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Giant magnetotransport and magnetostructural phenomena in hole-doped manganese oxides
MATERIALS SCIENCE & ENGINEERING ELSEVIER
Materials Science and Engineering B31 (1995) 187-191
B
Giant magnetotransport and magnetostructural phenomena in holedoped manganese oxides T. T o k u r a a'b, A. U r u s h i b a r a b, Y. M o r i t o m o a, A . A s a m i t s u a, Y. T o m i o k a a, T. A r i m a b, G . K i d o c aJoint Research Center for Atom Technology (JRCAT), Tsukuba, Ibaraki 325, Japan bDepartment of Physics, University of Tokyo, Tokyo 113, Japan
CNational Research Institute for Metals, Tsukuba, lbaraki 305, Japan
Abstract
Giant magnetotransport phenomena have been investigated for single crystals of perovskite-type La~ xSrxMnO3 and related compounds. Magnetoresistance (MR) with negative sign shows a universal curve as a function of the magnetization, the body of which is quantitatively accounted for by the mean-field theory of the Kondo lattice model with strong ferromagnetic (Hund) coupling. Furthermore, magnetic field-induced structural change has been found between the two types of slightly distorted perovskite structure perhaps due to the strong coupling between the electronic and lattice degrees of freedom in these hole-doped Mn-oxides. Keywords: Giant magnetotransport; Magnetostructural phenomena; Magnetization; Manganese oxides
1. Introduction
Discovery of high-temperature superconductivity in hole-doped copper oxide compounds has aroused renewed and extended interest in the correlated dynamics of spins and charges near the Mott transition in 3D electron transition metal oxides systems. In the course of the study, the electronic properties of holedoped manganese oxides, R~ xA~MnO3 (R being the rare-earth ions and A the divalent ions, i.e., A = Ca, Sr, Ba and Pb) have been revisited and colossal magnetotransport phenomena have been and are being (re)discovered [1-5]. T o correlate the charge dynamics with the spin dynamics, a high quality sample is needed in particular for the 3D transition metal oxide systems. This is partly because the ceramics sample suffers from the scattering of the spin-polarized carrier at the grain boundaries where a modification of the microscopic magnetic structure probably takes place. Another important fact to be noticed is that electronic properties and structures in such a strongly correlated electron system as the hole-doped Mn-oxides critically depend on the band filling (nominal hole concentration), and hence a finely filling-controlled sample is indispensable for a detailed investigation. For this purpose, we have grown single crystals of various Mn-oxides, R1_xAxMnO3, with band filling controlled E l s e v i e r S c i e n c e S.A.
SSDI 0 9 2 1 - 5 1 0 7 ( 9 4 ) 0 8 0 1 0 - 0
by the floating zone method [5]. In fact, the m e t a l insulator p h e n o m e n a as well as the magnetic field effect have been found to differ considerably from those of the ceramic samples and to critically depend on the band filling as well as on the ionic size of the Rand A-ions. H e r e , we will give an overview of our recent experimental investigation on the magnetotransport and magnetostructural p h e n o m e n a of the filling-controlled compound La 1 xSrxMnO3.
2. Metal-insulator phenomena in La]_xSrxMnO 3
Let us begin with a quick review of fundamental electronic features in hole-doped manganese oxides such as Lal_xSrxMnO 3. As shown in Fig. 1, the parent antiferromagnetic insulator L a M n O 3 involves Mn 3÷ 3 1 ions with t2geg(S = 2) configuration. Among the four 3 3D electrons on the Mn site, the t2g electrons may be viewed as a single local spin (s = 3 / 2 ) because of relatively poor hybridization of t2g orbitals with oxygen 2p states, while the eg1 state hybridized strongly with the O 2p states is either itinerant or localized as the case may be. The end insulator L a M n O 3 is a Mott insulator due to the strong correlations for the eg electrons. However, the deviation of the eg band filling (n = l - x ) from the integer value (n = 1), which corresponds to the so-called 'hole-doping' in the
188
T. Tokura et al. / Materials Science and Engineering B31 (1995) I87-191
----T-- +
t2g
crystal field splitting 4, (10Dq)
'',t
i il
---'~-- +
(a) LaMnO 3 +"~_ _'~+
:::,
--~
lOO
eg
8
86
+
4-(b) 4-Laa.xSrxMnO 4- +3 (T
0
' ' La 1_xSrxMnO 3 x=0.175 H=OT
4
H'=t 5T
•
8T~
50 "~
15T....2.~
40 "~ 30 <1 20 10
~
MoCOS!
