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Journal of the Physical Society of Japan
Vol. 67, No. 4, April, 1998, pp. 1083-1085

Letters

Anomalous Phonon-Spin Scattering in La1−x Srx MnO3
Manabu Ikebe, Hiroyuki Fujishiro and Yoshiyuki Konno
Department of Materials Science and Technology, Faculty of Engineering, Iwate University,
4-3-5 Ueda, Morioka 020-8551(Received December 24, 1997)

The temperature dependence of the phonon thermal conductivity κ(T ) of La1−x Srx MnO3
(0.11 ≤ x ≤ 0.20) polycrystals was measured from 60 K to 360 K. For x = 0.155 and 0.170, κ(T )is markedly reduced over a wide temperature range near the ferromagnetic transition temperature
Tc . The analyses of κ(T ) have revealed the existence of anomalously strong phonon scattering
caused by the coupling of spin ﬂuctuations and the lattice instability in this double exchange
coupled system.
KEYWORDS: La1−x Srx MnO3 , thermal conductivity, ferromagnetic order, phonon-spin scattering, sound velocity

Carrier-doped lanthanum manganites with perovskitebased structures have renewed the interest of researchers.
Although numerous studies were performed a long time
ago,1-3) the physical properties of these compounds arequite novel even at present.4-8) The electrical resistivity
ρ(T ) of La1−x Srx MnO3 conspicuously decreases below
the ferromagnetic transition temperature Tc , sometimes
displaying a kind of metal-insulator transition. This has
been explained as being due to the double exchangemechanism between doped carriers and localized Mn
spins, resulting in the eﬀective ferromagnetic interaction
between Mn3+ and Mn4+ ion pairs. The fundamental
properties of this system may be explained by the double exchange mechanism, but dramatic physical properties such as colossal magnetoresistance4-8) and magnetic
ﬁeld induced structural transition9, 10) are staged as aresult of the competition between various mechanisms,
i.e., the antiferromagnetic superexchange, Jahn-Teller effect, structural instability due to ion-radius mismatching, charge-ordering and orbit-ordering besides the double exchange mechanism. However, the most importantcompetition might be that between the electron system(itinerant or localized) and the lattice system. In a previous paper,11) we reported sound velocity (vs ) anomalies associated with the charge-ordering and the magnetic ordering in La1−x Srx MnO3 . In this paper, we report an anomalous enhancement in phonon scattering by
spin ﬂuctuations in a wide temperature range around Tc .
The anomalous phonon scattering enhancement occurs
for the Sr concentration x near the structural instability
from the orthorhombic to rhombohedral phase.
La1−x Srx MnO3 (x = 0.11–0.20) samples were prepared from stoichiometric mixtures of La2 O3 , SrCO3 and
Mn3 O4 powders. The mixtures were calcined at 1000◦ C
for 24 h in air, pressed into pellets and then sintered at
1500◦ C for 8 h in air. The measured densities of eachsample are higher than 80% of that of the ideal one. All
the samples were conﬁrmed by means of X-ray diﬀraction
to be in a single phase at room temperature. The thermal
conductivity κ(T ) was measured by a steady-state heat
ﬂow method between 60 K and 360 K. For κ(T ) measurement below 280 K, a Giﬀord-McMahon cycle helium

refrigerator was used. For measurement above 280 K, thesamples were remounted on a temperature controlled hot
stage in a vacuum chamber. AuFe(0.07 at.%)-chromeland chromel-alumel thermocouples were used as thermometers for below and above 280 K, respectively.
Figures 1(a)–1(c) show the thermal conductivity of
La1−x Srx MnO3 for x = 0.170, 0.155 and 0.145 as afunction of temperature T . The sound velocity vs (T ),taken from ref. 11, is also shown in these ﬁgures. We
can see that both κ(T ) and vs (T ) take local minimums
at around Tc . Tc was determined from the sharp drop inthe ρ(T ) curves of respective samples,11) which is almostin accordance with that determined from the magnetization measurement. As shown in ref. 11, the electricalresistivity of these samples is rather large and the electronic contribution κe of heat conduction estimated fromWiedemann-Franz law is negligible. Thus the heat conduction is almost entirely due to phonons in the present
La1−x Srx MnO3 .
The phonon thermal conductivity is generally givenby12)
3ρ0 nRT 3 2 ΘD /T x4 ex
κph =v
τph dx,
(1)
s
3
M ΘD
(ex − 1)20
where ρ0 is the mass density, M the mass of one mole,n (=5 for La1−x Srx MnO3 ) the number of atoms in thechemical formula, R the gas constant, ΘD the Debye temperature and x is the reduced phonon frequency. Assuming a kind of Matthiessen’s rule for the phonon scattering
rate, the phonon relaxation time τph is given by12)
−1
−1
τph
= τb−1 + d(T x) + s(T x)2 + p(T x)4 + Δτmag.

