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Neither Goodenough ionic model nor Zener polaron model for Bi0.5Ca0.5Mn1−xNixO3−δ system
Available online at www.sciencedirect.com
Solid State Sciences 10 (2008) 476e480 www.elsevier.com/locate/ssscie
Neither Goodenough ionic model nor Zener polaron model for Bi0.5Ca0.5Mn1xNixO3d system O. Toulemonde*, I. Skovsen, F. Mesguich, E. Gaudin ICMCB, CNRS, Universite´ Bordeaux I, 87, Av. Dr. A. Schweitzer, 33608 PESSAC Cedex, France Received 10 October 2007; received in revised form 7 February 2008; accepted 11 February 2008 Available online 21 February 2008
Abstract The magnetic susceptibilities of three Bi0.5Ca0.5MnO3d compounds synthesised by three different methods were characterised and analysed. Large magnetic Mnx clusters (x 4) were considered to explain the high value of the CurieeWeiss constant. Unlike previous studies on similar systems, Goodenough ionic model or Zener polaron model is not suitable. In all cases, cluster behaviour is observed at low field and at low temperature. The influence of the oxygen stoichiometry and the homogeneity of the cation distribution depending on the method of the synthesis used is discussed. Finally, the effects of nickel doping on the magnetic properties were studied and the cluster behaviour was confirmed. The distribution in size of the clusters depends on the amount of nickel and it induces a glassy magnetic behaviour. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Charge ordered manganites; Magnetic transition; Bismuth manganites
1. Introduction Mixed valent manganites T1xDxMnO3 with T trivalent and D divalent elements derived from perovskite structure are studied extensively for their fascinating properties like charge/orbital ordering, colossal magnetoresistance and magnetic phase separation. Among the charge ordered compounds that are often found near half doping x w 0.5 a certain controversy arises. At the so-called TCO temperature at which the charge/orbital ordering occurs does a Goodenough purely ionic model [1] or a Zener polaron model [2] describe the ordered state? From a structural point of view, Pbnm orthorhombic structures are observed above TCO. In contrast, far below TCO powder X-ray synchrotron diffraction pattern shows appearance of satellite peaks in the X-ray patterns [3]. This superstructure was correlated to an ordered distribution of Mn3þ and Mn4þ ions in the lattice which suggests a Goodenough model. However, atomic positions could also suggest a distribution of pairs
* Corresponding author. Tel.: þ33 5 40 00 22 82; fax: þ33 5 40 00 27 61. E-mail address: [email protected] (O. Toulemonde). 1293-2558/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.02.015
of manganese with a mixed valence state of 3.5þ resulting in a so-called Zener polaron order (ZP) [2]. From a magnetic point of view a paramagnetic regime is observed far above TCO. Considering T0.5D0.5MnO3 with an ionic distribution of Mn3þ (S ¼ 3/2) and Mn4þ (S ¼ 2) ions, the effective paramagnetic moment mionic is estimated to eff 4.42 mB for a spin only value. If a ZP pair model is considered, an eg electron is shared by two Mn to form the MneOeMn unit called polaron. The (teg)3 core electrons of the polaron are parallel reflecting a local double exchange phenomena. It results in a total spin value ST ¼ S1 þ S2 ¼ 7/2 where S1 ¼ 1/2 is the spin value of the eg electron ferromagnetically coupled to the core spins with S2 ¼ 2 3/2. Assuming that the angular moment has been quenched the effective paramagnetic moment for a ZP pair is then mZP eff ¼ 7.94 mB. This analysis was recently used to interpret anomalies at the charge order temperature on the magnetic susceptibility of Y0.5Ca0.5MnO3 compound which shows a magnetic transition from ionic paramagnetic to pairs paramagnetic entities [2]. For Bi0.75Sr0.25MnO3 compound a change in meff at TCO was also explained by the presence of ZP pairs [4]. Such magnetic anomalies were already observed in Bi1/2(Ca1ySry)1/2MnO3 series (0 < y < 1) but not discussed [5]. It consequently becomes interesting to extract
O. Toulemonde et al. / Solid State Sciences 10 (2008) 476e480
