Effect of aluminium for manganese substitution upon the GMR properties of the praseodymium manganites Christine Martin, Antoine Maignan and Bernard Raveau Laboratoire CRISMAT, URA CNRS 1318 associde au CNRS ISMRA, Universitk de Caen, 6 Bd du Mardchal Juin, 14050 Caen Cedex, France The substitution of A1 for Mn in the GMR perovskites Pr0.7Ca0.2sr0.1MnO3 pro. 7CaO. 1sr O.ZMnO3 and Pro~,Sro~,Mn03has been studied. For the first two series of compounds that belong to the type I GMR manganites [ferromagnetic (FM)-paramagnetic (PM) transition] T, decreases significantly as the aluminium content increases, by about 12 K per percent of A1 atoms introduced into the Mn sites, whereas the maximum magnetoresistance ratio Ro/HHis not modified dramatically, reaching lo3at 71 K for 6% A1 per Mn in a magnetic field of 7 T.This increase of Ro/RHis, in fact, correlated to the decrease of T', i.e. to the increase of the semiconductive character of the samples. For the series Pro.,Sro~,Mnl -,Al,O,, which belongs to type I1 [antiferromagnetic (AFM)-FM-PM transitions], an increase of TNfrom the AFM to the FM state is observed by A1 doping: TNincreases from 136 K for x =0 to 170 K for x =0.06. In contrast, T, decreases as x increases. The manganese perovskites have been studied extensively since the discovery of giant magnetoresistance (GMR) properties in these compounds more than seven years ago.' Among these compounds, the praseodymium phases Pr, -,A,MnO, (A= Ca, Sr, Ba) are of great interest owing to their exceptional Two kinds of effects can be distinguished in these materials, according to the x value.The first (type I) effect appears generally for 0.2 <x <0.5 and corresponds to the transition from a ferromagnetic (FM) metallic state to a paramagnetic (PM) semiconducting state as T increases.' The second effect (type 11) was observed for a particular x value (x=0.5),* and is explained in terms of charge ordering. This type I1 effect corresponds to a transition from an antiferromag- netic (AFM) semiconducting state to a ferromagnetic (FM) metallic state as Tincreases. The study of the crystal chemistry of these phases has shown that two factors are of paramount importance for their GMR properties: the mean size of the interpolated cation and the hole carrier density determined by the Mn"'/Mn'' rati0.~-'9~ It was indeed shown that the transition temperature, T,, of the FM-PM transition increases as the mean size of the interpolated cation increases, whereas the transition tempera- ture TNof the AFM-FM transition increases as the mean size of the interpolated cation decreases.Such effects, which corre- spond to the modification of the overlap of the d orbitals of manganese and oxygen p orbitals, play a key role in the control of the transition temperatures of the phases and to optimize their GMR properties. In contrast, very little is known about the influence of the presence of foreign elements in the Mn sites upon the GMR properties of these compounds.For this reason, the study of the substitution of aluminium for manganese in the phase Pr0.7( Ca,Sr),,,Mn03 and Pro.5Sro.5Mn03 was undertaken. Besides the regular decrease of Tc by aluminium doping, the most important feature that is shown herein for the first time deals with the substantial increase of the transition temperature TN in the type I1 manganites Pro.5Sro.,Mn03 by aluminium doping. The samples were prepared by solid-state reaction, by mixing Pr6011, SrC03, CaO, MnO, and A1203 in stoichiometric proportions. The powders were first heated for 12 h at 9OO0C, and after regrinding they were pressed into bars (2x 2 x 10 mm3) and sintered for 12 h at 1200°C and 1500°C successively. All the compounds were tested for purity by X- ray diffraction (XRD) and electron diffraction (ED).The resistances were measured by the four-probe method on bars in earth magnetic field and in a field of 7 T; the samples were first zero-field cooled and then the magnetic field was applied. The measurements were recorded by increasing the tempera- ture from 5 to 300 K. The magnetization curves were recorded with a vibrating-sample magnetometer, after zero-field cooling down to 5 K, a magnetic field was applied (1.45 T and 100 G for the Pro.7Cao.3 -,Sr,,Mnl -,A1,03 and Pro.,SrO.,Mnl -xAl,03 series, respectively) and the temperature was increased to 280 K. The substitution of aluminium for manganese in these praseodymium perovskites influences dramatically the GMR properties, so that only substitutions at very low level, i.e.<16% of aluminium, were studied. Under these conditions, the structure of the substituted perovskite is not significantly different from the pristine one. The powder XRD patterns of all the samples evidence a single phase with the perovskite structure. The systematic ED investigation performed on more than 100 microcrystals confirms the high purity of the different samples. The two series of oxides Pro~7(Ca,Sr)o~3Mnl -,Al,03 and Pro~,Sro.,Mnl -,Al,03 exhibit different behaviour and for this reason will be examined separately. The resistance us. temperatures curves in zero magnetic field for the type I GMR perovskites Pro~7Cao~2Sro~lMnl -,Al,03 [Fig. 1(a)] and Pro~7Cao~lSro~2Mnl -xAl,03 [Fig.1 (b)] show that the introduction of small amounts of aluminium does not modify the shape of the curves, which are all characterized by a maximum at T= T,, for x<O.16. Thus the transition from a metallic to a semiconducting state, as T increases, still exists in the presence of aluminium. Nonetheless, the resistance is increased significantly by aluminium substitution, so that for the higher x values, the samples do not exhibit metallic behaviour at low temperature. However, the transition tem- perature T,,, decreases rapidly as the aluminium content increases, by about 12 K per percent of A1 atoms introduced onto the manganese sites. Indeed for both series Pr0.7Ca0.2Sr0.1Mn1 -xA1x03 and Pr0.7Ca0.1Sr0.2Mn1 -xAlxO3, TmaXdecreases linearly us.x (Table 1). The magnetization us. temperature curves in a magnetic field of 1.4T for the series Pro.,Cao~2Sro,lMnl-xAl,03 (Fig. 2) confirm that this evolution of the resistance corresponds to a FM-PM transition as T increases: the Curie temperature, T,, coincides with T,,,. The magnetic moment per Mn atom, although it decreases slightly, remains rather close to the theoretical value of 3.70~~ (Table 1). J. Muter. Chern., 1996,6(7), 1245-1248 1245 =o 12 1 X=O 08 x=O 04 1 o2 10' -.--..--+.-1 oo -. lo-' 1 0 50 100 150 200 250 300 TIK Fig. 1 Temperature dependence of R for different x values for the Pro 7Cao ,Sr0 lMn, -xAl,03 samples (a) and for the Pro 7Cao lSro ,Mn, -,AlXO3 samples (b) Table 1 Magnetic moment per Mn atom and T,,, observed in Pro $ao ,Sr0 lMn,-,Al,O, and in Pro 7Cao lSro ,Mn, -,Al,03 Pro 7Cao ,Sr0 ,Mn, -,AlXo3 0 3.75 146 0.01 3.68 134 0.02 3.58 116 0.04 3.46 91 0.06 3.21 58 Pro 7Cao ,Sr0 ,Mn, -,Al,03 0 3.57 217 0.04 3.55 171 0.08 3.10 115 0.12 2.95 72 J 0 50 100 150 200 250 300 TIK Fig. 2 Temperature dependence of magnetization for different x values (labelled on the graph) for the Pro ,Ca, ,Sr0 ,Mn, -,Al,03 series in a magnetic field of 1.45 T The resistance us.temperature curves in a magnetic field of 7 T [Fig. 3(a)] show that the aluminium substitution, if it decreased T,, does not diminish or enhance dramatically the Ro/RH ratio [Fig. 3(b)]. In fact, a complex behaviour is 1246 J.Muter. Chem., 1996, 6(7), 1245-1248 lo'.