2 m
./
t2g
(c)
La,.xSr×MnO 3 (T>Tc)
H (d)
correlated insulators, may cause the insulator-to-metal transition. A distinct feature of such a barely metallic state in the manganese oxides is that there is a strong exchange interaction (Hund coupling) between the itinerant eg electrons and local t2g spins. We show in Fig. 2 the electronic phase diagram of La, ,SrxMnO 3 near the metal-insulator critical composition. As previously clarified for the ceramics samples [6], the x < 0.1 phase can be assigned to the spin-canted antiferromagnetic state below TN, while the x > 0 . 1 phase is ferromagnetic below T~. The ferromagnetic transition temperature Tc increases with x ( > 0 . 1 ) and reaches the maximum (370 K) around x = 0.3. As can be seen in the zero-field (H = 0) curves of Fig. 3 (x = 0.175), the resistivity shows a sudden decrease below Tc, though the p-T curve of the samples with x = 0.1-0.15 shows an upturn again with further lowering temperature below T~ due to some ,
,
,
,
i,
P.I.
400
~
,
,
i
P.M. Tc
:
260
'
Temperature(K)
localization effect. A r o u n d x = 0 . 2 the compound shows a metallic behavior below Tc, while the resistivity above T c is nonmetallic, i.e., dp/dT < 0. As shown in the phase diagram (Fig. 2), the compound becomes metallic (dp/dT>O) even above T c when the hole concentration x( = l - n ) exceeds 0.3. Temperature dependence of the resistivity at zero field can be interpreted in terms of the following well-established scenario [7-11]: A conspicuous resistivity drop observed below T c is due to the increase of the carrier mobility which results from decrease in the carrier scattering by thermal spin fluctuation as well as decrease of the hole-carrier effective mass. The theory of the double exchange interaction [8] indicates that the electron (hole) transfer (tij) between the neighboring sites depends on the relative angle (A0ij) of the local (tZg) spins in such a manner as tij : tocos(AOij/2), as schematically shown in Fig. 1. Thus, the ferromagnetic spin arrangement reduces the randomness of the transfer interaction and hence enhances the hole mobility. This is the main cause of the rapid change of the resistivity below T c in accord with the onset of the ferromagnetic spin moment. The nonmetallic behavior of the x ~< 0.3 sample above T~ may be interpreted as due to the carrier localization by the randomness of the hole (eg state) transfer (off-diagonal disorder effect) which is caused by the random (paramagnetic) spin configuration of t2gelectrons.
F.M. 3. Magnetoresistance
( ., .. 100 CN.I.! F.I. i 0
,
!
E 200 I-
'
Lal.xSrxMnO 3 (T>Tc)
Fig. 1. Schematics for the electronic structures of La] Sr~MnO 3.
,
o
~.~
Fig. 3. Temperature dependence of resistivity of La] xSrxMnO 3 crystal under a magnetic field. ©, magnitude of magnetoresistance at 15T.
"'. ~1~-'"
500
~
90 30 ~" 70 °
i
:,
0.1
i:
La 1_~SrxMnO 3
,
0.2
,
,
0.3 X
i
i
0.4
I
L
0.5
,
0.6
Fig. 2. Electronic phase diagram of La] xSrxMnO3: CN.|. (spincanted antiferromagnetic insulator), F.I. (ferromagnetic insulator), P.I. (paramagnetic insulator), P.M. (paramagnetic metal), and F.M. (ferromagnetic metal).
