Structure, magnetic and transport properties of Ti-substituted La$_{0.7}$Sr$_{0.3}$MnO$_{3}$
M. S. Kim
Ti-substituted perovskites, La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$, with 0$\leq$ x $\leq$ 0.20, were investigated by neutron diffraction, magnetization, electric resistivity, and magnetoresistance(MR) measurements. All samples show a rhombohedral structure (space group $R\overline{3}c$) from %%@ 10 K to room temperature. At room temperature, the cell parameters $a$, $c$ and the %%@ unit cell volume increase with increasing Ti content. However, at 10 K, the cell %%@ parameter $a$ has a maximum value for $x=0.10$, and decreases for $x>0.10$, while the unit cell volume remains nearly constant for $x>0.10$. The average (Mn,Ti)-O bond length increases up to %%@ x=0.15, and the (Mn,Ti)-O-(Mn,Ti) bond angle decreases with increasing Ti content to its %%@ minimum value at x=0.15 at room temperature. Below the Curie temperature T$_{C}$, the resistance exhibits metallic behavior for the $x\leq 0.05$ samples. A metal(semiconductor) to insulator transition is observed for the $x\geq 0.10$ %%@ samples. A peak in resistivity appears below T$_{C}$ for all samples, and shifts to a %%@ lower temperature as x increases. The substitution of Mn by Ti decreases the $2p-3d$ hybridization between O and Mn ions, reduces the bandwidth $W$, and increases %%@ the electron-phonon coupling. Therefore, the T$_C$ shifts to a lower temperature %%@ and the resistivity increases with increasing Ti content. A field-induced shift of the resistivity maximum occurs at $x$ $\leq$ 0.10. The maximum MR effect is about 70$\%$ for La$_{0.7}$Sr$_{0.3}$Mn$_{0.8}$Ti$_{0.2}$O$_{3}$. The separation of %%@ T$_C$ and the resistivity maximum temperature T$_{\rho,max}$ enhances the MR effect in %%@ these compounds due to the weak coupling between the magnetic ordering and the %%@ resistivity as compared with La$_{0.7}$Sr$_{0.3}$MnO$_{3}$.
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\begin{document}

\title{Structure, magnetic and transport properties of Ti-substituted 
La$_{0.7}$Sr$_{0.3}$MnO$_{3}$}

\author{M. S. Kim}
\affiliation{Department of Physics, University of Missouri-Rolla, Rolla,
MO 65409}
\affiliation{Graduate Center for Materials Research, University of 
Missouri-Rolla, Rolla, MO 65409}
\author{J. B. Yang}
\affiliation{Department of Physics, University of Missouri-Rolla, Rolla,
MO 65409}
\affiliation{Graduate Center for Materials Research, University of 
Missouri-Rolla, Rolla, MO 65409}
\author {Q. Cai}
\affiliation{Department of Physics, University of Missouri-Columbia, Columbia,
MO 65211}
\author{X. D. Zhou}
\affiliation{Graduate Center for Materials Research, University of 
Missouri-Rolla, Rolla, MO 65409}
\author{W. J. James}
\affiliation{Graduate Center for Materials Research, University of 
Missouri-Rolla, Rolla, MO 65409}
\affiliation{Department of Chemistry, University of Missouri-Rolla, Rolla,
MO 65409} 
\author{W. B. Yelon}
\affiliation{Graduate Center for Materials Research, University of 
Missouri-Rolla, Rolla, MO 65409}
\affiliation{Department of Chemistry, University of Missouri-Rolla, Rolla,
MO 65409}
\author{P. E. Parris}
\affiliation{Department of Physics, University of Missouri-Rolla, Rolla,
MO 65409}
\author{D. Buddhikot}
\affiliation{Tata Institute of Fundamental Research, Colaba, Mumbai 400-005, %%@
India}
\author{S. K. Malik}
\affiliation{Tata Institute of Fundamental Research, Colaba, Mumbai 400-005, %%@
India}

\begin{abstract}

Ti-substituted perovskites, La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$,
with 0$\leq$ x $\leq$ 0.20, were investigated by neutron diffraction, 
magnetization, electric resistivity, and magnetoresistance(MR) measurements. 
All samples show a rhombohedral structure (space group $R\overline{3}c$) from %%@
10 K to
room temperature. At room temperature, the cell parameters $a$, $c$  and the %%@
unit cell
 volume increase with increasing Ti content. However, at 10 K, the cell %%@
parameter $a$ 
has a maximum value for 
$x=0.10$, and decreases for $x>0.10$, while the unit cell volume remains nearly
constant for $x>0.10$. The average (Mn,Ti)-O bond length increases up to %%@
x=0.15, and 
the (Mn,Ti)-O-(Mn,Ti) bond angle decreases with increasing Ti content to its %%@
minimum 
value at x=0.15 at room temperature. Below the Curie temperature T$_{C}$, the 
resistance exhibits metallic behavior for the $x\leq 0.05$ samples. A 
metal(semiconductor) to insulator transition is observed for the $x\geq 0.10$ %%@
samples. 
A peak in resistivity appears below T$_{C}$ for all samples, and shifts to a %%@
lower 
temperature as x increases. The substitution of Mn by Ti decreases the $2p-3d$ 
hybridization between O and Mn ions, reduces the bandwidth $W$, and increases %%@
the 
electron-phonon coupling. Therefore, the T$_C$ shifts to a lower temperature %%@
and the 
resistivity increases with increasing Ti content. A field-induced shift of the 
resistivity maximum occurs at $x$ $\leq$ 0.10. The maximum MR effect is about 
70$\%$ for La$_{0.7}$Sr$_{0.3}$Mn$_{0.8}$Ti$_{0.2}$O$_{3}$. The separation of %%@
T$_C$ and 
the resistivity maximum temperature T$_{\rho,max}$ enhances the MR effect in %%@
these 
compounds due to the weak coupling between the magnetic ordering and the %%@
resistivity as 
compared with La$_{0.7}$Sr$_{0.3}$MnO$_{3}$.  
\end{abstract}