-oo 0 50 100 150 200 250 1000 L , 'A 1 , , ,I I I I I I I I I i, x=o.o1 200 0 50 100 150 200 TIK Fig. 3 Temperature dependence of (a) R at H =O and 7 T for the Pro 7Cao ,Sr0 ,Mn0 94A10 0603sample, and (b)the ratio Ro/R, for the Pro 7Cao $r0 ,Mn, -,AlXO3 series observed. For the series Pro &ao 2Sro ,Mn, -xAlx03, the maxi- mum Ro/RH ratio first decreases from 275 at 151 K for x=O to 76 for x=O.Ol at 141 K; it is then increased to 330 at 97 K for x=O.O4 and finally to lo3at 68 K for x=O.O6. In a similar manner, in the series Pro &ao ,Sr0 ,Mn, -,AlXO3, the maxi- mum Ro/RH ratio remains approximately constant, and close to 6-8, from x=O at 217 K to x=0.04 at 167 K; it increases then up to 100 at 72 K for x =0.12.Clearly, in each series, the fact that Ro/RH can be increased by one order of magnitude as x increases, is correlated to the decrease of T,,,, i.e. to the increase of the resistance of the material in a zero magnetic field. The evolution of the resistivity of the type I1 GMR perov- skites Pro 5Sro ,Mnl -xAlx03 us. temperature in a zero magnetic field (Fig. 4) shows that, up to x =0.08 they exhibit a behaviour similar to that observed for Pro $1-0 5Mn03,8 characterized by a transition from a semiconducting to a metallic state as T increases. The corresponding magnetization us. temperature curves (Fig. 5) confirm that for x <0.08 this transition coincides with an AFM-FM transition as T increases and evidences an FM-PM transition at higher temperature.Thus, the transition temperature, TN,from the AFM semiconducting state to the 1 o2 10' 1 oo G U lo-' 1o-2 1o-~ 0 50 100 150 200 250 300 TIK Fig. 4 Temperature dependence of R for different x values (labelled on the graph) for the Pro ,Mn, -,AI,O3 series 0 30 0.25 0.20 2m 0 15 z 0 10 0.05 0.00 0 50 100 150 200 250 300 TIK Fig. 5 Temperature dependence of magnetization for different x values (labelled on the graph) for the Pr,,,Sr,,,Mn, -xAl,O, series in a magnetic field of 100 G FM metallic state, can be defined either by the inflexion on the R(T) curves (Fig. 4) or by the left branch of the M(T) curve (Fig. 5). In the same way, the transition temperature, Tc (or T,,,) can be obtained either from the right branch of the M(T) curve (Fig.5) or from the maximum of the R(T) curve (Fig. 4). The first interesting point concerns the increase of the resistance as x increases, so that for x=O.lO, it has become two orders of magnitude larger than for the pristine phase Pr,,,Sr,,,MnO, at 100 K. For xb0.10 the behaviour of the phase is that of a classical semiconductor, the resistivity increasing dramatically as shown for the x =0.12 phase (Fig. 4), whose resistivity is three orders of magnitude larger than that of the pristine phase at 100 K. The most important feature deals with the significant increases of TN,as the A1 content increases, from 136 K for x =0 to 170 K for x =0.06. The Curie temperature Tc (or T,,,) undergoes a corresponding decrease as x increases, similarly to the decrease of T, observed for the type I compositions ranging from 260 K for x=O to 190 K for x=O.O6.In other words, the substitution of aluminium for manganese favours the expansion of the AFM and PM semiconducting states at the expense of the FM metallic state which vanishes around x =0.08 (Fig. 5). A similar behaviour has been observed previously for the 0~mangani tes pr0.5 -~ ~ ~ ~ ~ (ref. 9) . 