- Email: [email protected]
Restoring the AFMI-FMM transition by calcium doping in the CMR manganites Pr0.52Sr0.48−xCaxMnO3
~,~ Journal ofsm magneti and magneti c materials
N ELSEVIER
Journal of Magnetism and Magnetic Materials 174 (1997) L5-L9
Letter to the Editor
Restoring the AFMI-FMM transition by calcium doping in the CMR manganites Pro.szSro.48_xCaxMnO 3 J. Wolfman*, Ch. Simon, B. Raveau Laboratoire CRISMAT, UMR 6508 CNRS, ISMRA, Bd. du Marechal Juin, 14050 Caen Cedex, France
Received 5 May 1997
Abstract A small deviation from the ratio Mn3 + : Mn 4 + = 1 in the manganite Pro.5 +ySro.s -yMnO3 leads to a disappearance of the A F M I - F M M transition, the A F M I state being replaced by a F M M state. The present study of the series Pro.52Sro.gs-xCaxMnO3 shows for the first time that the A F M I - F M M transition can be restored for a Mn 3 + : Mn 4+ ratio different from 1 by calcium doping. This result is interpreted by the fact that the decrease of the average size of the A-site cation, (rA), and of the mismatch, 02, favors the A F M I state at the expense of the F M M state. The deviation from the perfect charge ordering of the i : 1 compound is then compensated by these two effects. The application of a magnetic field of 7 T shows that all the compositions exhibit a CMR effect. The maximum Ro/R7 r ratio is equal to 3500 at 127 K for the Pro.52Sr0.z6Ca22MnO 3 compound.
PACS: 75.25. + z; 75.30.Cr; 75.30.Kz Kevwords: CMR; Manganites; Antiferromagnetism; Ferromagnetism; Metal insulator transition
T h e p e r o v s k i t e s L n ~ _ x A x M n O 3 with Ln = lant h a n i d e a n d A = Ca, Sr, Ba have d r a w n a large interest these p a s t few years o w i n g to their colossal m a g n e t o r e s i s t a n c e ( C M R ) properties. A m o n g these oxides, the m a t e r i a l s c o r r e s p o n d i n g to the specific c o m p o s i t i o n x = 0.5 exhibit p a r t i c u l a r t r a n s p o r t a n d m a g n e t i c properties. Besides the F M M (fer-
*Corresponding author. E-mail: [email protected]; fax: + 33 2 3195 1600.
r o m a g n e t i c metallic) to P M I ( p a r a m a g n e t i c insulating) t r a n s i t i o n o b s e r v e d for all the C M R m a n g a n i t e s , these phases also d i s p l a y an A F M I (antiferromagnetic insulating) to F M M t r a n s i t i o n at low t e m p e r a t u r e . This is the case for the m a n g a nites Pro.sSro.sMnO3 [1] a n d N d o . s S r 0 . s M n O 3 [2], where an A F M state was i n t e r p r e t e d in terms of charge o r d e r i n g (CO), i.e. due to an o r d e r i n g of the M n 3+ a n d M n 4+ species at low t e m p e r a t u r e [3]. T h o u g h the A F M o r d e r can vary from one p h a s e to the o t h e r as shown for P r o . s S r o . s M n O 3
0304-8853/97/$17.00 ~2 1997 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 3 6 7 - 3
LETTER TO THE EDITOR
L6
J. Wo//htan et al. /Journal of Magnetism and Magnetic Materials" 174 (1997)L5 L9
(A type) and Ndo.sSro.sMnO3 (CE type) [4], it is presumed to be due to a 1 : 1 ordering of the Mn 3 + and Mn ~+ species. Consequently, a Mn 3+ : Mn 4÷ ratio equal to 1 appears to be a very crucial condition to observe the A F M ! F M M transition. It has effectively been shown that a slight deviation from the 1 : 1 stoichiometry in Pro.5-xSro.5+xMnO3 or Lao.s-xCao.5 +xMnO3 [4, 51, suppresses charge ordering and leads to a F M M state at low temperature. An important issue that has not been answered to date, concerns the possibility of observing the A F M I - F M M transition for Mn 3+ : M n 4÷ ratios different from 1. Such a possibility, if it exists, requires the stabilization of the A F M I state at low temperature. This stabilization should be favored by a decrease of the average size of the A-site cation, as was demonstrated for the substitution of smaller calcium for strontium in the chargeordered phase Pro.sSro.sMnO3, wherein the A F M I state is stabilized at the expense of the F M M state [61. In order to answer such a question, we have studied the substitution of calcium for strontium in the system Pro.52Sro.48-~CaxMnOs. The present paper shows for the first time the existence of an A F M I F M M transition in manganites having a Mn 3 ÷ : Mn 4 + ratio different from 1. It demonstrates that size and mismatch effects can be used to counteract the charge effect and to restore such a transition in the C M R manganites. The compounds Pro.52Sro.