\pacs{75.50Ee, 61.10Nz, 76.80+y, 81.40-Z}
\maketitle

\section{Introduction}

The A$_{1-x}$A'$_x$MnO$_3$ perovskites are interesting systems because of the %%@
anomalous 
magnetic and transport properties exhibited by them such as colossal %%@
magnetoresistance 
(CMR), metal-insulator transitions, antiferromagnetic-ferromagnetic ordering, %%@
and 
lattice dynamics associated with phase transitions. 
\cite{searle,goodenough,kusters,helmolt,jin,unishibara,tomioka,tokura} Zener's %%@
double 
exchange (DE) interaction between Mn$^{3+}$ and Mn$^{4+}$ ions through charge %%@
carriers 
in the oxygen $2p$ orbitals was introduced in order to explain the coupling of %%@
magnetic 
and electronic properties in these compounds.\cite{zener,anderson,gennes,kubo} %%@
Undoped 
LaMnO$_3$ is an \emph{A}-type antiferromagnetic insulator. By substitution of %%@
La$^{3+}$ 
with a divalent cation, LaMnO$_3$ can be driven into a metallic and %%@
ferromagnetic 
state.  Both Mn$^{3+}$ and Mn$^{4+}$ ions possess a local spin (S=3/2) from %%@
their lower 
$t_{2g}^3$ orbitals, and Mn$^{3+}$ has an extra electron in the $e_g$ orbital %%@
which is 
responsible for conduction. The spin of the $e_g^1$ electron in Mn$^{3+}$ is 
ferromagnetically coupled to the local spin of $t_{2g}^3$ according to Hund's %%@
rule. Sr 
doping induces holes in the $e_g$ band near the Fermi energy, producing mobile %%@
holes 
and conduction. However, recent studies have shown that DE is not sufficient to %%@
explain 
the complex physics in these compounds, especially as regards  the lattice %%@
distortions 
coinciding with the emergence of CMR \cite{millis,millis2}. An understanding of %%@
the 
Sr-doped systems requires one to consider both DE interactions in the 
Mn$^{3+}$-O-Mn$^{4+}$ pairs and the strong electron-phonon coupling, including %%@
lattice 
polarons and dynamic Jahn-Teller(J-T) distortions.\cite{dai,radaelli} The %%@
polaron 
effect arising from J-T distortion was introduced to explain the electronic %%@
transport 
mechanism in the high temperature region, T $\approx$ T$_{C}$, where a strong 
electron-phonon interaction is required to reduce the kinetic energy of the %%@
conduction 
electrons. The local J-T distortion of the MnO$_{6}$ octahedron lowers the %%@
energy of 
the $e^1_{g}$ electron and the charge carrier can then be localized to form a %%@
lattice 
phonon. Therefore local lattice distortion above T$_{C}$ rapidly decreases %%@
electron 
hopping, thus increasing the resistivity.\cite{millis2} Recently, it was found %%@
that 
polaron hopping was also the dominant conduction mechanism below 
T$_{C}$.\cite{ju,dessau,kim,zhao} A sharp increase of polaron density at %%@
temperatures 
below T$_{C}$ leads to a charge carrier density collapse, which is related to %%@
the 
resistivity peak and the CMR of doped manganites.\cite{alexandrov}

In order to understand the unusual magnetic and transport properties of doped 
perovskites A$_{1-x}$D$_{x}$MnO$_{3}$, many studies have been carried out by %%@
doping the 
trivalent rare earth site(A site) with divalent atoms(Ca, Sr, Ba, 
etc).\cite{jin,neumeier,muroi,medarde,cao,xiaming} 
Other studies have also shown that substitution for Mn (B site) dramatically %%@
affects 
the magnetic and transport properties of perovskites.\cite{martin,ahn,blasco}
The B site modification has merit in that it directly affects the Mn network by 
changing the Mn$^{3+}$/Mn$^{4+}$ ratio and the electron carrier density.  %%@
Therefore in 
order to better understand the role of Mn and its local environment in 
La$_{0.7}$Sr$_{0.3}$MnO$_{3}$, we studied the effects of replacing some of the %%@
Mn with 
Ti. The structural, magnetic and electrical phase transitions and transport %%@
properties 
of La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ with 0 $\leq$x$\leq$ 0.20 have %%@
been 
investigated by neutron diffraction, magnetization, electric resistivity, and 
magnetoresistance measurements and the results are presented here.

\section{Experimental}
Samples of Ti-substituted La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$, with %%@
$0\leq 
x\leq 0.2$, were prepared using the conventional solid state reaction method. %%@
Highly
purified La$_{2}$O$_{3}$, SrCO$_{3}$, TiO$_{2}$, MnO were mixed in %%@
stoichiometric 
ratios, ground, and then pelletized under 10,000 psi pressure to a 1 cm %%@
diameter. The 
pelletized samples were fired at $1500^{\circ}C$ in air for 12 hours, then %%@
reground and 
sintered at $1250^{\circ}C$ for 24 hours in air. X-ray diffraction of the %%@
powders was 
carried out at room temperature using a SCINTAG diffractometer with %%@
Cu-K$\alpha$
radiation. X-ray diffraction data indicated all samples to be single phase. %%@
Powder
neutron diffraction experiments were performed at the University of %%@
Missouri-Columbia 
Research Reactor(MURR) using neutrons of wavelength $\lambda$=1.4875 $\AA$. The %%@
data 
for each sample were collected between 2$\theta$ = 5.65 - 105.60$^{\circ}$ at %%@
300K and 
10 K. Refinement of the neutron diffraction data was carried out using the %%@
FULLPROF 
program\cite{full}, which permits multiple phase refinements as well as %%@
magnetic 
structure refinements. Magnetic measurements were conducted with a SQUID 
magnetometer(MPMS, Quantum design). The magnetization curves with zero-field 
cooling(ZFC) and field cooling(FC) were measured in an applied magnetic field %%@
of 50Oe.
Resistivity data were obtained using a physical properties measurement %%@
system(PPMS, 
Quantum design) with a standard four-point probe method.

