.425, 0.450, 0.475, and 0.500, are
characterized by a metal-like behavior at low temper-
atures, Fig. 1a. With the growth of x, the resistivity
increases and the metal-insulator transition point TMI
shifts toward lower temperatures (TMI was determined
as a point corresponding to the maximum temperature
derivative of ρ(T) below the resistivity peak). This can
be attributed to the narrowing of the electron bandwidth
and the weakening of ferromagnetic interaction.
sample with x = 0.525 is an insulator down to the lowest
After the16O →18O isotope substitution, only two
samples with the lowest Sr content (x = 0.425 and 0.450)
remain metal-like at low temperatures, the other become
insulating (Fig. 1b). These metallic samples have much
higher resistivity than those with16O. TMIdecreases by
40 K and 60 K for the samples with x = 0.425 and 0.450,
respectively. For x = 0.475 and 0.500, we observed the
metal-insulator transition induced by the oxygen isotope
substitution (see inset in Fig. 1a) similar to that reported
for La-Pr manganites . Thus, in this system, the16O
→18O isotope substitution leads to the changes in the
phase diagram: the weakening of the ferromagnetism and
the stabilization of the insulating (probably CO) phase.
A more pronounced thermal hysteresis in ρ(T) curves
for the18O samples is a manifestation of their enhanced
inhomogeneity. A relatively low magnetic field (H = 1 T)
transforms the samples with18O to the metal-like state
and suppresses the contribution from the high-resistivity
insulating state (for x=0.500 see inset in Fig. 1b).
The temperature dependence of the ac magnetic sus-
ceptibility χ′(T) for Sm1−xSrxMnO3system is presented
in Fig. 2. The steep growth of χ′(T) corresponds to the
onset of FM ordering; the Curie temperature TC was
determined as a point corresponding to the maximum
dχ′(T)/dT. For samples with16O and x = 0.425, 0.450,
and 0.475, χ′(T) behaves in a similar manner (as far as
the magnitude of χ′and TC are concerned). One sees
that these samples are essentially ferromagnetic. The
behavior of χ′at low temperatures (decrease after reach-
ing a maximum) is apparently connected with the effect
of magnetic domains, see Ref. 18.
For x = 0.500, the χ′value drops drastically, TCshifts
toward lower temperatures, this is a signature of the de-
creasing contribution of FM phase. For the composition
with x = 0.525 the susceptibility becomes very small,
and the ferromagnetism almost disappears. In the sam-
ples with18O, for compositions with x ≥ 0.450 the χ′
value is significantly lower in comparison to the samples
with16O, TC is also much lower and the hysteresis ap-
pears. The FM phase content decreases so steeply that
it becomes insufficient for the percolation in the samples
with x = 0.475 and 0.500, as it is clearly seen for the cor-
responding ρ(T) curves. The FM–CO phase boundary
shifts toward the CO state.
The temperature dependence of inverse ac magnetic
susceptibility 1/χ′(T) for Sm1−xSrxMnO3 manganite
samples with16O and18O is shown in insets in Figs. 2a
and 2b, respectively. Above the magnetic ordering tem-
perature, the behavior of 1/χ′(T) curves is rather com-
plicated suggesting the existence of an inhomogeneous
state even in the paramagnetic region. At relatively high
temperatures, T ∼ 250–300 K, all 1/χ′(T) are very close
to each other exhibiting a nearly linear growth of the
050100 150 200 250 300
050 100 150 200 250
050 100 150 200 250
ρ (Ω cm)
ρ (Ω cm)
ρ (Ω cm)
ρ (Ω cm)
FIG. 1: Temperature dependence of electrical resistivity for
Sm1−xSrxMnO3with x = 0.425 (1), 0.450 (2), 0.475 (3), 0.500
(4), and 0.525 (5). The results for samples with16O and18O
are presented at panels (a) and (b), respectively. The inset in
panel (a) illustrates the metal-insulator transition induced by
the oxygen isotope substitution for samples with x = 0.500.