~8-xCa~MnO3 were prepared as sintered ceramics by solid-state reaction (see for details Ref. [7]). The samples were characterized by X-ray diffraction using a Seifert powder X-ray diffractometer for Cu K~ radiation. The temperature dependence of the magnetization was measured in a field of 10 .2 T, after a zero-field cooling, with a vibrating sample magnetometer (Foner). The resistivity was measured decreasing the temperature by a classical four-probe method. The transport properties were studied, after zerofield cooling, under magnetic fields up to 7 T. The typical behavior of a charge ordered 1 : 1 Mn oxide is demonstrated by the resistivity curve registered in zero field while decreasing the temperature (see Fig. 1) of Pro.sSro.5MnO3. One observes a transition from a semiconducting state to a
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Fig. 1. Temperaturedependenceof the resistivityp for different valuesofx labeledon the graph for the Pro.szSro.4s xCaxMnO3 series and for Pro.sSr0.sMnOs. pseudo-metallic state at T = 276 K and is followed by a transition to an 'insulating' state as the temperature is further decreased. These electrical transitions are strongly correlated to the observed magnetic transitions. As the temperature is decreased, a transition from a paramagnetic (PM) state to a ferromagnetic (FM) state occurs at Tc and is followed by a transition to an antiferromagnetic (AFM) state at TN. The metallic behavior is a consequence of the ferromagnetic state via the double-exchange interaction [81. The 'insulating' state at low temperature corresponds with the onset of charge ordering (localization of the eg holes on the Mn sites) associated with an A-type A F M order [41 . The transport behavior of the compound Pro.5+ySro.5 yMnO3 is drastically different from that of Pro.sSro.sMnO3. This is shown for Pro.52Sro.48MnO3, whose resistivity is quite independent of the temperature in the [0, 300 K1 range (Fig. 1). No resistivity anomaly can be seen, implying that charge ordering is absent in this compound. This is confirmed by the temperature-dependent magnetization behavior (Fig. 2) for which a F M state is reached at 280 K and a smooth decrease of the moment arises with decreasing the temperature but there is no pure A F M at low temperature. The weak F M M state observed at low temperature for Pro.52Sro.48MnO 3 contrasts the A F M I
LETTER TO THE EDITOR
J. WolJ~nanet al. /Journal of Magnetism and Magnetic Materials 174 (1997) L5 L9
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T(K) Fig. 2. Temperature dependence of the magnetization M for different values of x labeled on the graph for the Pro.52Sro.~8 xCa,MnO3 series and for Pro.sSro.sMnO3 registered, after zero-field cooling, in a field of 10-2 Y.
state obtained for Pro.sSro.sMnO3, and supports the commonly held view that a small deviation of the Mn 3+ : Mn 4+ ratio from 1 is sufficient to destroy charge ordering. Similar behavior was previously emphasized for the Lal _xCa~MnO3 system [5], in which Lao.sCao.sMnO3 is an A F M insulator at low temperature whereas Lao.szCao.4sMnO3 is a F M metal. The substitution of calcium for strontium in Pro.52Sro.48 ~CaxMnO3 leads to a spectacular change of the magnetic and transport properties (Figs. l and 2). For x = 0.04, the latter are similar to those of x = 0. But for x = 0.08 a jump in the resistivity curve reappears at a temperature T c o = 175 K, and below 155 K, the resistivity remains constant (Fig. 1). This behavior is correlated with a rapid drop of the magnetic moment, under 1 0 - 2 T (Fig. 2), at 1 8 0 K followed by a slower decrease below 155 K. At low temperature, the magnetic moment reaches a value close to onethird of the value at T - - 2 0 0 K in the F M state. So a FM contribution remains and, despite the fact that the resistivity j u m p is as high as that of Pro.sSro.sMnO3, charge ordering is not completely restored since an A F M state is not reached at low temperature. For x = 0.1, the resistivity j u m p (Fig. 1) occurs at Tco = 180 K and R18o K and R5 K differ by 3 or-
L7