meff values from magnetic measurements in the new series Bi0.5Ca0.5Mn1xNixO3 (0 x 0.10) to test the validity of the different models. For the first time, the consequences of the synthesis method on the magnetic properties are presented and allow us to conclude that the cluster transition observed at low temperature is depending on both the cationic distribution on the perovskite A site and the oxygen stoichiometry. 2. Experimental 2.1. Powder synthesis The samples Bi0.5Ca0.5Mn1xNixO3d (x ¼ 0, 0.03, 0.5 and 0.10) were prepared by solid state route using stoichiometric amounts of CaCO3, MnO2, NiO and Bi2O3 (CaCO3 was stored in an oven at 250 C to avoid its hydration). The samples were ground under acetone in an agate mortar and annealed for 8 h at 900 C, then heated for 8 h at 1100 C twice with intermediate grinding. All samples were fired in air except Bi0.5Ca0.5Mn0.9Ni0.1O3 which was fired under oxygen flow. The samples Bi0.5Ca0.5Mn1xNixO3d (x ¼ 0, 0.015) were prepared by a liquid route. Stoichiometric amounts of Bi2O3, CaCO3, Mn(CH3COO)2$4H2O and Ni(NO3)2$6H2O were dissolved in concentrated nitric acid. The nitric acid was evaporated on a hotplate and the residue was heated with a Bunsen burner until it turned black. The remains were heated in the furnace at 500 C for 5 h under oxygen flow to remove any remaining nitric gases. Thereafter the samples were annealed at 900 C for 8 h and fired twice at 1100 C for 8 h in air with intermediate grinding. Bi0.5Ca0.5MnO3d was synthesised using the Pechini method [6]. Stoichiometric amount of Bi2O3, CaCO3 and Mn(CH3COO)2$4H2O was dissolved in concentrated nitric acid. Thereafter citric acid and ethylene glycol were P added with the molar ratio ncitric acid ¼ nethylene glycol ¼ 5 nreactant. The liquid was evaporated on a hotplate and the residue was heated with a Bunsen burner. The remains were heated in an oven at 300 C for 5 h to remove nitric gasses and CO2 produced during the esterification. The sample was thereafter annealed at 800 C for 8 h under oxygen flux. The resulting mixture was reground, pressed into pellets and sintered at 1100 C for 8 h under oxygen flux. For clarity, the polycrystalline samples of Bi0.5Ca0.5MnO3d prepared by conventional ceramic route, autocombustion reaction and ‘‘Pechini’’ reaction will be denoted as BiCaMn-S, BiCaMn-L, BiCaMn-P, respectively.
477
1000 Oe. The samples were first heated to 350 K and then cooled to 5 K in zero field at 5 K/min before the magnetic field was applied. Measurements using field cool cooling have also been already performed. 3. Results 3.1. Bi0.5Ca0.5MnO3d compounds Fig. 1 shows the room temperature XRD patterns for sample BiCaMn-P. The powder patterns were indexed using an orthorhombic cell within the space group Pbnm. It leads to the lattice parameters a ¼ 5.4163(2), b ¼ 5.4692(2) and ˚ . Almost the same cell parameters were c ¼ 7.5333(2) A obtained for BiCaMn-S and BiCaMn-L samples within the standard uncertainties. Such cell parameters are not expected around room temperature since the charge ordering is already established as seen in Fig. 2 that shows a discontinuity of the magnetic susceptibility measurements around 330 K. As previously discussed by Giot et al. [7], a doubling of the b cell parameter is observed below TCO. However, this doubling is not clearly evidenced for all crystallites. At room temperature, an average structure in between ordered (at low temperature) and disordered (at high temperature) structures would be observed. X-ray diffraction measurements far below TCO are needed to get better accuracy of the cell symmetry. As explained, the magnetic anomalies around room temperature are related to the charge ordering temperature. On the inverse magnetic susceptibility versus temperature curve (Fig. 3), when temperature decreases, they are followed up by a linear domain characteristic of a paramagnetic regime at lower temperature. A CurieeWeiss analysis of such linear domain (c ¼ C/(T qCW)) allows the determination of the critical paramagnetic temperature qCW and the Curie constant C as shown in the inset of Fig. 3. The decrease of qCW indicates an increase of the antiferromagnetic interactions. Such behaviour was already observed when qCW goes from 30 K to 10 K when Mn3þ/Mn4þ ratio decreases in the series
2.2. Characterisations High resolution X-ray diffraction. The instrument used was an X’Pert MPD PANalytical diffractometer with q 2q BraggeBrentano geometry with a primary monochromator and Cu Ka1 radiation (45 kV, 40 mA). The divergence and anti-scatter slits were both equal to 1 . Magnetic properties. Magnetisation measurements were made with a superconducting quantum interference device SQUID (MPMS, quantum design) using a dc current under
Fig. 1. Observed (crosses) and indexed (solid lines) XRD patterns for BiCaMn-P. The row of the reflection markers shows the positions of the allowed reflections for the space group Pbnm.