Based on the above arguments (Fig. 1), an intuitive view of the magnetoresistance (MR) effect can be obtained as follows. The applied magnetic field tends to align the local spin and then the conduction electron coupled strongly with the local spin gains a more itinerant character. In Fig. 3, we show the magnetic field effect on the resistivity (the longitudinal
T. Tokura et al. / Materials Science and Engineering B31 (1995) 187-191
MR; IHH ) for the x = 0 . 1 7 5 sample. The MR was found to be nearly isotropic; the transverse MR is only a few percent smaller in absolute magnitude than the longitudinal MR. An anomalously large MR effect is observed around T c for the both samples. The negative M R value defined as [p(0) -p(H)]/p(O) is plotted in Fig. 3 with open circles for H = 15 T. The MR value is as large as 0.9 around 300 K in the x = 0 . 1 7 5 sample. Such a giant MR immediately above T c may be interpreted as the field-induced nonmetal-metal transition. The MR magnitude apparently correlates with the change of the magnetization, which is large around the T c. In fact, the MR effect at 77 K for the fully spin ordered x = 0.175 sample is very small compared with that around To. (As for the origin of the anomaly of the MR around 220 K, see the argument in the next section.) We have also measured the field-induced magnetization of these samples above T~ up to B = 15 T with use of a pulse magnet [5]. Then, we have deduced the change of the resistivity as a function of the total magnetization which represents the averaged moment of the sum of the t2g local spins and forcedly aligned eg spin on the same Mn site. The results for the x = 0.15 and x = 0.175 sample are shown in Fig. 4, in which the magnetization on the abscissa is normalized by the classical value (Msa t = 2/zB) in the case of the n = 1 filling of the eg band. Looking at the p - M curve, one may notice that the experimental points taken at different temperatures (above Tc) show an approximately identical curve in spite of seemingly much different MR behaviors (see Fig. 3). Furthermore, the p - M curve, which was obtained by the temperature-dependence of the p(T) and M(T) data, is included in Fig. 5. The curve shows
1
,
,
i
,
,
"~o " ~ ~it ° ,1~o ~=° ~*
0.8
x=0.175
"~-=
0.6
~
i
x=0.15
0
.
4
,
,
i
274K o 304K [] 343K294K • 304K • 330K " ~K-
~
0.2 x=0.175 0(.1..)at 0.5T 0
0.2
0.4
0,6
M/Msat
0.8
Fig. 4. Magnitude of magnetoresistance for La~_xSrxMnO 3 crystals with x = 0 . 1 5 and 0.175 as a function of the normalized magnetization, M~,, being 2/xB. Temperature dependent resistivity of the x = 0.175 crystal is also plotted as a function of temperature-dependent magnetization.
Kondo / 1 coupling /
t magnetic field
189
\
el.- lattice "k, interaction
1 structural change
Fig. 5. Mutual coupling among the local spins, doped carrier and lattice degree of freedom that causes the magnetostructural phenomena.
a similar trace, indicating that a similar recovering process of the carrier mobility governs the decrease of p(T) below T c and the negative MR above T c. Nevertheless, a small but systematic discrepancy between the T-variational and H-variational curves is observed in the high-M region. This is probably contributed from the temperature-dependent scattering process which works even in the case of the same (static) magnetization. These p - M curves have been analyzed in terms of the Kondo lattice model with ferromagnetic coupling (Kondo interaction) [5,12]. Furukawa [12] derived the expression of the MR using the Kubo formula as a solution of the Kondo lattice model in the limit of the infinite dimension and classical local spins. The calculated results based on this mean-field solution reproduce the results of Fig. 5 quantitatively apart from the high-M region, as described in detail in Ref. [5]. A more explicit consideration of the effect of the quantum spin fluctuation would further improve our understanding of the giant MR phenomena of the Mnoxides.