\section{Results and Discussion}

Figure 1 shows the x-ray diffraction patterns of 
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ samples, with $0\leq 
x\leq 0.2$, at room temperature(RT). All the samples are single phase and all %%@
peak 
positions can be indexed to La$_{0.67}$Sr$_{0.33}$MnO$_{2.91}$(JCPDS 50-0308), %%@
space 
group $R$$\overline{3}$c. In order to investigate the details of the structural 
distortion and the magnetic interactions in these compounds, powder neutron %%@
diffraction 
measurements were performed at different temperatures. Figure 2 shows the %%@
neutron 
diffraction patterns of La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ with %%@
$x=0.0$, x=0.03 and 
0.15 measured at RT and 10 K. All patterns can be fitted with the %%@
$R$$\overline{3}$c 
rhombohedral space group (No. 167) in which the atomic positions are La(Sr): 6a 
(0,0,1/4), Mn(Ti): 6b (0,0,0); O18e (x,0,1/4). The $P1$ space group was used to %%@
fit the 
magnetic structure with collinear Mn magnetic moments because of its %%@
flexibility.   Refined structural and magnetic parameters are listed in Tables %%@
I and II for RT and 10K, respectively. For samples with $x \geq 0.10$, there is %%@
no magnetic ordering at RT since T$_{C}<$ RT, whereas for 
samples with $x \leq 0.10$, T$_{C}>$ RT magnetic ordering is observed. The %%@
arrows on 
the neutron diffraction patterns of the $x=0.0$ sample (Figure 2 ) indicate %%@
magnetic 
reflection peaks that are not present for the $x=0.20$ sample at RT. The peak 
intensities of the magnetic reflections decrease with Ti substitution at both %%@
RT and 10 
K. In addition, the refinement results confirm that the substituted Ti ions go %%@
into B 
sites, not into A sites, because the ionic radius of Ti$^{4+}$ (0.605$\AA$) %%@
lies 
between the ionic radius of Mn$^{4+}$ (0.530$\AA$) and Mn$^{3+}$ 
(0.645$\AA$)\cite{shannon}. The tolerance factor, which is the geometric %%@
measure of 
size mismatch of perovskites, 

\begin{equation}
	t=(r_{(La,Sr)}+r_{O})/[(r_{(Mn,Ti)}+r_{O})\sqrt{2}]
\end{equation}
 
decreases linearly from 0.928 for La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ to 0.921 for 
La$_{0.7}$Sr$_{0.3}$Mn$_{0.8}$Ti$_{0.2}$O$_{3}$, which is in the stable range %%@
of the 
perovskite structure 0.89$<$t$<$1.02.\cite{xiaming} Therefore, substitution of %%@
Mn by Ti 
does not change the crystal structure itself but changes the bond lengths and %%@
the bond 
angles of the MnO$_{6}$ octahedra.

Fig. 3 plots the lattice parameters $a$,$c$ and the unit cell volumes  of %%@
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ versus the Ti(Ti$^{4+}$) content %%@
at room temperature and 10 K. The lattice parameters $a$,$c$ and the unit cell %%@
volume increase with the Ti content at 
RT. At 10 K, the lattice parameter $a$ shows a maximum value at $x=0.10$ and %%@
then 
decreases as $x>0.10$, and the unit cell volume increases up to $x=0.10$ and %%@
becomes 
almost constant for $x>0.10$. The refined magnetic moment of the Mn atoms %%@
indicate that Mn atoms have a high spin state, and the 
average valence state of the Mn varies from 3d$^{3.5}$ to 3d$^{3.3}$ for x=0.0 %%@
and 
x=0.15, which suggests that the Ti atoms are in the Ti$^{4+}$ state. The values %%@
of the temperature factor B of oxygen increase with increasing Ti content which %%@
is consistent with the increase of the Mn-O bond length. This is likely related %%@
to the structural disorder in the position of oxygen atoms due to the %%@
substitution of Mn for Ti.

The average (Mn,Ti)-O bond length and (Mn,Ti)-O-(Mn,Ti) bond angle extracted %%@
from the 
Rietveld refinement at RT and 10K are shown in Figure 4.  
The bond length of La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ increases up %%@
to 
$x=0.15$ and remains constant thereafter for $x\geq 0.15$, while the bond angle 
decreases and attains an anomalous minimum value for $x=0.15$ at RT. 
At 10K, the bond length increases up to $x=0.10$ and remains constant for %%@
$x>0.10$, 
while the bond angle decreases with increasing $x$.
The bond length and the bond angle are closely related to the oxygen positions. 
Therefore, an increasing (Mn-Ti)-O bond length and a decreasing %%@
(Mn,Ti)-O-(Mn,Ti) bond 
angle are strongly correlated. The changes in bond length and bond angle of %%@
MnO$_{6}$ 
compensate one another to diminish the internal strain induced by Ti$^{4+}$.
Since the exchange interaction between Mn-Mn depends on both the bond angle and %%@
the 
bond distance, the decrease in bond angle and the increase in bond length %%@
decrease the 
Mn-Mn exchange interaction which leads to a lower magnetic ordering temperature 
T$_C$(see later discussion of M-T curves).