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Fig. 2. Magnetic susceptibility of BiCaMn-P compounds deduced from ‘‘zero field cooled warming’’ and ‘‘field cooled cooling’’ measurements under low and high applied magnetic fields. For clarity, the high field measurement was shifted up by 2.25 103 emu/mol.
Bi1xCaxMnO3 (0.4 x 0.6) [7]. Since the estimated qCW is the lowest in the case of BiCaMn-P compound and since an optimised oxidising process (lowest annealed and sintered temperature, oxygen flux) was indeed used for this compound, one can conclude that the antiferromagnetic interactions are weakened when oxygen content decreases. If one now considers the Curie constant C, it increases from BiCaMn-L to BiCaMn-P. The ideal Curie constant for an ionic model is given by C ¼ N m2eff/3kB (N is the number of magnetic entities which is equal in the ionic case to the Avogadro number NA, mionic eff ¼ 4.42 mB and kB is the Boltzman constant) and is estimated to be about 2.44 emu K/mol. The ideal Curie constant for ZP pairs model is estimated to be about 3.94 emu K/mol (N ¼ NA/2, mZP eff ¼ 7.94 mB). The experimental values of C are much larger than the theoretically predicted ones. It suggests the existence of a stronger coupling between
Fig. 3. Temperature dependence of the inverse magnetic susceptibility of the samples Bi0.5Ca0.5MnO3d depicted as BiCaMn-P, BiCaMn-S, BiCaMn-L (see text). The results of the CurieeWeiss analysis (c ¼ C/(T qCW)) obtained in between 160 K and 230 K in the temperature range for which the first derivative is constant are given in the inset.
manganese within larger clusters than pairs. If Mn4 clusters are now considered with a total spin value of ST ¼ 7 and assuming that the angular moment is quenched, an effective paramag4 netic moment mMn ¼ 14:97 mB is expected for a Curie eff constant C ¼ 7 emu K/mol. In contrast to the previous predictions the experimental value of 6.80 emu K/mol for BiCaMn-P which again may present the highest oxygen content, is now close to the theoretically expected value for Mn4 cluster model. In fact, on one hand recent detailed analysis of Bi0.6Ca0.4MnO3 crystals suggests that the ordered state consists of alternating layers of two kinds of MnO6 octahedra in an Mn(1)eMn(2)eMn(1)eMn(2) sequence [7]. On the other hand, in the Bi0.75Sr0.25MnO3 compound, an ordered intergrowth of double stripes of antiferromagnetic orbital ordering LaMnO3-type and ferromagnetic orbital ordering YBaMn2O6type is observed [8]. The hypothesis is that Mn4 clusters may also originate from an intergrowth mode. Finally, the existence of large magnetic clusters was already reported in manganites [9]. Now let us discuss the sensitivity of the Mn4 cluster model to the oxygen stoichiometry. Indeed, for Bi0.5Ca0.5MnO3d the oxygen off-stoichiometry d will drastically enhance the number of free Mn3þ ions and consequently the Curie constant will vary for example from 7 to 4.33 emu K/mol for d ¼ 0 to d ¼ 0.05, respectively. Preliminary thermogravimetric and Inductively Coupled Plasma analysis performed on BiCaMnS estimate d to be 0.023 and Bi/Ca ratio equals close to 1, respectively. It results on a mixed valence state of 3.44þ for manganese to respect the charge balance. One mole of hypothetical Bi0.55Ca0.45MnO3 will give 0.45/4 ¼ 0.1125 mole of Mn4 clusters and 0.1 mol of free Mn3þ ions. As a result, a theoretical CurieeWeiss constant of 5.12 emu K/mol can be calculated which is close to the experimental value for the BiCaMn-S compound. Consequently, a lower content of oxygen will increase the number of free Mn3þ ions and decrease the CurieeWeiss constant. Based on the Goff model [8], a loss of the oxygen stoichiometry would induce disorder in the