The inset in panel (b) shows the evolution of resistivity with
increasing magnetic field for the18O sample with x = 0.500.
Solid and dashed lines correspond to cooling and heating, re-
Curie–Weiss type. With the temperature lowering, the
curves flatten, the flattening being more pronounced for
larger x. For x = 0.475, x = 0.500, we can see even a
plateau in 1/χ′(T). The onset temperature for the flat-
tening and deviation of 1/χ′(T) curves for different x
from each other seems to be related to the arising CO
correlations (TCO≈ 240 K). This value agrees with the
data of Refs. 14, 19. The behavior of 1/χ′(T) at T < TCO
can be interpreted in a picture of interacting and com-
peting FM and CO correlations, the latter growing with
x. Similar behavior was reported in Ref. 4.
At T > TC, the16O →18O isotope substitution does
not change the general features of 1/χ′(T) curves: the
deviations from the Curie–Weiss law begin at the same
temperature, below which the slope of 1/χ′(T) is virtu-
ally independent of the isotope content at a given value
of x. However, for the heavier isotope, TCbecomes lower
and 1/χ′increases. Thus, for the samples with18O, the
050100 150 200 250 300
0 50 100150200250
FIG. 2: Temperature dependence of ac magnetic susceptibil-
ity for Sm1−xSrxMnO3 with x = 0.425 (1), 0.450 (2), 0.475
(3), 0.500 (4), and 0.525 (5). The behavior of the inverse sus-
ceptibility is illustrated in the insets. The results for samples
with16O and18O are presented at panels (a) and (b). Solid
and dashed lines correspond to cooling and heating, respec-
plateau in 1/χ′(T) curves is observed within a wider tem-
perature range. We can argue that the AFM correlations
related to the charge ordering arise in the18O samples
at the same temperature TCO≈ 240 K, but exist down
to lower temperatures, i.e. the FM interaction becomes
weaker and the CO–FM equilibrium shifts toward the
CO state. In the samples with18O, the transition to
the FM state is characterized by a broader hysteresis in
comparison to that than for the16O samples.
From the data presented above one sees that the16O
→18O isotope substitution induces the metal-insulator
transition close to the crossover to an insulating state.
According to Ref. 13, for16O samples such a crossover
starts already at x = 0.4 and extends up to x = 0.6. As
follows from our data, the actual change of the ground
state occurs at x ∼ 0.5 (although an inhomogeneous state
with some traces of the FM phase may exist up to x =
The exact nature of an insulating phase for x ≥ 0.5
is not yet established with certainty, but the neutron
scattering data  show that it is an A-type antifer-
romagnet. This agrees with the general trend charac-
teristic of R1−xSrxMnO3 manganites where the A-type
“bad metal” state was observed for x ≈ 0.5 in com-
pounds with R = La  and R = Nd , and with
the theoretical considerations [22, 23]. In our case, this
state is insulating but the resistivity and the energy
gap in it at low temperatures are much smaller than
in (La1−yPry)0.7Ca0.3MnO3with y= 0.75, for which the
isotope-induced metal-insulator transition was first ob-
served in Ref. 9.Indeed, whereas the room temper-
ature resistivities of these two systems are compara-
ble (0.29 Ohm·cm in (SmSr)MnO3 vs 0.37 Ohm·cm in
(LaPr)CaMnO3), the resistivities at 60 K are already
much different:3.5·103Ohm·cm for SmSr system vs
2.9·107Ohm·cm for (LaPr)Ca one. Note that the sam-
ples of both systems had similar porosity and mean grain
size. The effective activation energies at lowest temper-
atures can be estimated as ∼15 meV for the SmSr case
but ∼120 meV for (LaPr)Ca.