ders of magnitude. The magnetic moment (Fig. 2) drops sharply below 190 K and then decrease very smoothly below 150 K. For x = 0.18, the j u m p on the resistivity curve (Fig. 1) occurs at a lower temperature than for x = 0.1 but is much more pronounced: the resistivity changes by 5 orders of magnitude between the metallic and the 'insulating' state at 5 K. The corresponding magnetization shows a rapid drop between 170 and 150 K. Below 120 K the magnetization remains constant at a very low value of 0.017 I~B/Mn atom under a field of 10 .2 T. A pure A F M state is nearly achieved as the F M contribution is very weak. These results demonstrate that the A F M I - F M M transition can be restored by calcium doping and imply that charge ordering also exists with a Mn 3÷ : Mn 4+ different from one. If one keeps in mind that the mean valence of manganese is constant in the series Pro.s2Sro.48-xCaxMnO3, then the size difference between the A-site cations must be driving the restoration of the A F M I - F M M transition. Substitution of Ca for Sr affects both the average size, ~rA), and the variance, a 2, of the A-site cation. It was recently shown for Pro.5Sro.5-xCaxMnO3 [6] that a decrease of (rA) leads to a decrease of Tc and to an increase of TN. Similarly, a decrease of a 2 was shown to increase Tc in Lno.TAo.3MnO3 [9], whereas an increase of a 2 in P r l - x C a ~ M n O 3 suppresses charge ordering [10]. In the present series Pro.s2Sro.4s xCa~MnO3, the decrease in both (rA) and a 2, as x increases, stabilizes the A F M state. In Fig. 3, Tc (filled symbols) and Tco (unfilled symbols) are plotted for the two series Pro.sSro.5 _~Ca~MnO3 [6] and Pro.s2Sro.48-xCa~MnO3. In the regions where Tc and Tco are defined for both series (i.e. all but region 2 for Pro.52Sro.4s-~CaxMnO3, where Tco is not defined owing to the absence of an insulating state), it is clear that the critical temperatures are not strongly dependent on the valence of manganese in the compounds, but are dominated by the cationic size effect. However, the minor differences in Tc of these two series may result from the unit cell expansion (~0.005 A) associated with the increase in the content of the larger Mn 3÷ cation in Pro.s2Sro.~8-xCaxMnO3 as compared to
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Fig. 3. Electronic and magnetic phase diagram in the (, temperature) plane for different values of x for the two series Pro.s2Sro.48 xCaxMnO3 and Pro.sSro.5 ~Ca~MnO3. Tc and Tco are deduced, respectively, from magnetization and resistivity curves. Note that, due to the very close ionic radius of Pr 3+ and Ca 2+, the compounds of the two series which have the same average size also have the same variance a 2 (see the double scale in Fig. 3). The nature of domains labeled 1 and 2 is precised in text.
Pro.sSro.5_xCaxMnO 3 [11], thus i n d i c a t i n g t h a t B-site size effects c a n n o t be c o m p l e t e l y ignored. W h i l e the valence state has little influence over the critical t e m p e r a t u r e s , this m i n o r v a r i a t i o n a w a y from a M n 3+ : M n 4+ ratio equal to one has a d r a m a t i c influence on the o b s e r v e d phases in the electronic m a g n e t i c phase d i a g r a m (Fig. 3). In dom a i n 1 (see Fig. 3), Pro.szSro.,~8-xCaxMnO3 is a f e r r o m a g n e t i c metal while P r o . s S r o . 5 - x C a x M n O 3 is an i n s u l a t o r with a c o m p l e x a n d n o t welldefined m a g n e t i c state. In d o m a i n 2, while the M n 3+ : M n 4+ = 1 series Pro.sSro.5-xCa~,MnO3 is a charge ordered AFM insulator, Pro.szSro.4s-~Ca~,MnO3 r e m a i n s a f e r r o m a g n e t i c metal. Thus, b o t h charge d i s o r d e r ( c o m p a r e d to the Mn3+ : M n 4 + = 1 c o m p o u n d ) a n d size disorder, a 2, lead to a s t a b i l i z a t i o n of the F M M state, in regions 1 a n d 2, respectively. It is interesting to observe that charge d i s o r d e r can d o m i n a t e the elect r i c a l - m a g n e t i c b e h a v i o r in region 1 where b o t h a n d a2 are small a n d one might expect insulat-
10 - 2
50 100 150 200 250 300
r (K) Fig. 4. Temperature dependence of the resistivity p in zero field and in a magnetic field of 7 T after a zero-field cooling for the compounds Pro.52Sro.3Cao.~sMnO3 (a) and Pr0.52Sro.26Cao.22MnO3 (b). The temperature dependence of the CMR ratio Ro {R7 T is also plotted for the two samples.