4. Magnetostructural phenomena The crystal lattice of Lal_xSrxMnO 3 undergoes the transition between the slightly distorted perovskitelike structures, i.e. the rhombohedral form and the orthorhombic one, with change of the composition x, and temperature. As well known, the major origin of such a structural transformation is the difference in the tolerance factor of the perovskite-like structure. Near the transition point, however, the coupling of the electronic energy with the lattice degree of freedom becomes significant. In particular, the increase of the d-electron transfer energy, or equivalently the increase in the kinetic energy of the conduction electrons, favors the rhombohedral structure rather than the orthorhombic one. In the orthorhombic form (the
T. Tokura et al. / Materials" Science and Engineering B31 (1995) 187-191
190
so-called GdFeO3-type structure), the transfer interaction is reduced owing to the M n - O - M n bond angle distortion. Our idea here is to cause the structural phase transition at a fixed temperature by application of an external magnetic field. As shown in Fig. 5, the applied magnetic field directly acts on the local/2g spin system and tends to align the spin moment. Then, as evidenced by the giant negative MR, the kinetic energy of the conduction electrons increases when the system locates near the magnetic transition temperature. In that case, the lattice structure may show a field-induced transformation. As a demonstrative example for this scenario, we show in Fig. 6 the temperature dependence of resistivity (in the warming run) near the structural phase transition temperature as a function of the magnetic field for La,_xSrxMnO 3 (x = 0.175) crystal. The abrupt decrease of resistivity with increase of temperature corresponds to the structural phase transition from the orthorhombic lattice (low-temperature phase) to the rhombohedral one (high-temperature phase), which was confirmed by the X-ray diffraction measurements. The inset shows the hysteresis loop of the zero-field resistivity in cooling and warming runs, reflecting the first-order nature of this lattice transition. With application of the magnetic field, the structural transition temperature decreases continuously, as probed by the distinct change of resistivity (Fig. 6). This means that at some fixed temperature the lattice structure can be transformed by the magnetic field. Furthermore, when the temperature is set within the hysteresis loop, the application or removal of the magnetic field can irreversibly switch the crystal structure, which was experimentally confirmed by the i
. l
Z|
,
I.
"t
H=OT
i
;
o°o
E'4 O (1~
H=OT 0
0
T(K)
.~
/¢<
+"
:
(13
5. Summary We have observed the giant magnet•transport and magnet•structural phenomena in filling-controlled single crystals of La~ xSrxMnO3 near the insulator-metal transition. The negative magnet•resistance as well as the temperature-dependent resistivity can be well scaled with the ferromagnetic magnetization, the body of which is quantitatively accounted for by the meanfield type theory for the Kondo lattice model. Furthermore, the distortion of the perovskite structure relates to the kinetic energy (Fermi energy) of the conduction electrons, which varies with the magnetization of the local tzg spins. This has led us to the observation that the structural phase transition temperature between the rhombohedral and orthorhombic phase is altered by application of the magnetic field. The effect will be more remarkable as the structural transition temperature approaches the magnetic transition temperature. The present manganate systems show even more variation in magnet•transport properties when the electron bandwidth is further controlled by appropriate elemental substitution of the perovskite A-site, which may provide a novel opportunity for the study of new spin-charge coupled phenomena.
.o"• ' " .
O~l
"~
o"
/
simultaneous measurements of the resistivity and the lattice dilation with use of the strain gauge. According to the scheme shown in Fig. 5, the field control of the lattice structure may be best achieved in the case of the structural transition temperature positions close to the magnetic transition temperature (To). This will be realized with very fine tuning of the composition (x) or by appropriate choice of the A-site element and give even more remarkable magnet•structural phenomena in perovskite Mn-oxide systems.
Z/i
3
~
rr
...
'*"
..
.
"'. 3T
87
e• i e* e
t~
o~o
•
..,...-
::"" .•.o
..
el•
.,"
..
""
..
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#
.,'* .-" 15T ----" II i
"...
Acknowledgements
".":"~-" "
.-" o•"
We are grateful to N. Furukawa for enlightening discussion. This work was partly supported by the N E D • and by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan.
it I
°lioo
p• o°
•,~i
200
I
I
I
I
220 240 Temperature(K)
Fig. 6. C h a n g e of the lattice transition temperature with the magnetic field as probed by the temperature dependence of resistivity for Lal_xSr~MnO 3 (x = 0,175) crystal.
References [1] R.M. Kusters, J. Singleton, D.A. Keen, R. M c G r e e v y and W. Hayes, Physica B, 15.5 (1989) 362. [2] K. Chabara, T. O h n o , M. Kasai and Y. Kozono, Appl. Phys. Len., 6.3 (1993) 1990.