The electronic bandwidth $W$ has been used to discuss magnetic and transport %%@
properties 
of perovskites with varied A-site doping.\cite{radaelli,hwang}. The empirical %%@
formula 
of the bandwidth $W$ for $ABO_{3}$-type perovskites using the tight binding 
approximation\cite{medarde} is
\begin{equation} 
	W \propto \frac{cos\omega}{(d_{Mn-O})^{3.5}},
\end{equation} 
 where $\omega = \frac{1}{2} ( \pi -\langle Mn-O-Mn \rangle)$ and d$_{Mn-O}$ is %%@
the 
Mn-O bond length. The calculated values of ${cos\omega}/{(d_{Mn-O})^{3.5}}$ %%@
using the  
refinement results are shown in Figure 4(c). We assumed the bandwidth $W$ is %%@
proportional
to the values of ${cos\omega}/{(d_{Mn-O})^{3.5}}$. It is found that the %%@
bandwidth $W$ 
decreases with increasing Ti content. Further, the bandwidth at RT is smaller %%@
than the 
bandwidth at 10K for a given Ti content. The evolution of the bandwidth follows %%@
the 
change in the $\langle Mn-O-Mn \rangle$ bond angle. The decrease in bandwidth %%@
reduces 
the overlap between the O-$2p$ and the Mn-$3d$ orbitals, which in turn %%@
decreases the 
exchange coupling of Mn$^{3+}$-Mn$^{4+}$, and the magnetic ordering temperature %%@
T$_C$ 
as well. 
For a charge-transfer insulator, the band gap energy $E_{g}$ in the insulating %%@
phase 
can be written as $E_{g}=\Delta-W$, where $\Delta$ is the charge-transfer %%@
energy and 
$W$ is the O-$2p$-like bandwidth. 
In practice, $\Delta$ changes little in the La$_{1-x}$Sr$_{x}$MnO$_{3}$ system %%@
and thus 
the bandwidth $W$ becomes a main factor in tuning the band gap %%@
energy.\cite{harrison} 
For the La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ compounds, the decrease %%@
in 
bandwidth $W$ increases the band gap, $E_{g}$, and leads to the metal to %%@
insulator 
transition for $x > 0.10$.

Figure 5 shows the magnetization versus temperature (M-T) curves measured under 
field-cooled (FC) and zero field-cooled (ZFC) conditions in a magnetic field of %%@
50 Oe 
for the $x=0.05, 0.10$, and 0.15 samples. A sharp paramagnetic to ferromagnetic 
transition is observed at a critical temperature T$_{C}$. Figure 6 shows the %%@
Curie 
temperatures, T$_{C}$, of La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ for %%@
differing 
Ti content. The decrease in T$_{C}$ is obviously related to the changes in %%@
bandwidth as 
seen in Fig. 4(c). The T$_{C}$ drops at a rate of about 10K per Ti. %%@
$\lambda$-shaped 
magnetization curves in ZFC emerge for $x \geq 0.10$ samples. The existence of 
$\lambda$-shaped curves under ZFC may be evidence of the formation of %%@
ferromagnetic 
clusters with a spin glass state. The Ti substitution weakens the exchange %%@
interaction 
and breaks the Mn-O-Mn network, and creates short range ordered ferromagnetic %%@
clusters. 
As more Ti is substituted, more inhomogeneous clusters are formed, which leads %%@
to a 
broadening of the paramagnetic to ferromagnetic phase transition peak. A %%@
similar 
phenomenon has been observed in the La$_{0.7}$Ca$_{0.3}$Mn$_{1-x}$Ti$_x$O$_3$ 
system.\cite{xiaming}

Magnetization versus field (M-H) curves of 
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ at different temperatures are %%@
plotted in 
Figure 7.
At 20 K, all samples reach a nearly constant value of magnetization under a %%@
field, H = 
0.6 T. 
The estimated magnetic moments of the $x=$ 0.0, 0.05, 0.10 and 0.15 samples %%@
from 
magnetization data at 20 K are 3.79, 3.54, 3.24, and 2.49 $\mu_{B}$ per Mn %%@
atom, 
respectively. These moment values are in good agreement with the neutron %%@
diffraction 
refinement results (see Table II). The theoretically estimated magnetic moments %%@
of Mn 
from its valence state taking into account the dilution effect of Ti$^{4+}$, %%@
are 3.70, 3.55, 
3.40 and 3.35 $\mu_{B}$ respectively. This suggests that the decrease of  %%@
magnetization 
with increasing Ti content is not only due to the dilution of magnetic %%@
Mn$^{4+}$ atoms 
but also due to the weakening of exchange coupling by the cluster formation.

Figure 8 shows the resistivity as a function of temperature under different %%@
applied 
fields for La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ compounds with $x %%@
=0.0$, 
0.05, 0.10 and x=0.15. In the temperature range 4 - 300 K, the resistivity of %%@
the samples increases as the Ti content increases. The resistivity of %%@
0.05$\leq$ $x\leq $ 0.10 shows a maximum value at temperature 
T$_{\rho,max}$ below T$_{C}$, and then decreases as the temperature decreases. %%@
Finally 
the resistivity increases again as T decreases further for $x\geq 0.10$. The %%@
difference 
between T$_{C}$ and T$_{\rho, max}$ becomes larger as the Ti content increases %%@
and 
T$_{\rho,max}$ is lower than T$_{C}$. This behavior is quite different from %%@
that 
observed in the Ti-substituted La$_{0.7}$Ca$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ %%@
series 
which exhibit large differences between T$_{C}$ and T$_{\rho,max}$\cite{cao}, %%@
and 
T$_{\rho,max}$ is higher than T$_{C}$.\cite{xiaming}.
For the $x \leq 0.15$ sample, a metal(semiconductor) to insulator transition %%@
(MIT) 
appears in the low temperature region. The field-induced shift of maximum %%@
resistivity 
to higher temperature appears for the $x \leq 0.10$ samples, and becomes %%@
negligible for 
$x \geq 0.15$. 
The suppression of the resistivity by the applied magnetic field occurs over %%@
the entire 
temperature range for all samples. At T $>$ T$_{C}$, the suppression of the %%@
resistivity 
becomes weaker. According to the DE mechanism, the mobility of the charge %%@
carriers 
$e_{g}$ electrons improves if the localized spins are polarized. The applied %%@
field 
aligns the canted electron spins which should reduce the scattering of %%@
itinerant 
electrons with spins and thus the resistivity is reduced. Therefore an applied %%@
magnetic 
field competes with the thermal fluctuations and maintains magnetic ordering %%@
around  
T$_{C}$ for the $x\leq 0.10$ samples, and thus shifts the T$_{\rho,max}$ to %%@
higher 
temperatures. 