ing b e h a v i o r similar to the P r o . s S r o . s - x C a x M n O 3 system. It s h o u l d be n o t e d that T o k u r a et al. [11] observed the reversed effect of on charge o r d e r ing in the series ( N d l y S m j o . s S r o . s M n O 3 , i.e. charge o r d e r i n g d i s a p p e a r s for a sufficiently low value of (rA>. However, in the ' N d Sin' series, c~2 increases as (rA> decreases leading to a n t a g o n i s t effects, whereas in o u r system a2 a n d (rA> vary in the same manner. Thus, the d i s a p p e a r a n c e of charge o r d e r i n g in the ' N d Sm' system can be c o n s i d e r e d to arise from an increase of a2, which at a certain value of y becomes p r e d o m i n a n t . Therefore, in b o t h systems charge o r d e r i n g d i s a p p e a r s for larger values of cr2. Samples which e x h i b i t e d the largest v a r i a t i o n of their resistivity with t e m p e r a t u r e (at low t e m p e r ature) were investigated for C M R by m e a s u r i n g their t r a n s p o r t p r o p e r t i e s b o t h in the absence and in the presence of a m a g n e t i c field. All the samples exhibited C M R . The a p p l i c a t i o n of a m a g n e t i c field
Wolfman et al. /Journal of Magnetism and Magnetic Materials 174 (1997) L5 L9
shifts Tco toward lower temperatures, as exemplified by the data for the sample x = 0.18 shown in Fig. 4a, and this shift gives rise to the CMR. For the x = 0.18 sample, the Ro T/R7 ~ ratio maximizes at a value of 2000 at T = 126K, while for the x = 0.2 (not shown) and x = 0.22 (Fig. 4b) samples it maximizes at 1400 (T = 130 K) and 3300 (T = 128 K), respectively. The difference between the transition temperatures Tco for the x - - 0 . 1 8 sample in Figs. 1 and 4, registered while decreasing and increasing the temperature, respectively, is due to the first-order nature of the charge-ordering transition. The values of the Ro v/R7 "r ratios in the present system can be directly compared to those of the Pro.sSro.5-xCaxMnO3 series [6]. The maximum value of this ratio for the latter series was found to be 3000 at T = 150K, for the compound Pro.sSro.41Cao.o9MnO3. This value is of the same order of magnitude as for the present compound Pro.szSro.z6Cao.22MnO3 but is obtained at a slightly higher temperature. This is somewhat surprising since the resistivity jump between the metallic and the insulating state is 10 times greater in Pro.52Sro.26Cao.zzMnO3 than in Pro.sSro.~lCao.o9MnO3. In these type-2 CMR oxides, melting of the charge-ordered insulating state by the applied magnetic field accounts for their CMR and, therefore, one might have anticipate a larger CMR effect in the former compound. However, that this is not the case can be understood by taking into account the disorder induced by the different radii of the A-site cations (size mismatch). The variance ~2 is greater for Pro.sSro.41Cao.o9MnO3 (0"2= 0.00414) than for Pro.szSro.z6Cao.z2MnO3 (a z = 0.00358), which implies that the charge-ordered state is more favoured in the latter. This is reflected by the resistivity jump between the insulating and the metallic states,
L9
Pro.52Sro.26Cao.22MnO 3 having a 10 times larger difference than Pro.5Sro.41Cao.09MnO3. Moreover, the less favored charge-ordered state in the compound Pro.sSro.glCao.09MnO3 is less stable, so it is more easily melted by the application of the magnetic field, giving rise to a larger Ro v/R7 v ratio. In conclusion, the ability to restore the A F M I - F M M transition in the manganites having a Mn3+ : M n ~+ ratio different from 1 has been shown for the first time. This study shows that the deviation from the ideal 1:1 mixed valence of manganese can be compensated by controlling the average size of the A-site cation (rA) and the mismatch effect 0 "2 in these compounds. Clearly, perfect charge ordering is not necessary to stabilize the AFMI state, although it plays an important role.