intergrowth of the distinctive blocks. Several experiments such as X-ray Photoelectron Spectroscopy, microprobe, ICP and thermogravimetric analysis are under progress to determine the Bi/Ca ratio and the d value for BiCaMn-L and BiCaMnP samples. The magnetic anomalies observed at 130 K are ascribed to the Ne´el temperatures (TN) as shown in Fig. 3. For the three compounds no significant evolution of TN is evidenced as already observed for TCO. Our results are in the range of the already published Ne´el temperature in between 120 K and 150 K for Bi1xCaxMnO3 (0.4 x 0.6) samples [7,10]. Finally, the minima at low temperature correspond to cluster transitions. They are displaced to lower temperature for higher applied magnetic field until vanishing under 5 T (Fig. 2). It is a clear indication that they are not due to a true magnetic phase transition. At low applied magnetic field the net difference in field cooled and zero field cooled measurements increases significantly at the cusp (Fig. 2). The cluster transition is less pronounced for BiCaMn-L than for BiCaMn-P that it is likely due to the change in oxygen stoichiometry. In fact, such magnetic anomalies and the existence of
O. Toulemonde et al. / Solid State Sciences 10 (2008) 476e480
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ferromagnetically ordered clusters were already pointed out when Bi0.6Ca0.4MnO3 thin films were deposited at a reduced O2 partial pressure [11]. In addition, the cluster behaviour could also come from an inhomogeneous distribution of the perovskite A site cations. Macroscopic Bi3þ rich region may enhance ferromagnetic domains in contrast to macroscopic Ca2þ rich region which would enhance antiferromagnetic domains. The distribution of the Bi3þ and Ca2þ perovskite A cations is in fact improved using a Pechini method. Both variations of the amplitude of cluster anomalies and of the critical paramagnetic temperature qCW suggest the existence of ferromagnetic coupling between clusters. It reveals that the long-range antiferromagnetic ordering in between clusters is decreased by the free Mn3þ ions and the macroscopic inhomogeneities of the cationic distribution.
3.2. Bi0.5Ca0.5Mn1xNixO3d When nickel is introduced on the perovskite B site of the BiCaMn-S compounds, the charge ordering state is expected to become unstable [12]. First, Fig. 4 shows the cell parameter evolution with Ni2þ ion content at room temperature for powder patterns indexed using an orthorhombic cell. When Ni2þ content increases, the convergence of the cell parameters suggests a continuous melting of the charge ordering. However this convergence is not associated with a structural transition from orthorhombic to tetragonal unit cell in contrast to what was proposed by Xiong et al. [13]. Indeed, in our case some peaks in the X-ray diffraction pattern cannot be indexed using a tetragonal setting. The melting of the charge ordering by nickel doping is further supported by the magnetisation measurements. First, a continuous reduction of TCO from w330 K for BiCaMn-S to w260 K for Bi0.5Ca0.5Mn0.9Ni0.1O3 is observed in Fig. 5. The almost linear decrease of TCO as a function of doping level agrees with the results obtained with chromium doping of the same compound [13]. However, TCO decreases for nickel doping by about 6 K per % Ni, whereas 1% of Cr doping decreases TCO by about 8 K. In fact Cr3þ and Ni2þ
Fig. 4. Lattice parameters as a function of nickel content obtained at room temperature. Solid lines are guides for the eyes.
Fig. 5. Temperature dependence of the magnetic susceptibility of the samples Bi0.5Ca0.5Mn1xNixO3d. The results of the CurieeWeiss analysis (c ¼ C/ (T qCW)) obtained below TCO are given in the inset.