T. Tokura et al. / Materials Science and Engineering B31 (1995) 187-191 [3] R. von Helmolt, J. Wocker, B. Holzapfel, Schultz and K. Samwer, Phys. Rev. Lett., 71 (1993) 2331. [4] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh and L.H. Chen, Science, 264 (1994) 413. [5] Y. Tokura, A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido and N. Furukawa, J. Phys. Soc. Jpn., in press.
[6] [7] [8] [9] [10] [11] [12]
191
E.O. Wollan and W.C. Koehler, Phys. Rev., 100 (1955) 545. C. Zener, Phys. Rev., 82 (1951) 403. P.W. Anderson and H. Hasegawa, Phys. Rev., 100 (1955) 675. P.-G. de Gennes, Phys. Rev., 118 (1960) 141. C.W. Searle and S.T. Wang, Can. J. Phys., 47 (1969) 2703. K. Kubo and N. Ohata, J. Phys. Soc. Jpn., 33 (1972) 21. N. Furukawa, J. Phys. Soc. Jpn., to be published.
Materials Science and Engineering B31 (1995) 187-191
B
Giant magnetotransport and magnetostructural phenomena in holedoped manganese oxides T. T o k u r a a'b, A. U r u s h i b a r a b, Y. M o r i t o m o a, A . A s a m i t s u a, Y. T o m i o k a a, T. A r i m a b, G . K i d o c aJoint Research Center for Atom Technology (JRCAT), Tsukuba, Ibaraki 325, Japan bDepartment of Physics, University of Tokyo, Tokyo 113, Japan
CNational Research Institute for Metals, Tsukuba, lbaraki 305, Japan
Abstract
Giant magnetotransport phenomena have been investigated for single crystals of perovskite-type La~ xSrxMnO3 and related compounds. Magnetoresistance (MR) with negative sign shows a universal curve as a function of the magnetization, the body of which is quantitatively accounted for by the mean-field theory of the Kondo lattice model with strong ferromagnetic (Hund) coupling. Furthermore, magnetic field-induced structural change has been found between the two types of slightly distorted perovskite structure perhaps due to the strong coupling between the electronic and lattice degrees of freedom in these hole-doped Mn-oxides. Keywords: Giant magnetotransport; Magnetostructural phenomena; Magnetization; Manganese oxides
1. Introduction
Discovery of high-temperature superconductivity in hole-doped copper oxide compounds has aroused renewed and extended interest in the correlated dynamics of spins and charges near the Mott transition in 3D electron transition metal oxides systems. In the course of the study, the electronic properties of holedoped manganese oxides, R~ xA~MnO3 (R being the rare-earth ions and A the divalent ions, i.e., A = Ca, Sr, Ba and Pb) have been revisited and colossal magnetotransport phenomena have been and are being (re)discovered [1-5]. T o correlate the charge dynamics with the spin dynamics, a high quality sample is needed in particular for the 3D transition metal oxide systems. This is partly because the ceramics sample suffers from the scattering of the spin-polarized carrier at the grain boundaries where a modification of the microscopic magnetic structure probably takes place. Another important fact to be noticed is that electronic properties and structures in such a strongly correlated electron system as the hole-doped Mn-oxides critically depend on the band filling (nominal hole concentration), and hence a finely filling-controlled sample is indispensable for a detailed investigation. For this purpose, we have grown single crystals of various Mn-oxides, R1_xAxMnO3, with band filling controlled E l s e v i e r S c i e n c e S.A.
SSDI 0 9 2 1 - 5 1 0 7 ( 9 4 ) 0 8 0 1 0 - 0
by the floating zone method [5]. In fact, the m e t a l insulator p h e n o m e n a as well as the magnetic field effect have been found to differ considerably from those of the ceramic samples and to critically depend on the band filling as well as on the ionic size of the Rand A-ions. H e r e , we will give an overview of our recent experimental investigation on the magnetotransport and magnetostructural p h e n o m e n a of the filling-controlled compound La 1 xSrxMnO3.