References [1] Y. Tomioka, A. Asamitsu, Y. Moritomo, H. Kuwahara, Y. Tokura, Phys. Rev. Lett. 74 (1995) 5108. [2] H. Kuwahara, Y. Tomioka, A. Asamitsu, Y. Moritomo, Y. Tokura, Science 270 (1995) 961. [31 K. Knizek, Z. Jirak, E. Pollert, F. Zounova, S. Vratislav, J. Solid State Chem. 100 (1992) 292. [4] H. Kawano, R. Kajimoto, H. Yoshizawa, Y. Tomioka, H. Kuwahara, Y. Tokura, Phys. Rev. Lett. 78 (1997) 4353. [5] P. Schiffer, A.P. Ramirez, W. Bao, S.-W. Cheong, Phys. Rev. Lett. 75 (1995) 3336. [6] J. Wolfman, Ch. Simon, M. Hervieu, A. Maignan, B. Raveau, J. Solid State Chem. 123 (1996) 413. [7] J. Wolfman, A. Maignan, Ch. Simon, B. Raveau, J. Magn. Magn. Mater. 159 (1996) 299. [8] P.G. De Gennes, Phys. Rev. 118 (1960) 141. [9] L.M. Rodriguez-Martinez, J.P. Anfield, Phys. Rev. B 54 (1996) Rl5622. [10] A. Sundaresan, A. Maignan, B. Raveau, Phys. Rev. B 56 (1997) 5092. [11] E. Pollert, S. Krupicka, E. Kuzmicova, J. Phys. Chem. Solids 43 (1992) t 137.
N ELSEVIER
Journal of Magnetism and Magnetic Materials 174 (1997) L5-L9
Letter to the Editor
Restoring the AFMI-FMM transition by calcium doping in the CMR manganites Pro.szSro.48_xCaxMnO 3 J. Wolfman*, Ch. Simon, B. Raveau Laboratoire CRISMAT, UMR 6508 CNRS, ISMRA, Bd. du Marechal Juin, 14050 Caen Cedex, France
Received 5 May 1997
Abstract A small deviation from the ratio Mn3 + : Mn 4 + = 1 in the manganite Pro.5 +ySro.s -yMnO3 leads to a disappearance of the A F M I - F M M transition, the A F M I state being replaced by a F M M state. The present study of the series Pro.52Sro.gs-xCaxMnO3 shows for the first time that the A F M I - F M M transition can be restored for a Mn 3 + : Mn 4+ ratio different from 1 by calcium doping. This result is interpreted by the fact that the decrease of the average size of the A-site cation, (rA), and of the mismatch, 02, favors the A F M I state at the expense of the F M M state. The deviation from the perfect charge ordering of the i : 1 compound is then compensated by these two effects. The application of a magnetic field of 7 T shows that all the compositions exhibit a CMR effect. The maximum Ro/R7 r ratio is equal to 3500 at 127 K for the Pro.52Sr0.z6Ca22MnO 3 compound.
PACS: 75.25. + z; 75.30.Cr; 75.30.Kz Kevwords: CMR; Manganites; Antiferromagnetism; Ferromagnetism; Metal insulator transition
T h e p e r o v s k i t e s L n ~ _ x A x M n O 3 with Ln = lant h a n i d e a n d A = Ca, Sr, Ba have d r a w n a large interest these p a s t few years o w i n g to their colossal m a g n e t o r e s i s t a n c e ( C M R ) properties. A m o n g these oxides, the m a t e r i a l s c o r r e s p o n d i n g to the specific c o m p o s i t i o n x = 0.5 exhibit p a r t i c u l a r t r a n s p o r t a n d m a g n e t i c properties. Besides the F M M (fer-
*Corresponding author. E-mail: [email protected]; fax: + 33 2 3195 1600.