Figure 9 shows the typical temperature dependence of the magnetoresistance [MR %%@
= 
$(\rho_{0}-\rho_{H}) / \rho_{0} \times 100$] of 
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ samples with 0$\leq$ x $\leq$ %%@
0.20 under an 
applied field of 1 and 3T. The maximum magnetoresistance increases with %%@
increasing Ti concentration. For example, the maximum MR values are 30$\%$, %%@
55$\%$ under 3T for 
$x=0.05$, $0.15$, respectively. The temperature of the MR peak shifts to a %%@
lower 
temperature, approximately 15 K per Ti. It is known that in A-site, %%@
electron-doped 
A$_{1-x}$A$'$$_x$MnO$_3$(x=0.3) compounds, the metal-insulator transition %%@
temperature 
T$_{MI}$ coincides with the T$_c$, and the metal-insulator transition is %%@
strongly 
coupled with the magnetic ordering transition. Therefore, a strong variation of %%@
the 
electrical resistivity up to several orders of magnitude, namely the colossal 
magnetoresistance (CMR) effect, occurs upon application of a magnetic field %%@
near 
T$_{C}$. However, for La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ compounds, %%@
the 
T$_{C}$ is different from the metal to insulator transition temperature (MIT). %%@
The 
application of a magnetic field has much more effect on the change of electric 
resistivity when compared to La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ due to the weak %%@
coupling 
between the MIT and the magnetic ordering. An enhancement of the MR effect is %%@
observed 
in these compounds, similar to that in 
La$_{0.7}$Ca$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$\cite{cao,xiaming} and 
Pr$_{1-x}$(Ca,Sr)$_{x}$MnO$_{3}$ compounds.\cite{maignan}
	
	The change of the electronic properties of Ti-substituted 
La$_{0.7}$Ca$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ compounds is strongly related to %%@
the 
electron phonon coupling \cite{millis2}. Accordingly, in the 
La$_{1-x}$Sr$_{x}$MnO$_{3}$ system, the strong electron-phonon coupling %%@
localizes the 
conduction band electron as a polaron, due to the competition between the self %%@
trapping 
energy E$_{J,T}$ and the electron itinerant energy. The electron-phonon %%@
coupling 
constant $\lambda$ = E$_{J,T}$/$t$, where $t$ is the electron hopping parameter %%@
which is proportional to the electronic bandwidth $W$. As mentioned above, the %%@
substitution of 
Mn by Ti decreases the overlap of the O-$2p$ and Mn-$3d$ orbitals due to the %%@
decrease 
in $W$, thus increasing the electron-phonon coupling. This results in a shift %%@
of 
T$_{C}$ to lower temperatures and an increase of resistivity with increasing Ti 
content. As a consequence, one should consider a possible dependence of %%@
E$_{J,T}$ on Ti 
content. We cannot rule out the contribution from E$_{J,T}$, even though our %%@
data 
indicate that all the observed T$_{C}$ and resistivity changes can be %%@
explained, at 
least qualitatively, by the change in $W$. Especially, for $x \geq 0.15$, the 
electron-phonon coupling becomes very strong, and the insulator behavior occurs %%@
below 
T$_{C}$ as shown in Figure 8. The changes in bandwidth $W$ are not large enough %%@
to 
account for the dramatic changes in resistivity, and therefore, E$_{J,T}$ might %%@
be 
contributing significantly to the change of resistivity in these samples.        

	It has been proposed that, above T$_{C}$, charge may be localized in the %%@
form of 
J-T polarons.\cite{alexandrov} At T $\geq$ T$_{C}$, the resistivity of the CMR 
materials can be explained by the activated adiabatic polaron equation %%@
\cite{emin}

 \begin{equation}
	\rho=AT \exp (E_{hop}/kT)
\end{equation} 

 Figure 10 shows the plot of ln($\rho$/T) as a function 
of 1/T for La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ compounds with %%@
$x=$0.00, 0.05, 0.10, 0.15 and 0.20 in the high temperature region with zero %%@
field resistivity data. Resistivity of all the samples 
shows a similar slope at T $\geq$ T$_{C}$, which can be fitted well with the %%@
small 
polaron model indicating the formation of a polaron. The polaron hopping energy 
$E_{hop}$ is calculated from the slopes. The calculated values of $E_{hop}$ are %%@
14.5 49.8, 
132.0, 138.3, and 152.5 $meV$ for $x=0.00, 0.05, 0.10, 0.15$, and 0.20, %%@
respectively. The increase of $E_{hop}$ is due to the substitution of Mn by Ti %%@
which depletes the oxygen $p$ holes and leads to an increase in the polaron %%@
binding energy. This further confirms 
that Ti substitution at Mn enhances the electron-phonon interaction, which %%@
decreases 
$W$ and increases $E_{hop}$ at high temperatures. The calculated polaron %%@
hopping energy 
shows a large variation between the $x=0.05$ and the $x\geq 0.10$ samples. This %%@
is in 
good agreement with the sharp increase in resistivity and its temperature %%@
dependence. 
Some studies have also suggested that polaron hopping is the prevalent %%@
mechanism to 
explain resistivity below T$_{C}$.\cite{hundley,pickett}. However, we were %%@
unable to 
fit the resistivity data of the Ti-doped La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ samples %%@
below 
T$_{C}$ with several other polaron models such as the semiconducting %%@
model,\cite{mott} 
the variable range hopping(VHR) polaron model,\cite{sun} and the adiabatic %%@
polaron 
hopping model. Only the VHR polaron model works reasonably well for the low %%@
temperature 
region, T $<$ 75K, for the insulating state, $x=0.10$ sample. There may be %%@
other 
contributions, such as ferromagnetic clusters, which would increase the %%@
resistivity of 
the compound.