2. Metal-insulator phenomena in La]_xSrxMnO 3
Let us begin with a quick review of fundamental electronic features in hole-doped manganese oxides such as Lal_xSrxMnO 3. As shown in Fig. 1, the parent antiferromagnetic insulator L a M n O 3 involves Mn 3÷ 3 1 ions with t2geg(S = 2) configuration. Among the four 3 3D electrons on the Mn site, the t2g electrons may be viewed as a single local spin (s = 3 / 2 ) because of relatively poor hybridization of t2g orbitals with oxygen 2p states, while the eg1 state hybridized strongly with the O 2p states is either itinerant or localized as the case may be. The end insulator L a M n O 3 is a Mott insulator due to the strong correlations for the eg electrons. However, the deviation of the eg band filling (n = l - x ) from the integer value (n = 1), which corresponds to the so-called 'hole-doping' in the
188
T. Tokura et al. / Materials Science and Engineering B31 (1995) I87-191
----T-- +
t2g
crystal field splitting 4, (10Dq)
'',t
i il
---'~-- +
(a) LaMnO 3 +"~_ _'~+
:::,
--~
lOO
eg
8
86
+
4-(b) 4-Laa.xSrxMnO 4- +3 (T
0
' ' La 1_xSrxMnO 3 x=0.175 H=OT
4
H'=t 5T
•
8T~
50 "~
15T....2.~
40 "~ 30 <1 20 10
~
MoCOS!
2 m
./
t2g
(c)
La,.xSr×MnO 3 (T>Tc)
H (d)
correlated insulators, may cause the insulator-to-metal transition. A distinct feature of such a barely metallic state in the manganese oxides is that there is a strong exchange interaction (Hund coupling) between the itinerant eg electrons and local t2g spins. We show in Fig. 2 the electronic phase diagram of La, ,SrxMnO 3 near the metal-insulator critical composition. As previously clarified for the ceramics samples [6], the x < 0.1 phase can be assigned to the spin-canted antiferromagnetic state below TN, while the x > 0 . 1 phase is ferromagnetic below T~. The ferromagnetic transition temperature Tc increases with x ( > 0 . 1 ) and reaches the maximum (370 K) around x = 0.3. As can be seen in the zero-field (H = 0) curves of Fig. 3 (x = 0.175), the resistivity shows a sudden decrease below Tc, though the p-T curve of the samples with x = 0.1-0.15 shows an upturn again with further lowering temperature below T~ due to some ,
,
,
,
i,
P.I.
400
~
,
,
i
P.M. Tc
:
260
'
Temperature(K)
localization effect. A r o u n d x = 0 . 2 the compound shows a metallic behavior below Tc, while the resistivity above T c is nonmetallic, i.e., dp/dT < 0. As shown in the phase diagram (Fig. 2), the compound becomes metallic (dp/dT>O) even above T c when the hole concentration x( = l - n ) exceeds 0.3. Temperature dependence of the resistivity at zero field can be interpreted in terms of the following well-established scenario [7-11]: A conspicuous resistivity drop observed below T c is due to the increase of the carrier mobility which results from decrease in the carrier scattering by thermal spin fluctuation as well as decrease of the hole-carrier effective mass. The theory of the double exchange interaction [8] indicates that the electron (hole) transfer (tij) between the neighboring sites depends on the relative angle (A0ij) of the local (tZg) spins in such a manner as tij : tocos(AOij/2), as schematically shown in Fig. 1. Thus, the ferromagnetic spin arrangement reduces the randomness of the transfer interaction and hence enhances the hole mobility. This is the main cause of the rapid change of the resistivity below T c in accord with the onset of the ferromagnetic spin moment. The nonmetallic behavior of the x ~< 0.3 sample above T~ may be interpreted as due to the carrier localization by the randomness of the hole (eg state) transfer (off-diagonal disorder effect) which is caused by the random (paramagnetic) spin configuration of t2gelectrons.
F.M. 3. Magnetoresistance
( ., .. 100 CN.I.! F.I. i 0
,
!