r o m a g n e t i c metallic) to P M I ( p a r a m a g n e t i c insulating) t r a n s i t i o n o b s e r v e d for all the C M R m a n g a n i t e s , these phases also d i s p l a y an A F M I (antiferromagnetic insulating) to F M M t r a n s i t i o n at low t e m p e r a t u r e . This is the case for the m a n g a nites Pro.sSro.sMnO3 [1] a n d N d o . s S r 0 . s M n O 3 [2], where an A F M state was i n t e r p r e t e d in terms of charge o r d e r i n g (CO), i.e. due to an o r d e r i n g of the M n 3+ a n d M n 4+ species at low t e m p e r a t u r e [3]. T h o u g h the A F M o r d e r can vary from one p h a s e to the o t h e r as shown for P r o . s S r o . s M n O 3
0304-8853/97/$17.00 ~2 1997 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 3 6 7 - 3
LETTER TO THE EDITOR
L6
J. Wo//htan et al. /Journal of Magnetism and Magnetic Materials" 174 (1997)L5 L9
(A type) and Ndo.sSro.sMnO3 (CE type) [4], it is presumed to be due to a 1 : 1 ordering of the Mn 3 + and Mn ~+ species. Consequently, a Mn 3+ : Mn 4÷ ratio equal to 1 appears to be a very crucial condition to observe the A F M ! F M M transition. It has effectively been shown that a slight deviation from the 1 : 1 stoichiometry in Pro.5-xSro.5+xMnO3 or Lao.s-xCao.5 +xMnO3 [4, 51, suppresses charge ordering and leads to a F M M state at low temperature. An important issue that has not been answered to date, concerns the possibility of observing the A F M I - F M M transition for Mn 3+ : M n 4÷ ratios different from 1. Such a possibility, if it exists, requires the stabilization of the A F M I state at low temperature. This stabilization should be favored by a decrease of the average size of the A-site cation, as was demonstrated for the substitution of smaller calcium for strontium in the chargeordered phase Pro.sSro.sMnO3, wherein the A F M I state is stabilized at the expense of the F M M state [61. In order to answer such a question, we have studied the substitution of calcium for strontium in the system Pro.52Sro.48-~CaxMnOs. The present paper shows for the first time the existence of an A F M I F M M transition in manganites having a Mn 3 ÷ : Mn 4 + ratio different from 1. It demonstrates that size and mismatch effects can be used to counteract the charge effect and to restore such a transition in the C M R manganites. The compounds Pro.52Sro.~8-xCa~MnO3 were prepared as sintered ceramics by solid-state reaction (see for details Ref. [7]). The samples were characterized by X-ray diffraction using a Seifert powder X-ray diffractometer for Cu K~ radiation. The temperature dependence of the magnetization was measured in a field of 10 .2 T, after a zero-field cooling, with a vibrating sample magnetometer (Foner). The resistivity was measured decreasing the temperature by a classical four-probe method. The transport properties were studied, after zerofield cooling, under magnetic fields up to 7 T. The typical behavior of a charge ordered 1 : 1 Mn oxide is demonstrated by the resistivity curve registered in zero field while decreasing the temperature (see Fig. 1) of Pro.sSro.5MnO3. One observes a transition from a semiconducting state to a
.
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Fig. 1. Temperaturedependenceof the resistivityp for different valuesofx labeledon the graph for the Pro.szSro.4s xCaxMnO3 series and for Pro.sSr0.sMnOs. pseudo-metallic state at T = 276 K and is followed by a transition to an 'insulating' state as the temperature is further decreased. These electrical transitions are strongly correlated to the observed magnetic transitions. As the temperature is decreased, a transition from a paramagnetic (PM) state to a ferromagnetic (FM) state occurs at Tc and is followed by a transition to an antiferromagnetic (AFM) state at TN. The metallic behavior is a consequence of the ferromagnetic state via the double-exchange interaction [81. The 'insulating' state at low temperature corresponds with the onset of charge ordering (localization of the eg holes on the Mn sites) associated with an A-type A F M order [41 . The transport behavior of the compound Pro.5+ySro.5 yMnO3 is drastically different from that of Pro.sSro.sMnO3. This is shown for Pro.52Sro.48MnO3, whose resistivity is quite independent of the temperature in the [0, 300 K1 range (Fig. 1). No resistivity anomaly can be seen, implying that charge ordering is absent in this compound. This is confirmed by the temperature-dependent magnetization behavior (Fig. 2) for which a F M state is reached at 280 K and a smooth decrease of the moment arises with decreasing the temperature but there is no pure A F M at low temperature. The weak F M M state observed at low temperature for Pro.52Sro.48MnO 3 contrasts the A F M I
LETTER TO THE EDITOR
J. WolJ~nanet al. /Journal of Magnetism and Magnetic Materials 174 (1997) L5 L9