\section{Summary}

	The magnetic and electronic transport properties of Ti-substituted 
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ have been systematically %%@
investigated. 
All the Ti-substituted La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ %%@
compositions have 
a rhombohedral structure, (space group $R\overline{3}c$). The correlation %%@
between 
ferromagnetic T$_{C}$ and T$_{\rho,max}$ becomes weaker and spin glass clusters %%@
are 
expected in the low temperature region with increasing Ti substitution. The %%@
resistivity 
in the high temperature region suggests the formation of localized polarons %%@
that affect 
the strong correlation between local structural changes and the MIT. The %%@
decrease of the 
bandwidth $W$ decreases the overlap between the O-$2p$ and Mn-$3d$ orbitals, %%@
which in 
turn decreases the exchange coupling of Mn-Mn and the magnetic ordering %%@
temperature 
T$_c$ as well. Our studies indicate that Ti substitution at Mn enhances the 
electron-phonon interaction in these compounds, which decreases the bandwidth %%@
and 
increases the resistivities in the entire temperature range. 

\begin{acknowledgements}
The authors thank Aranwela Hemantha for invaluable help in magnetoresistance 
measurements. The support by DOE under DOE contract \#DE-FC26-99FT400054 is 
acknowledged. 
\end{acknowledgements}

\newpage
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\bibitem{goodenough} J. B. Goodenough, "Magnetism and the Chemical Bond", %%@
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New-York/London (1963).

\bibitem{kusters} R. M. Kusters, J. Singleton, D. A. Keen, R. McGreevy, W. %%@
Hayes, 
Physica(Amsterdam). {\bf 155B}, 362 (1989).

\bibitem{helmolt} R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. %%@
Samwer, Phys. 
Rev. Lett. {\bf 71}, 2331 (1993).

\bibitem{jin} S. Jin, T. H. Tiefel, M. McCormack, R. A. Fastnacht, R. Ramesh, %%@
L. H. 
Chen,  Science. {\bf 264}, 413 (1994).

\bibitem{unishibara} A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. %%@
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Tokura, Phys. Rev. B {\bf 51}, 
14103 (1995)
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\bibitem{tomioka} Y. Tomioka, A. Asamitsu, H. Kuwahara, Y. Moritomo, Y. Tokura, %%@
Phys. 
Rev. B {\bf 53}, R1689 (1996).

\bibitem{tokura} Y. Tokura, "Fundamental features of colossal
magnetoresistive manganese oxides", In: Y. Tokura (Ed.),
Contribution to Colossal Magnetoresistance Oxides, Monographs in
Condensed Matter Science, Gordon \& Breach, London (1999).

\bibitem{zener} C. Zener, Phys. Rev. {\bf 82}, 403 (1951).

\bibitem{anderson} P. W. Anderson, and H. Hasegawa, Phys. Rev. {\bf 100}, 675 %%@
(1955).

\bibitem{gennes} P. G. de Gennes, Phys. Rev. {\bf 118}, 141 (1960).

\bibitem{kubo} K. Kubo, N. Ohata, J. Phys. Soc. Japan {\bf 33}, 21 (1972).

\bibitem{millis} A. J. Millis, P. B. Littlewood, and B. I. Shraiman, Phys. Rev. %%@
Lett. 
{\bf 74}, 5144 (1995).

\bibitem{millis2} A. J. Millis, B. I. Shraiman, and R. Mueller, Phys. Rev. %%@
Lett. {\bf 
77}, 175 (1996).

\bibitem{dai} P. Dai, J. Zhang, H. A. Mook, S. H. Liou, P.A. Dowben, and E. W. %%@
Plummer, 
Phys. Rev. B {\bf 54}, R3694 (1996).

\bibitem{radaelli} P. G. Radaelli, G. Iannone, M. Marezio, H. Y. Hwang, S-W. %%@
Cheong, J. 
D. Jorgensen, and D. N. Argyriou, Phys. Rev. B. {\bf 56}, 8265 (1997).

\bibitem{ju} H. L. Ju, H. C. Sohn, and K. M. Krishnan, Phys. Rev. Lett. {\bf %%@
79}, 3230 
(1997).

\bibitem{dessau}  D. S. Dessau, T. Saitoh, C. H. Park, Z. X. Shen, P. Villella, %%@
N. 
Hamada, Y. Moritomo, and Y. Tokura, Phys. Rev. Lett. {\bf 81}, 192 (1998).

\bibitem{kim} K. H. Kim, J. H. Jung, and T. W. Noh, Phys. Rev. Lett. {\bf 81}, %%@
1517 
(1998).

\bibitem{zhao} G. M. Zhao, V. Smolyaninova, W. Prellier, and H. Keller, Phys. %%@
Rev. 
Lett. {\bf 84}, 6086 (2000).

\bibitem{alexandrov} A. S. Alexandrov and A. M. Bratkovsky, Phys. Rev. Letter. %%@
{\bf 
82}, 141(1999).

\bibitem{neumeier}J. J. Neumeier, K. Andres, and K. J. McClellan, Phys. Rev. B, %%@
{\bf 
59}, 1701 (1999).

\bibitem{muroi} M. Muroi, R. Street, and P. G. McComick, J. Appl. Phys. {\bf %%@
87}, 3424 
(2000). 

\bibitem{medarde} M. Medarde, J. Mesot, P. Lacorre, S. Rosenkranz, P. Fischer, %%@
and K. 
Gobrecht,  Phys. Rev. B {\bf 52}, 9248 (1995).