5 and Pr,,,Sr0,,-,Ca,Mn0, (ref. 10). It was explained as the conse- quence of size effect: the decrease of the mean size of the interpolated cation (Pr, Y, Ca) hinders the overlapping of the Mn-0-Mn orbitals and weakens significantly the hole delocal- ization and consequently benefits to the semiconducting AFM and PM phases. In the case of the aluminium substitution, the smaller size of aluminium compared to manganese may not be the prominent factor that governs this property, but rather its electronic configuration.Owing to the absence of d electrons on the aluminium sites, the substitution of Al"' for Mn"' tends to break the hole propagation in the manganese-oxygen lattice. Nevertheless it is quite remarkable that for an occupancy factor of 6% of the Mn sites by aluminium the metallic conductivity still exists. The application of a magnetic field of 7T confirms the negative magnetoresistance properties of these compounds [Fig. 6(u)]. One only observes a small decrease of the R,/R, ratio as aluminium is substituted for manganese [Fig.6(b)]. Moreover, this decrease of R,/R, is regular and correlated to the increase of TN.In a zero-field cooled sample, one observes maximum R,/R, ratios of 18 at 61 K, 11 at 105 K, 7 at 144 K and 3 at 173 K for x=O, 0.01, 0.04 and 0.08, respectively. 10' a: loo lo-' 1o-2 50 100 150 200 250 300--0 \ i c I-% 10cr" 5 0 50 100 150 200 250 300 T/K Fig. 6 Temperature dependence of (a) R at H=O and 7 T for the Pro,5Sro~5Mno,96Al,,0403sample, and (b) the ratio R,/R, for the Pr,,,Sr,,,Mn, -xAl,03 series In contrast to the evolution of TN,the evolution of R,/RH is very different from that observed for the manganites Pro., ~,Y,Sr,~,MnO, (ref. 9) and Pro.5Sro.5 -,Ca,Mn03'o.One indeed observes that the introduction of aluminium does not modify or decrease only slightly the R,/R, ratio as x increases in the solid solution Pr,.SSro.,Mn, -,AlXO3, whereas the R,/RH ratio can be increased by two orders of magnitude by yttrium or calcium doping of Pr,~,SrO,,MnO3. Such a different behav- iour can be explained by the fact that the introduction of aluminium onto the Mn sites tends to decouple the Mn-0-Mn magnetic interactions owing to the absence of d electrons for this element. This study has shown that the substitution of aluminium for manganese in the praseodymium manganites does not deteriorate their GMR properties, although it decreases T,. One indeed observes a significant increase of the R,/R, ratio for the type I Pr,~,(Sr,Ca),~,Mn, -,Al,O, phases, whereas the ratio is only slightly decreased in the case of the type I1 GMR phases PT,.~ST,.~M~~ -,Al,03.But the most remarkable feature ~3~which is shown for the first time deals with the fact that the transition temperature TNfrom the AFM semiconducting to the FM metallic state can be increased by 40 K by doping the Mn sites with aluminium. Such results suggest that substi- tutions on the manganese by foreign cations should play a key role for the optimisation of the GMR properties of the manganites, depending upon their electronic configuration. A systematic study of the doping of the manganese sites by other cations, transition and post-transition elements, is in progress in order to understand their role in the GMR properties of these materials.References 1 R. M. Kusters, J. Singleton, D. A. Keen, R. M. Greevy and W. Hayes, Physica B, 1989,155,362. 2 B. Raveau, A. Maignan and V. Caignaert, J. Solid State Chem., 1995,117,424. 3 H. Y. Hwang, S. W. Cheong, P. G. Radaelli, M. Marezio and B. Battlogg, Phys. Rev. Lett., 1995,75,914. 4 A. Maignan, Ch. Simon, V. Caignaert and B. Raveau, Solid State Commun., 1995,96,623. 5 A. Maignan, V. Caignaert, Ch. Simon, M. Hervieu and B. Raveau, J.Muter. Chem., 1995,5, 1089. J. Muter. Chem., 1996,6(7), 1245-1248 1247 6 7 8 V Caignaert, E Suard, A Maignan, Ch Simon and B Raveau, J Magn Magn Muter, 1996,152, L5 A Maignan, Ch Simon, V Caignaert and B Raveau, Z Phys B, 1996,99,305 Y Tomioka, A Asamitsu, Y Montomo, H Kowahara and 9 10 J Wolfman, A Maignan, Ch Simon and B Raveau, J Magn Magn Muter, submitted J Wolfman, Ch Simon, M Hervieu, A Maignan and B Raveau, J Solid State Chem ,1996, in press Y Tokura, Phys Rev Lett, 1995,74,5108 Communication 6/02498J, Received 10th Aprzl, 1996 1248 J Muter Chem , 1996,6(7), 1245-1248