E 200 I-
'
Lal.xSrxMnO 3 (T>Tc)
Fig. 1. Schematics for the electronic structures of La] Sr~MnO 3.
,
o
~.~
Fig. 3. Temperature dependence of resistivity of La] xSrxMnO 3 crystal under a magnetic field. ©, magnitude of magnetoresistance at 15T.
"'. ~1~-'"
500
~
90 30 ~" 70 °
i
:,
0.1
i:
La 1_~SrxMnO 3
,
0.2
,
,
0.3 X
i
i
0.4
I
L
0.5
,
0.6
Fig. 2. Electronic phase diagram of La] xSrxMnO3: CN.|. (spincanted antiferromagnetic insulator), F.I. (ferromagnetic insulator), P.I. (paramagnetic insulator), P.M. (paramagnetic metal), and F.M. (ferromagnetic metal).
Based on the above arguments (Fig. 1), an intuitive view of the magnetoresistance (MR) effect can be obtained as follows. The applied magnetic field tends to align the local spin and then the conduction electron coupled strongly with the local spin gains a more itinerant character. In Fig. 3, we show the magnetic field effect on the resistivity (the longitudinal
T. Tokura et al. / Materials Science and Engineering B31 (1995) 187-191
MR; IHH ) for the x = 0 . 1 7 5 sample. The MR was found to be nearly isotropic; the transverse MR is only a few percent smaller in absolute magnitude than the longitudinal MR. An anomalously large MR effect is observed around T c for the both samples. The negative M R value defined as [p(0) -p(H)]/p(O) is plotted in Fig. 3 with open circles for H = 15 T. The MR value is as large as 0.9 around 300 K in the x = 0 . 1 7 5 sample. Such a giant MR immediately above T c may be interpreted as the field-induced nonmetal-metal transition. The MR magnitude apparently correlates with the change of the magnetization, which is large around the T c. In fact, the MR effect at 77 K for the fully spin ordered x = 0.175 sample is very small compared with that around To. (As for the origin of the anomaly of the MR around 220 K, see the argument in the next section.) We have also measured the field-induced magnetization of these samples above T~ up to B = 15 T with use of a pulse magnet [5]. Then, we have deduced the change of the resistivity as a function of the total magnetization which represents the averaged moment of the sum of the t2g local spins and forcedly aligned eg spin on the same Mn site. The results for the x = 0.15 and x = 0.175 sample are shown in Fig. 4, in which the magnetization on the abscissa is normalized by the classical value (Msa t = 2/zB) in the case of the n = 1 filling of the eg band. Looking at the p - M curve, one may notice that the experimental points taken at different temperatures (above Tc) show an approximately identical curve in spite of seemingly much different MR behaviors (see Fig. 3). Furthermore, the p - M curve, which was obtained by the temperature-dependence of the p(T) and M(T) data, is included in Fig. 5. The curve shows
1
,
,
i
,
,
"~o " ~ ~it ° ,1~o ~=° ~*
0.8
x=0.175
"~-=
0.6
~
i
x=0.15
0
.
4
,
,
i
274K o 304K [] 343K294K • 304K • 330K " ~K-
~
0.2 x=0.175 0(.1..)at 0.5T 0
0.2
0.4
0,6
M/Msat
0.8
Fig. 4. Magnitude of magnetoresistance for La~_xSrxMnO 3 crystals with x = 0 . 1 5 and 0.175 as a function of the normalized magnetization, M~,, being 2/xB. Temperature dependent resistivity of the x = 0.175 crystal is also plotted as a function of temperature-dependent magnetization.
Kondo / 1 coupling /
t magnetic field
189
\
el.- lattice "k, interaction
1 structural change
Fig. 5. Mutual coupling among the local spins, doped carrier and lattice degree of freedom that causes the magnetostructural phenomena.
a similar trace, indicating that a similar recovering process of the carrier mobility governs the decrease of p(T) below T c and the negative MR above T c. Nevertheless, a small but systematic discrepancy between the T-variational and H-variational curves is observed in the high-M region. This is probably contributed from the temperature-dependent scattering process which works even in the case of the same (static) magnetization. These p - M curves have been analyzed in terms of the Kondo lattice model with ferromagnetic coupling (Kondo interaction) [5,12]. Furukawa [12] derived the expression of the MR using the Kubo formula as a solution of the Kondo lattice model in the limit of the infinite dimension and classical local spins. The calculated results based on this mean-field solution reproduce the results of Fig. 5 quantitatively apart from the high-M region, as described in detail in Ref. [5]. A more explicit consideration of the effect of the quantum spin fluctuation would further improve our understanding of the giant MR phenomena of the Mnoxides.