(2)

Here τb is the scattering time due to grain boundaries
and d, s and p refer to the strength of the phonon scattering by dislocations, sheet-like faults and point defects,
respectively.13) In the following, we try to quantify thestrength of the enhancement in phonon scattering around
Tc by the use of eqs. (1) and (2).
As can be seen in Figs. 1(a)–1(c), the sound velocity vsshows a considerable change around Tc . By the changeof vs (Δvs ), κph (T ) is aﬀected in two competitive ways;(i) through the factor vs2 in eq. (1) and (ii) through the1083

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Letters1084

Fig. 1. (a) Data of thermal conductivity κ(T ) (◦) and sound velocity vs (T )11) of La1−x Srx MnO3 for x = 0.170. Solid line represents the base line of κ(T ) which is estimated by use of theparameter values in Table I. The dotted line represents Δvs corrections to κ(T ) due to the vs2 factor in eq. (1). (b) Similar dataand curves to (a) for x = 0.155. (c) Similar data and curves for
x =0.145.

change of ΘD . The temperature range in which κ(T ) of
the present samples exhibits peculiar reductions belongs
to a rather high-temperature region (Tc ≥ ΘD /2, ΘD ∼=450 K). In such a case, κph (T ) is mainly aﬀected by the
Table I.

x =0.170
x =0.155
x =0.145

factor vs2 in eq. (1). This correction due to the factor
vs2 to κ(T ) base lines is presented by the dotted line in
Figs. 1(a)–1(c). The κ(T ) base lines given by the solidlines are calculated on the basis of eqs. (1) and (2) using
the parameter values in Table I. The corrections forκ(T ) given by the dotted line arefar too small to explain
the observed reduction in κ(T ). In order to remedy the−1,
discrepancy, we introduced an additional term, Δτmag−1
in eq. (2) and calculated Δτmag values necessary to ﬁt theexperimental κ(T ) by the use of eq. (1). The results of−1
values are shown in Fig. 2. The peaks
calculated Δτmag−1−1
are centered at Tc and the values of Δτmagof Δτmagseem to become larger with the order of increasing x
for the present three samples. Figure 3 shows κ(T ) of
La1−x Srx MnO3 for x = 0.135 and 0.20. The reduction in
κ(T ) at Tc is very weak, although it is still clearly noticed
for La0.80 Sr0.20 MnO3 . The κ(T ) of La0.89 Sr0.11 MnO3
was also measured but the corresponding anomaly near
Tc was almost completely wiped out.
Next, we brieﬂy discuss the origin and the mechanism−1.
which result in the enhanced phonon scattering Δτmag
Since the enhancement occurs in a temperature range
centered around Tc , it is quite clear that some mechanism closely related to spin magnetic moments is theorigin of the enhancement. The observed enhancement−1
is very large; by assuming vs ≈ 4 × 105 cm/s
Δτmag
−1
≈ 10−12 (the maximum value in Fig. 2), theand Δτmagphonon mean free path limited by this scattering is as
small as ∼40 Å. Such a strong spin-phonon scattering
has not been observed and usual spin-phonon interaction through the spin-orbit coupling is unlikely to be theorigin. One possible scenario for the enhanced phonon
scattering by the spin system may be as follows.
The La1−x Srx MnO3 system with a perovskite based
structure has a lattice instability and the crystal structure changes from orthorhombic to rhombohedral withincreasing x at around x = 0.17.6) Our La0.83 Sr0.17 MnO3
sample actually shows the structural transition at about
T = 180 K as indicated by the hysteretic behavior invs (T ) and ρ(T ).11) The La1−x Srx MnO3 sample considered here is orthorhombic below 180 K and rhombohedral above 180 K for x = 0.170 and orthorhombic
for x = 0.145 and 0.155 over the entire temperature
range.6, 11) The structural instability from orthorhombic
to rhombohedral is intimately related with the stabilityof ferromagnetism through the double exchange mechanism and the ferromagnetic order favors a rhombohedral
lattice.14) This interplay of lattice structure and ferromagnetism is considered to be the origin of the characteristic ﬁeld-induced structural transition observed inthis system.9, 10) The observed anomalous enhancementin phonon scattering may similarly result from the strong
coupling between the lattice and the spin system via the
double exchange interaction.