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state obtained for Pro.sSro.sMnO3, and supports the commonly held view that a small deviation of the Mn 3+ : Mn 4+ ratio from 1 is sufficient to destroy charge ordering. Similar behavior was previously emphasized for the Lal _xCa~MnO3 system [5], in which Lao.sCao.sMnO3 is an A F M insulator at low temperature whereas Lao.szCao.4sMnO3 is a F M metal. The substitution of calcium for strontium in Pro.52Sro.48 ~CaxMnO3 leads to a spectacular change of the magnetic and transport properties (Figs. l and 2). For x = 0.04, the latter are similar to those of x = 0. But for x = 0.08 a jump in the resistivity curve reappears at a temperature T c o = 175 K, and below 155 K, the resistivity remains constant (Fig. 1). This behavior is correlated with a rapid drop of the magnetic moment, under 1 0 - 2 T (Fig. 2), at 1 8 0 K followed by a slower decrease below 155 K. At low temperature, the magnetic moment reaches a value close to onethird of the value at T - - 2 0 0 K in the F M state. So a FM contribution remains and, despite the fact that the resistivity j u m p is as high as that of Pro.sSro.sMnO3, charge ordering is not completely restored since an A F M state is not reached at low temperature. For x = 0.1, the resistivity j u m p (Fig. 1) occurs at Tco = 180 K and R18o K and R5 K differ by 3 or-
L7
ders of magnitude. The magnetic moment (Fig. 2) drops sharply below 190 K and then decrease very smoothly below 150 K. For x = 0.18, the j u m p on the resistivity curve (Fig. 1) occurs at a lower temperature than for x = 0.1 but is much more pronounced: the resistivity changes by 5 orders of magnitude between the metallic and the 'insulating' state at 5 K. The corresponding magnetization shows a rapid drop between 170 and 150 K. Below 120 K the magnetization remains constant at a very low value of 0.017 I~B/Mn atom under a field of 10 .2 T. A pure A F M state is nearly achieved as the F M contribution is very weak. These results demonstrate that the A F M I - F M M transition can be restored by calcium doping and imply that charge ordering also exists with a Mn 3÷ : Mn 4+ different from one. If one keeps in mind that the mean valence of manganese is constant in the series Pro.s2Sro.48-xCaxMnO3, then the size difference between the A-site cations must be driving the restoration of the A F M I - F M M transition. Substitution of Ca for Sr affects both the average size, ~rA), and the variance, a 2, of the A-site cation. It was recently shown for Pro.5Sro.5-xCaxMnO3 [6] that a decrease of (rA) leads to a decrease of Tc and to an increase of TN. Similarly, a decrease of a 2 was shown to increase Tc in Lno.TAo.3MnO3 [9], whereas an increase of a 2 in P r l - x C a ~ M n O 3 suppresses charge ordering [10]. In the present series Pro.s2Sro.4s xCa~MnO3, the decrease in both (rA) and a 2, as x increases, stabilizes the A F M state. In Fig. 3, Tc (filled symbols) and Tco (unfilled symbols) are plotted for the two series Pro.sSro.5 _~Ca~MnO3 [6] and Pro.s2Sro.48-xCa~MnO3. In the regions where Tc and Tco are defined for both series (i.e. all but region 2 for Pro.52Sro.4s-~CaxMnO3, where Tco is not defined owing to the absence of an insulating state), it is clear that the critical temperatures are not strongly dependent on the valence of manganese in the compounds, but are dominated by the cationic size effect. However, the minor differences in Tc of these two series may result from the unit cell expansion (~0.005 A) associated with the increase in the content of the larger Mn 3÷ cation in Pro.s2Sro.~8-xCaxMnO3 as compared to
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Pro.sSro.5_xCaxMnO 3 [11], thus i n d i c a t i n g t h a t B-site size effects c a n n o t be c o m p l e t e l y ignored. W h i l e the valence state has little influence over the critical t e m p e r a t u r e s , this m i n o r v a r i a t i o n a w a y from a M n 3+ : M n 4+ ratio equal to one has a d r a m a t i c influence on the o b s e r v e d phases in the electronic m a g n e t i c phase d i a g r a m (Fig. 3). In dom a i n 1 (see Fig. 3), Pro.szSro.,~8-xCaxMnO3 is a f e r r o m a g n e t i c metal while P r o . s S r o . 5 - x C a x M n O 3 is an i n s u l a t o r with a c o m p l e x a n d n o t welldefined m a g n e t i c state. In d o m a i n 2, while the M n 3+ : M n 4+ = 1 series Pro.sSro.5-xCa~,MnO3 is a charge ordered AFM insulator, Pro.szSro.4s-~Ca~,MnO3 r e m a i n s a f e r r o m a g n e t i c metal. Thus, b o t h charge d i s o r d e r ( c o m p a r e d to the Mn3+ : M n 4 + = 1 c o m p o u n d ) a n d size disorder, a 2, lead to a s t a b i l i z a t i o n of the F M M state, in regions 1 a n d 2, respectively. It is interesting to observe that charge d i s o r d e r can d o m i n a t e the elect r i c a l - m a g n e t i c b e h a v i o r in region 1 where b o t h
10 - 2
50 100 150 200 250 300
r (K) Fig. 4. Temperature dependence of the resistivity p in zero field and in a magnetic field of 7 T after a zero-field cooling for the compounds Pro.52Sro.3Cao.~sMnO3 (a) and Pr0.52Sro.26Cao.22MnO3 (b). The temperature dependence of the CMR ratio Ro {R7 T is also plotted for the two samples.
