Phonon Raman scattering in A
2
Mn
2
O
7
„ ATl,In,Y…
E. Granado, P. G. Pagliuso, J. A. Sanjurjo, and C. Rettori
Instituto de Fı ´sica ‘‘Gleb Wataghin,’’ UNICAMP, 13083-970, Campinas-SP, Brazil
M. A. Subramanian
Dupont Central Research and Development, Experimental Station, Wilmington, Delaware 19880
S.-W Cheong
Lucent Technology, Bell Laboratories, Murray Hill, New Jersey 07974
S. B. Oseroff
San Diego State University, San Diego, California 92182
Received 8 March 1999
Raman scattering of optical phonons in polycrystalline A
2
Mn
2
O
7
( A =Y, In, and Tl) are reported as a
function of temperature. For A =In and Tl a hardening of the two lowest energy modes is observed below T
c
that we associate to a phonon modulation of the exchange interaction. These data suggest superexchange as the
mechanism responsible for magnetic ordering in these systems. All but one of the Raman modes in the In- and
Tl-based compounds appear at lower frequencies when compared with those in the Y-based one. This fre-
quency shift may be due to the strong hybridization of the TlIn 6 s (5 s ) orbital with O 2 p and Mn 3 d , claimed
by recent band-structure calculations on these materials. S0163-18299901733-6
I. INTRODUCTION
The ‘‘colossal’’ magnetoresistive CMR manganese-
based compound, Tl
2
Mn
2
O
7
with pyrochlore structure, has
recently attracted much attention.
1
It was proposed that the
large resistivity change and the ferromagnetic-paramagnetic
FM-PM transition at T
c
120 K Ref. 2 results from the
presence of a significant amount of Mn
3 +
due to a large
oxygen deficiency in this material.
1
However, detailed struc-
tural neutron-scattering analysis in a Tl
2
Mn
2
O
7
sample indi-
cated a negligible amount of oxygen deficiency, as well as
Mn-O distances that are inconsistent with Mn
3+
/Mn
4 +
mixed valence.
3,4
Besides, neutron-scattering spin dynamics
studies showed well defined spin waves with a gapless dis-
persion relation in the FM phase, different from what was
observed for CMR perovskites.
5
On this ground it was pro-
posed that CMR in Tl
2
Mn
2
O
7
results from the Mn
4+
/Mn
4 +
superexchange SE interaction and strong spin-fluctuation
scattering of charge carriers above and near T
c
.
6,7
It may be
of interest to compare the metallic Tl-based material
(0 K) 1 m cm Ref. 8 with the isostructural
In
2
Mn
2
O
7
compound that is also FM with T
c
125 K,
2
but
insulating. For completeness, the study of Y
2
Mn
2
O
7
com-
pound, that is an insulator with a much weaker magnetic
coupling ( T
c
15 K),
9,10
may be useful to understand the
origin of the differences between these compounds. The me-
tallic behavior of the Tl-based compound was recently attrib-
uted to the hybridization of the TlIn 6 s (5 s ) orbitals with O
2 p and Mn 3 d .
11
Also, it was suggested that the strong FM
coupling for Tl
2
Mn
2
O
7
and In
2
Mn
2
O
7
was associated to that
hybridization.
10
Nevertheless, the actual mechanism for the
magnetic coupling in these materials, particularly for the Tl
and In-based compounds, is still a subject of discussion.
In this work we present phonon Raman scattering data in
the A
2
Mn
2
O
7
system A=Y,In,Tl. For the In- and Tl-based
compounds, a hardening of two of the low-energy phonons is
observed below T
c
. This is interpreted in terms of a phonon
modulation of the exchange interaction. These results sug-
gest a similar magnetic coupling mechanism for the
Tl
2
Mn
2
O
7
and In
2
Mn
2
O
7
compounds. Except for the high-
frequency stretching peak, all Raman modes show decreas-
ing frequencies when going from Y- to In- and Tl-based
compounds. A simple model, based on the structural differ-
ences between these compounds, cannot explain this behav-
ior. It is possible that the strong hybridization of the TlIn
6 s (5 s ) orbitals with O 2 p and Mn 3 d found by band-
structure calculations may be responsible for this
behavior.
10,11
II. EXPERIMENT
Polycrystalline A
2
Mn
2
O
7
( A =Tl,In,Y) were prepared by
solid-state reaction of MnO
3
and A
2
O
3
under hydrostatic hy-
drothermal conditions in sealed gold tube at 500 °C and 2
kbar pressure.
9
The magnetization measurements were done
with a MPMS Quantum Design magnetometer in the range
of 2–300 K. The Raman spectra were excited using the
514.5- and 488-nm lines of an Ar
+
laser, with a power less
than 10 mW focused in a spot of 50- m diameter. The
scattered light was analyzed by a JY T-64000 spectrometer
with a CCD detector. The measurements as a function of
temperature were done attaching the samples on a cold finger
of a closed-cycle He refrigerator. The spectrometer resolu-
tion was better than 1 cm
-1
and the temperature accuracy
was 1 K. All the Raman measurements were taken on
as-grown surfaces of the ceramic samples.
PHYSICAL REVIEW B 1 SEPTEMBER 1999-I VOLUME 60, NUMBER 9
PRB 60 0163-1829/99/609/65134/$15.00 6513 ©1999 The American Physical Society