\bibitem{cao} D. Cao, F. Bridges, M. Anderson, A. P. Ramirez, M. Olapinski, M. %%@
A. 
Subramanian, C. H. Booth, and G. H. Kwei, Phys. Rev. B. {\bf 64}, 184409 %%@
(2001).

\bibitem{xiaming} Xiaming Liu, Xiaojun Xu, and Yuheng Zhang, Phys. Rev. B {\bf %%@
62},  
15112 (2000).

\bibitem{martin} C. Martin, A. Maignan, and B. Raveau, J. Mater. Chem. {\bf 6}, %%@
1245 
(1996).

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81}, 
5505 (1997).

\bibitem{blasco} J. Blasco, J. Garcia, J. M. de Teresa, M. R. Ibarra, J. Perez, %%@
P. A. 
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\bibitem {full}  Rodriguez-Carvajal J., Program: FULLPROF, Version 3.5d.

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Condens. 
Matter {\bf 12}, 4975 (2000).

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Diffr., 
Theor. Gen. Crystallogr. {\bf 32}, 
751 (1976).

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Batlogg, 
Phys. Rev. Letter. {\bf 75}, 914 (1995).

\bibitem{harrison} W. A. Harrison, {\em {The electronic structure and %%@
properties of 
solids}} (Freeman, San Francisco, 1980).

\bibitem{maignan} A. Maignan, C. Martin, and B. Raveau, Z. Phys. B {\bf 102}, %%@
19 
(1997).

\bibitem{emin} D. Emin and T. holstein, Ann. Phys. (N.Y.) {\bf 53}, 439 (1969).

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Marquina, J. Garcia, J. Stankiewicz, and C. Ritter, Phys. Rev. B {\bf 56}, 8252 %%@
(1997).

\bibitem{hundley} M. F. Hundley, M. Hawley, R. H. Heffner, Q. X. Jia, J. J. %%@
Neumeier, 
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(1995).

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\end{thebibliography}

\newpage
\begin{table}
\caption{Refined parameters for 
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ compound with %%@
\emph{R$\overline{3}$c} 
space-group at room temperature(T=300K). Numbers in parentheses are statistical %%@
errors. 
$a$ 
and $c$ are the lattice parameters. $m$ is magnetic moment. $V$ is the unit %%@
cell 
volume. $B$ is the isotropic temperature parameter. 
$\chi^2$ is [R$_{wp}$/R$_{exp}$]$^2$ where R$_{wp}$ is the residual error of %%@
the 
weighted profile.}

\begin{ruledtabular}
\begin{tabular}{ccccccc}
Composition   & 0.00     & 0.03        & 0.05         & 0.10        & 0.15       %%@
& 0.20 \\ 
\hline
$a$ ($\AA$)    & 5.5038(2) & 5.5107(1) & 5.5157(2) & 5.5225(2) & 5.5306(2) & %%@
5.5310(2) \\
$c$ ($\AA$)    & 13.3553(5) & 13.3635(4)& 13.3699(5)& 13.3845(5)& 13.4032(6) %%@
&13.4124(6) \\
$V$ ($\AA^{3}$) & 350.364(18) & 351.445(16) & 352.261(18) & 353.508(21) & %%@
355.042(22) &355.341(23)\\
\hline
$m$ ($\mu_{B}$) & 2.514(28) & 2.121(33)   & 1.022(63)   & 0.0  & 0.0   & 0.0  %%@
\\
$\chi^{2}$ ($\%$) & 2.81  & 3.28    & 3.60    & 3.23   & 4.64  & 2.98 \\
\hline
O, 18e,x  & 0.5422(2)  & 0.5437(2) & 0.5448(2)  & 0.5460(2) & 0.5469(2) & %%@
0.5461(2)\\
B($\AA^{2}$), La(Sr),6a & 0.882(33)  & 0.8124(33)   & 0.873(35)    & 1.030(42)   %%@
& 0.975(43)   & 1.149(40) \\
B($\AA^{2}$), Mn(Ti),6b & 0.423(54) & 0.556(56)   & 0.574(59)    & 0.364(66)   %%@
& 0.394(67)   & 0.404(63) \\
B($\AA^{2}$), O,18e & 1.221(25)  & 1.248(25)   & 1.306(27)    & 1.501(37)   & %%@
1.475(37)   & 1.586(35)

\end{tabular}
\end{ruledtabular}
\end{table}

\newpage
\begin{table}
\caption{ Refined parameters for 
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ compound with %%@
\emph{R$\overline{3}$c} 
space-group at low temperature(T=10K). Numbers in parentheses are statistical %%@
errors. 
$a$ and $c$ are the lattice parameters. $m$ is magnetic moment. $V$ is the unit %%@
cell 
volume. $B$ is the isotropic temperature parameter. $\chi^2$ is 
[R$_{wp}$/R$_{exp}$]$^2$ where R$_{wp}$ is the residual error of the weighted %%@
profile.}

\begin{ruledtabular}
\begin{tabular}{ccccccc}
Composition  & 0.00   & 0.03        & 0.05         & 0.10        & 0.15       & %%@
0.20 \\ 
\hline
$a$ ($\AA$) & 5.4811(1) & 5.4940(1) & 5.4989(1) & 5.5116(1) & 5.5089(2) & %%@
5.5053(3) \\
$c$ ($\AA$) & 13.2756(3)    & 13.3037(4)& 13.3137(4)& 13.3354(4)& 13.3421(6) %%@
&13.3746(10) \\
$V$ ($\AA^{3}$) & 345.397(13) & 347.756(14) & 348.644(16) & 350.820(15) & %%@
350.652(22) &351.056(35)\\
\hline
$m$ ($\mu_{B}$) & 3.443(25) & 3.461(27)   & 3.506(32)   & 3.422(28)  & %%@
3.282(36)   & 2.913(51)  \\
$\chi^{2}$ ($\%$) & 3.23  & 3.51    & 2.89    & 2.69   & 3.72  & 4.90 \\
\hline
O, 18e,x  & 0.5431(1)  & 0.5442(2) & 0.5448(1)  & 0.5467(1) & 0.5469(2) & %%@
0.5472(2)\\
B($\AA^{2}$), La(Sr),6a & 0.167(26) & 0.240(28)   & 0.302(33)    & 0.318(28)   %%@
& 0.449(37)   & 0.269(48) \\
B($\AA^{2}$), Mn(Ti),6b &  0.127(45) & 0.230(49)   & 0.271(57)    & 0.169(47)   %%@
& 0.291(62)   & 0.174(81) \\
B($\AA^{2}$), O,18e & 0.328(21) & 0.536(23)   & 0.595(27)    & 0.649(22)   & %%@
0.926(28)   & 0.945(36)