Fitting parameters used to determine κph (T ) base lines for various La1−x Srx MnO3 samples.

τb−1 (s−1 )d (K−1 s−1 )s (K−2 s−1 )p (K−4 s−1 )ΘD (K)

ρ0 (g/cm3 )3.2 × 108
3.7 × 108
4.2 × 108

1.6 × 1010
1.8 × 1010
2.1 × 1010

3.2 × 105
3.7 × 105
4.2 × 105

3.2 × 101
3.7 × 101
4.2 × 101

450
450
450

5.28
5.28
5.31

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Letters−1
Fig. 2. The strength of additional phonon scattering Δτmag
vs−1
reduced temperature T /Tc . Δτmag
is estimated on the basis ofeqs. (1) and (2) (see text).

Fig. 3. Thermal conductivity κ(T ) of La1−x Srx MnO3 for x =0.135 and 0.20. Anomalous reduction in κ(T ) is small at these
Sr concentrations. The electrical resistivity ρ(T ) is also shownfor x =0.20 (see ref. 11 for ρ(T ) of x =0.135).

It is well-known that many types of relaxation effects show characteristic anomalies near the second orderphase transition Tc .15) This is caused by the divergent
ﬂuctuations of the order parameter and the ﬂuctuationsof the spin magnetic moments also intensely develop at
around the ferromagnetic transition temperatures. Associated with the ﬂuctuations of the magnetic shortrange order in La1−x Srx MnO3 , the crystal structure may
also tend to ﬂuctuate locally between orthorhombic andrhombohedral lattices. From the viewpoint of phonontransport, the local ﬂuctuations of lattice structure may
act as eﬀective phonon scattering centers, providing a
resultant strong phonon-spin scattering mechanism in
La1−x Srx MnO3 crystals. The local structural ﬂuctuations are expected to be stronger for the Sr concentrationx near the orthorhombic to rhombohedral lattice transition and will become weaker for x far from the transi−1
is the largest fortion line. In Fig. 2, we see that Δτmagrhombohedral La0.83 Sr0.17 MnO3 which just crosses the−1
arelattice transition line at 180 K. The values of Δτmagalso large for orthorhombic La0.845 Sr0.155 MnO3 with Srconcentration pretty near the structural transition. For
x = 0.145, which is fairly separated from the lattice tran−1
values are much suppressed in comparsition line, Δτmagison with x = 0.170 and x = 0.155 samples. Thus the1085

anomalous phonon-spin scattering shows a clear correlation with the structural instability in the La1−x Srx MnO3
system at around x = 0.170. This local lattice ﬂuctuation model bears some common features to the twolevel model of amorphous solids,16) where phonons arescattered by tunneling motions of atoms between two
metastable atomic sites. In the present model, the motion between two atomic sites (orthorhombic and rhombohedral) is mediated by ﬂuctuations of the ferromagnetic order.
In summary, our thermal conductivity measurement
revealed anomalously strong phonon scattering around
Tc (“giant” phonon-spin scattering) in La1−x Srx MnO3
crystals with Sr composition x in the vicinity of theorthorhombic to rhombohedral structural transition.
We have proposed one possible mechanism for “giant”
phonon-spin scattering based on the double exchangemechanism. The proposed mechanism reﬂects, however,
only one phenomenological aspect of the problem. Theoretical studies which more rigorously take account of various competitive or cooperative mechanisms in this system,especially the (dynamical) Jahn-Teller eﬀect are urgently required. It is also worth noting that the “giant”
phonon-spin scattering occurs over a very wide temperature range around Tc in spite of the three-dimensional
nature of the La1−x Srx MnO3 system.
The authors wish to thank professor T. Fukase at Tohoku University for valuable discussions. The sound velocity measurement cited in this letter was performed
under the inter-university cooperative research programof the Institute for Material Research, Tohoku University. This work is partially supported by a Grant-in-Aid
for Scientiﬁc Research from the Ministry of Education,
Science, Sports and Culture (No. 09640414).
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