ing b e h a v i o r similar to the P r o . s S r o . s - x C a x M n O 3 system. It s h o u l d be n o t e d that T o k u r a et al. [11] observed the reversed effect of
Wolfman et al. /Journal of Magnetism and Magnetic Materials 174 (1997) L5 L9
shifts Tco toward lower temperatures, as exemplified by the data for the sample x = 0.18 shown in Fig. 4a, and this shift gives rise to the CMR. For the x = 0.18 sample, the Ro T/R7 ~ ratio maximizes at a value of 2000 at T = 126K, while for the x = 0.2 (not shown) and x = 0.22 (Fig. 4b) samples it maximizes at 1400 (T = 130 K) and 3300 (T = 128 K), respectively. The difference between the transition temperatures Tco for the x - - 0 . 1 8 sample in Figs. 1 and 4, registered while decreasing and increasing the temperature, respectively, is due to the first-order nature of the charge-ordering transition. The values of the Ro v/R7 "r ratios in the present system can be directly compared to those of the Pro.sSro.5-xCaxMnO3 series [6]. The maximum value of this ratio for the latter series was found to be 3000 at T = 150K, for the compound Pro.sSro.41Cao.o9MnO3. This value is of the same order of magnitude as for the present compound Pro.szSro.z6Cao.22MnO3 but is obtained at a slightly higher temperature. This is somewhat surprising since the resistivity jump between the metallic and the insulating state is 10 times greater in Pro.52Sro.26Cao.zzMnO3 than in Pro.sSro.~lCao.o9MnO3. In these type-2 CMR oxides, melting of the charge-ordered insulating state by the applied magnetic field accounts for their CMR and, therefore, one might have anticipate a larger CMR effect in the former compound. However, that this is not the case can be understood by taking into account the disorder induced by the different radii of the A-site cations (size mismatch). The variance ~2 is greater for Pro.sSro.41Cao.o9MnO3 (0"2= 0.00414) than for Pro.szSro.z6Cao.z2MnO3 (a z = 0.00358), which implies that the charge-ordered state is more favoured in the latter. This is reflected by the resistivity jump between the insulating and the metallic states,
L9
Pro.52Sro.26Cao.22MnO 3 having a 10 times larger difference than Pro.5Sro.41Cao.09MnO3. Moreover, the less favored charge-ordered state in the compound Pro.sSro.glCao.09MnO3 is less stable, so it is more easily melted by the application of the magnetic field, giving rise to a larger Ro v/R7 v ratio. In conclusion, the ability to restore the A F M I - F M M transition in the manganites having a Mn3+ : M n ~+ ratio different from 1 has been shown for the first time. This study shows that the deviation from the ideal 1:1 mixed valence of manganese can be compensated by controlling the average size of the A-site cation (rA) and the mismatch effect 0 "2 in these compounds. Clearly, perfect charge ordering is not necessary to stabilize the AFMI state, although it plays an important role.
References [1] Y. Tomioka, A. Asamitsu, Y. Moritomo, H. Kuwahara, Y. Tokura, Phys. Rev. Lett. 74 (1995) 5108. [2] H. Kuwahara, Y. Tomioka, A. Asamitsu, Y. Moritomo, Y. Tokura, Science 270 (1995) 961. [31 K. Knizek, Z. Jirak, E. Pollert, F. Zounova, S. Vratislav, J. Solid State Chem. 100 (1992) 292. [4] H. Kawano, R. Kajimoto, H. Yoshizawa, Y. Tomioka, H. Kuwahara, Y. Tokura, Phys. Rev. Lett. 78 (1997) 4353. [5] P. Schiffer, A.P. Ramirez, W. Bao, S.-W. Cheong, Phys. Rev. Lett. 75 (1995) 3336. [6] J. Wolfman, Ch. Simon, M. Hervieu, A. Maignan, B. Raveau, J. Solid State Chem. 123 (1996) 413. [7] J. Wolfman, A. Maignan, Ch. Simon, B. Raveau, J. Magn. Magn. Mater. 159 (1996) 299. [8] P.G. De Gennes, Phys. Rev. 118 (1960) 141. [9] L.M. Rodriguez-Martinez, J.P. Anfield, Phys. Rev. B 54 (1996) Rl5622. [10] A. Sundaresan, A. Maignan, B. Raveau, Phys. Rev. B 56 (1997) 5092. [11] E. Pollert, S. Krupicka, E. Kuzmicova, J. Phys. Chem. Solids 43 (1992) t 137.