The insulating behavior of SmSr samples be either re-
lated to the granular nature of our samples as contrasted
with the single crystals studied in , or may be due to
a formation of some weak superstructure of the CO type
. As follows from the results of , the A-type AFM
single crystals show metal-like behavior in the ab-plane,
but are insulating in c-direction. This can lead to an
insulating behavior with small activation energy in our
Another factor may be a possible instability of a metal-
lic state. Most probably, the occupied orbitals in the A-
type SmSr insulating phase for x ≥ 0.5 are of x2− y2
type [21, 22, 23], whereas these are ordered 3x2− r2
and 3y2− r2in (LaPr)0.7Ca0.3MnO3, the latter having
the conventional charge ordering with the CE magnetic
structure. The existence of a small energy gap in the
A-type SmSr system may be related to the instability of
a metallic state: the Hubbard subband of x2− y2type
will have spectrum ε(k) = −2t(coskx+ cosky) and it
will be half filled at x = 0.5, which would give a nested
Fermi surface. This can lead to a charge-density-wave
state with the opening of a small gap. This should also
give a superstructure with the wavevector q = (1
the same as in the usual CO state, but with much weaker
distortions. It would be interesting to look for such weak
The present results show that the isotope substitution
can drastically change the properties of the system when
it is close to a FM-I crossover independent of the detailed
nature of the I phase. For the (LaPr)Ca system, this I
state is a ”strong” insulator with the charge ordering and
the CE magnetic ordering, whereas here, in SmSr system,
it is a weak insulator of the A-type. Nevertheless the
effect of the isotope substitution is similar.
The detailed mechanism of the isotope effect on the
properties of manganites is not yet completely clear, but
most probably it is connected with the decrease in the
0,425 0,450 0,475 0,500 0,525
TC, TN, TCO (K)
FIG. 3: The Curie, N´ eel, and charge ordering temperatures
for Sm1−xSrxMnO3 samples under study.
symbols correspond to the samples with16O and18O, respec-
tively. ⊗, ♦, ⊠ and ⊕, △, ⊞ are the data from  and 
for TC, TN, TCO.
Open and solid
electron bandwidth for heavier isotopes either due to
zero-point oscillations or to polaronic effects [17, 25].
This is consistent with the change of TCO and TC with
the isotope content shown in Fig. 3. The CO state is
relatively insensitive to the isotope composition, which
is quite natural if the mechanism of CO (or CDW) is
predominantly electron-lattice interaction  (as is well
known, the dimensionless electron-phonon coupling con-
stant λ does not depend on the mass of ions). On the
contrary, TC in the double exchange model scales with
the bandwidth, and the decrease in TC with the growth
of oxygen mass (Fig. 3) is consistent with this interpre-
Summarizing, we studied the effect of the oxygen iso-
tope substitution on the properties of Sm1−xSrxMnO3in
the most interesting concentration range 0.4 ≤ x ≤ 0.6.
It was shown that close to a crossover from the ferromag-
netic metallic state (x ≤ 0.5) to an antiferromagnetic in-
sulator (x ≥ 0.5) all the properties change drastically –
up to the fact that for x = 0.475 and 0.5 one even induces
the metal-insulator transition by the16O →18O substitu-
tion. Most probably this substitution transforms the FM
state to an A-type antiferromagnet. We speculate that
the relevant orbitals in this state are x2− y2ones and
the energy gap appears due to the formation of a charge
density wave in the x2−y2band. We conclude that this
transition is induced by the decrease of the bandwidth for
heavier ions. The results obtained, together with the ear-
lier observed isotope-induced metal-insulator transition
in (La1−yPry)0.7Ca0.3MnO3, show that the isotope sub-
stitution is a powerful tool both for modifying the prop-
erties of manganites close to a crossover between different
states and for the study of their physical characteristics.
We are grateful to A. N. Taldenkov and A. V. In-
yushkin for helpful discussions and to A.I. Kurbakov for
providing us with his neutron scattering data prior to
publication. The work was supported by grants of INTAS
(01-2008), CRDF (RP2-2355-MO-02), NWO (097-008-
017), and RFBR (01-02-1624, 02-02-16078, 02-03-33258
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