Q. Zhan et al.: The effect of doping Ag on the microstructure of La2/3Sr1/3MnO3 films
a field of only 4 kOe, which is larger by a factor of two
compared with the undoped one.27 This large enhance-
ment could be attributed to the existence of Ag and poly-
crystalline clusters in the doped films. The ferromagnetic
spin alignment would show disordered status to some
extent at the LSMO grain boundaries and AgO-LSMO
interfaces in the polycrystallinelike clusters. Nonmag-
netic metal Ag existing at the LSMO grain boundaries
could also weaken the ferromagnetic interaction between
grains and cause Mn spin disorder at the interfaces. They
serve as a strong scatter centers for the highly spin-
polarized conducting Mn3+ eg electrons and lead to
a high zero-field electrical resistance. Application of a
moderate magnetic field can readily align the spin and
drop the resistivity significantly. As a result, a large MR
value at low magnetic field was obtained.
V. CONCLUSIONS
Cross-section and plan-view observations of changes
in microstructure of LSMO films with different Ag-
doping level were given in detail. The effect of doping
Ag on the microstructure of the LSMO films depended
on the substrate temperature during deposition, and the
effect was significant when the substrate temperature
was low (i.e., 400 °C). With decrease in the substrate
temperature from 750 to 400 °C, epitaxial property of the
LSMO films deteriorated. In the doped LSMO films de-
posited at 400 °C, large polycrystalline clusters consist-
ing of LSMO, AgO, and Ag grains were formed, and Ag
existed at the LSMO grain boundaries in its elemental
state. This modified microstructure due to doping Ag
could lead to an enhanced low-field MR value.
FIG. 9. (a) EDS of a large cluster in the doped film, where Ag peaks were
clearly visible. (b) Nanometer-beam EDS analysis with a spatial resolu-
tion of 2–3 nm inside a LSMO grain, showing the absence of Ag.
When the substrate temperature was 400 °C, the de-
composition of AgO deposited at the substrate surface
could also occur. As discussed above, the decomposed
Ag atoms agglomerated at the LSMO grain boundaries.
At the same time, the diffusion of Ag atoms increased the
mobility of atoms in LSMO grains, leading to the ob-
served larger grain sizes. On the other hand, since the
substrate temperature is not high enough, the decompo-
sition of AgO could be incomplete, with some large AgO
particles kept in the films. Due to the existence of these
large AgO particles, the growth of the subsequent LSMO
grains, especially those lay on the AgO particles, could
not be epitaxial with respect to the LAO substrate. As a
result, LSMO grains with different orientations were
formed, giving the polycrystalline clusters.
ACKNOWLEDGMENT
The authors gratefully acknowledge the support from
the National Natural Science Foundation of China under
Grant No. 50071063.
REFERENCES
The magnetotransport properties were measured at a
low-field of 4 kOe for all of the films.27 The MR values
were similar for the doped and undoped LSMO films
deposited at 750 °C, with a maximum of approximately
3% at the vicinity of Tc and declined with decreasing
temperature. The MR values can even be ignored at
77 K. It is thus clear that the addition of Ag has no
obvious effect on the magnetotransport properties of the
epitaxial films deposited at 750 °C. This may be attrib-
uted to highly aligned grain boundaries in epitaxial films.
However, low-field MR of the Ag-doped LSMO
films grown at Ts ס
400 °C show a large enhancement
compared to the undoped ones. In particular, the MR
value at 77 K for doped films could be as large as 12% in
1. J.M.D. Coey, J. Appl. Phys. 85, 5576 (1999).
2. H.R. Khan, Mater. Sci. Forum 373–376, 93 (2001).
3. C. Zener, Phys. Rev. 82, 403 (1951).
4. H.Y. Hwang, S-W. Cheong, N.P. Ong, and B. Batlogg, Phys. Rev.
Lett. 77, 2041 (1996).
5. A. Gupta, G.Q. Gong, G. Xiao, P.R. Duncombe, P. Lecoeur,
P. Trouilloud, Y.Y. Wang, V.P. Dravid, and J.Z. Sun, Phys.
Rev. B 54, 15629 (1996).
6. X.W. Li, A. Gupta, G. Xiao, and G.Q. Gong, Appl. Phys. Lett. 71,
1124 (1997).
7. J.Y. Gu, C. Kwon, M.C. Robson, Z. Trajanovic, K. Ghosh, and
R.P. Sharma, Appl. Phys. Lett. 70, 1763 (1997).
8. H.L. Ju and H. Sohn, Solid State Commun. 102, 463 (1997).
9. T. Walter, K. Do¨rr, K-H. Mu¨ller, B. Holzapfel, D. Eckert,
M. Wolf, D. Schla¨fer, L. Schultz, and R. Gro¨tzschel, Appl. Phys.
Lett. 74, 2218 (1999).
2718
J. Mater. Res., Vol. 17, No. 10, Oct 2002
Downloaded: 14 Mar 2015
IP address: 130.102.42.98