4. Magnetostructural phenomena The crystal lattice of Lal_xSrxMnO 3 undergoes the transition between the slightly distorted perovskitelike structures, i.e. the rhombohedral form and the orthorhombic one, with change of the composition x, and temperature. As well known, the major origin of such a structural transformation is the difference in the tolerance factor of the perovskite-like structure. Near the transition point, however, the coupling of the electronic energy with the lattice degree of freedom becomes significant. In particular, the increase of the d-electron transfer energy, or equivalently the increase in the kinetic energy of the conduction electrons, favors the rhombohedral structure rather than the orthorhombic one. In the orthorhombic form (the
T. Tokura et al. / Materials" Science and Engineering B31 (1995) 187-191
190
so-called GdFeO3-type structure), the transfer interaction is reduced owing to the M n - O - M n bond angle distortion. Our idea here is to cause the structural phase transition at a fixed temperature by application of an external magnetic field. As shown in Fig. 5, the applied magnetic field directly acts on the local/2g spin system and tends to align the spin moment. Then, as evidenced by the giant negative MR, the kinetic energy of the conduction electrons increases when the system locates near the magnetic transition temperature. In that case, the lattice structure may show a field-induced transformation. As a demonstrative example for this scenario, we show in Fig. 6 the temperature dependence of resistivity (in the warming run) near the structural phase transition temperature as a function of the magnetic field for La,_xSrxMnO 3 (x = 0.175) crystal. The abrupt decrease of resistivity with increase of temperature corresponds to the structural phase transition from the orthorhombic lattice (low-temperature phase) to the rhombohedral one (high-temperature phase), which was confirmed by the X-ray diffraction measurements. The inset shows the hysteresis loop of the zero-field resistivity in cooling and warming runs, reflecting the first-order nature of this lattice transition. With application of the magnetic field, the structural transition temperature decreases continuously, as probed by the distinct change of resistivity (Fig. 6). This means that at some fixed temperature the lattice structure can be transformed by the magnetic field. Furthermore, when the temperature is set within the hysteresis loop, the application or removal of the magnetic field can irreversibly switch the crystal structure, which was experimentally confirmed by the i
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5. Summary We have observed the giant magnet•transport and magnet•structural phenomena in filling-controlled single crystals of La~ xSrxMnO3 near the insulator-metal transition. The negative magnet•resistance as well as the temperature-dependent resistivity can be well scaled with the ferromagnetic magnetization, the body of which is quantitatively accounted for by the meanfield type theory for the Kondo lattice model. Furthermore, the distortion of the perovskite structure relates to the kinetic energy (Fermi energy) of the conduction electrons, which varies with the magnetization of the local tzg spins. This has led us to the observation that the structural phase transition temperature between the rhombohedral and orthorhombic phase is altered by application of the magnetic field. The effect will be more remarkable as the structural transition temperature approaches the magnetic transition temperature. The present manganate systems show even more variation in magnet•transport properties when the electron bandwidth is further controlled by appropriate elemental substitution of the perovskite A-site, which may provide a novel opportunity for the study of new spin-charge coupled phenomena.
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simultaneous measurements of the resistivity and the lattice dilation with use of the strain gauge. According to the scheme shown in Fig. 5, the field control of the lattice structure may be best achieved in the case of the structural transition temperature positions close to the magnetic transition temperature (To). This will be realized with very fine tuning of the composition (x) or by appropriate choice of the A-site element and give even more remarkable magnet•structural phenomena in perovskite Mn-oxide systems.
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We are grateful to N. Furukawa for enlightening discussion. This work was partly supported by the N E D • and by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan.
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