\end{tabular}
\end{ruledtabular}
\end{table}


\newpage
\begin{figure}
%\includegraphics{fig1.eps}
\caption{X-ray diffraction patterns of %%@
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ 
(0$\leq$ x $\leq$ 0.20) at room temperature.}
\end{figure}

\begin{figure}
%\includegraphics{fig2.eps}
\caption{Neutron diffraction patterns of
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ (x=0.0, x=0.03 and x=0.20) at 10 %%@
K and RT. (The
bottom curves(Yobs-Ycal) are the difference between experimental
data and refinement data. The vertical bars indicate the
magnetic(bottom) and Bragg(top) peak positions). Arrows indicate some of major %%@
magnetic diffraction peaks.}
\end{figure}

\begin{figure}
%\includegraphics{fig3.eps}
\caption{Lattice parameter $a$, $c$, and volume of 
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ versus Ti content at room %%@
temperature and at 10 K.}
\end{figure}

\begin{figure}
%\includegraphics{fig4.eps}
\caption{Average (Mn,Ti)-O bond lengths(a), (Mn,Ti)-O-(Mn,Ti) bond angles(b) %%@
and  
electronic bandwidth parameter ${cos\omega}/{(d_{Mn-O})^{3.5}}$(c), of 
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ at room temperature and at 10 K.}
\end{figure}

\begin{figure}
%\includegraphics{fig5.eps}
\caption {The magnetization versus temperature(M-T) curves of 
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ (x=0.05, 0.10, 0.15)  measured %%@
under 
field cooling (FC) and zero field cooling (ZFC) conditions in a magnetic field %%@
of 50 Oe 
.}
\end{figure}

\begin{figure}
%\includegraphics{fig6.eps}
\caption{The Curie temperature (T$_{C}$), and the temperature of maximum %%@
resistivity 
(T$_{\rho, max}$) of La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ compounds %%@
with 
0$\leq$ x $\leq$ 0.20.}
\end{figure}

\begin{figure}
%\includegraphics{fig7.eps}
\caption{Field dependent magnetization of 
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ (0$\leq$ x $\leq$ 0.20) at %%@
different temperatures.} 
\end{figure}

\begin{figure}
%\includegraphics{fig8.eps}
\caption{Electric resistivity $\rho$ versus temperature for 
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ compounds (x=0.0(a), 0.05(b), %%@
0.10(c), and 
0.15(d)) in applied magnetic field H=0, 1, 3, and 5 T. Arrows indicate the %%@
T$_{\rho, 
max}$. The inset in (d) is the plot of  resistivity of x=0.15 compound (with %%@
log scale) 
in H= 0 T.}
\end{figure}

\begin{figure}
%\includegraphics{fig9.eps}
\caption{Temperature dependence of magnetoresistance of 
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ (0$\leq$ x $\leq$ 0.20) compounds %%@
in the 
magnetic field of H=1, 3 T.}
\end{figure}

\begin{figure}
%\includegraphics{fig10.eps}
\caption{ln($\rho$/T) versus 1/T plots in the high temperature region of 
La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Ti$_{x}$O$_{3}$ (0$\leq$ x $\leq$ 0.20) compounds %%@
. 
Dot line is the fitting line .}
\end{figure}

\end{document}

cond-mat0502235.extracted_bib
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\bibitem{anderson} P. W. Anderson, and H. Hasegawa, Phys. Rev. {\bf 100}, 675 %%@
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\bibitem{gennes} P. G. de Gennes, Phys. Rev. {\bf 118}, 141 (1960).

\bibitem{kubo} K. Kubo, N. Ohata, J. Phys. Soc. Japan {\bf 33}, 21 (1972).

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\bibitem{kim} K. H. Kim, J. H. Jung, and T. W. Noh, Phys. Rev. Lett. {\bf 81}, %%@
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\bibitem{muroi} M. Muroi, R. Street, and P. G. McComick, J. Appl. Phys. {\bf %%@
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\bibitem{medarde} M. Medarde, J. Mesot, P. Lacorre, S. Rosenkranz, P. Fischer, %%@
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\bibitem{martin} C. Martin, A. Maignan, and B. Raveau, J. Mater. Chem. {\bf 6}, %%@
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\bibitem{shannon} R. D. Shannon, Acta Crystallogr. Sect. A: Cryst. Phys., %%@
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\bibitem{hwang} H. Y. Hwang, S-W. Cheong, P.G. Radaelli, M. Marezio, and B. %%@
Batlogg, 
Phys. Rev. Letter. {\bf 75}, 914 (1995).

\bibitem{harrison} W. A. Harrison, {\em {The electronic structure and %%@
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\bibitem{maignan} A. Maignan, C. Martin, and B. Raveau, Z. Phys. B {\bf 102}, %%@
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\bibitem{emin} D. Emin and T. holstein, Ann. Phys. (N.Y.) {\bf 53}, 439 (1969).

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Marquina, J. Garcia, J. Stankiewicz, and C. Ritter, Phys. Rev. B {\bf 56}, 8252 %%@
(1997).

\bibitem{hundley} M. F. Hundley, M. Hawley, R. H. Heffner, Q. X. Jia, J. J. %%@
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