destroy the charge ordering by two different mechanisms on Ln0.5Ca0.5MnO3 [14,15]. Second, it can be seen in Fig. 5 that the cluster behaviour observed in the Bi0.5Ca0.5MnO3 compounds vanishes whereas a broader glass transition appears at slightly higher temperature. The glass-like transition is evidenced by the difference between field cooled and zero field cooled measurements (not shown). The inter-clusters Mnx (x 4) magnetic exchange is essentially antiferromagnetic in the undoped phase as shown by the negative sign of qCW. In contrast ferromagnetic superexchange Niþ2eO(2p)2eMn4þ is observed in the doped phases which is enhanced with doping. qCW increases and even becomes positive when Ni content increases (inset Fig. 5). It confirms previous X-ray magnetic circular dichroism study showing an increase of the XMCD signal with chromium doping [15]. In the doped compounds, the cluster glass transition is consequently due to a change of the nature of the inter-cluster magnetic exchange interactions. From the linear parts of the inverse magnetisation graph the CurieeWeiss constants of the samples were estimated. Both theoretical and experimental values are presented in the inset of Fig. 5. The experimental values for Curie constants decrease continuously and are higher than the theoretically expected values considering an ionic model. Note that the temperature range of the paramagnetic behaviour increases with Ni doping. It supports a disordered insertion of the nickel ions on the perovskite B site. Secondly, the anomaly on the magnetic susceptibility at TCO becomes nearly suppressed for Bi0.5Ca0.5Mn0.9Ni0.1O3d. A magnetic phase segregation is then observed below TCO when nickel cations are introduced. However, the Curie Constants are still large compared to the ionic model reflecting the existence of magnetic clusters MnxNiy around Ni2þ. It indicates that no long-range ferromagnetic ordering can be induced by doping the Bi0.5Ca0.5MnO3d compound in contrast to Ln0.5Ca0.5MnO3d (Ln ¼ rare earth) [12]. Indeed, our results suggest a competition between ferromagnetic and antiferromagnetic inter-cluster interaction and support the intergrowth block mode which is destroyed by the doping giving a glassy magnetic behaviour. The intrinsic
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nature of Bi0.5Ca0.5MnO3d compound avoids the opportunity to induce long-range ferromagnetism by doping.
size of the clusters magnetic time relaxation measurements are needed.
4. Concluding remarks
References
Bi0.5Ca0.5Mn1xNixO3d compounds were synthesised following different methods in order to point out the literature incoherencies in the series Bi0.5Ca0.5MnO3d and to test the validity of the Zener polaron pairs theory. X-ray diffraction powder patterns collected at room temperature were indexed using an average orthorhombic cell. A preliminary Curiee Weiss analysis of the paramagnetic regime that is observed in between TN w 130 K and TCO w 330 K was performed. Both Curie constant and qCW parameters behave linearly with the relative expected oxygen content which depends on the way of synthesis. Disordered Mn4 clusters are proposed to explain the paramagnetic domain. A competition between ferromagnetic and antiferromagnetic inter-cluster interactions is clearly established in Bi0.5Ca0.5MnO3d compounds depending on the distribution of the A site cations and the oxygen stoichiometry. Our work supports an intergrowth distinctive block mode as proposed by Goff and Attfield [8]. When nickel is introduced at the perovskite B site, a convergence of the cell parameters corroborated with DC magnetic susceptibility measurements suggests a continuous and disordered melting of the charge ordering. In contrast to Ln0.5Ca0.5MnO3d (Ln ¼ rare earth) no long-range ferromagnetic ordering is observed when nickel doping is introduced but glassy magnetic behaviour of MnxNiy clusters is supported. In order to obtain the (in)homogeneity in
[1] J.B. Goodenough, Phys. Rev. 100 (1955) 564. [2] A. Daoud-Aladine, J. Rodriguez-Carjaval, L. Pinsart-Gaudart, M.T. Fernandez-Diaz, A. Revcolevschi, Phys. Rev. Lett. 89 (2002) 097205. [3] P.G. Radaelli, D.E. Cox, M. Marezio, S.W. Cheong, Phys. Rev. B 55 (1997) 3015. [4] C. Frontera, J.L. Garcı´a-Mu~noz, C. Ritter, L. Ma~nosa, X. Capdevila, A. Calleja, Solid State Commun. 125 (2003) 277. [5] J.L. Garcı´a-Mu~noz, C. Frontera, M.A.G. Aranda, A. Llobet, C. Ritter, Phys. Rev. B 63 (2001) 064415. [6] M.P. Pechini, U.S. Patent No. 3,330,697, (July 11, 1967). [7] M. Giot, P. Beran, O. Pe´rez, S. Malo, M. Hervieu, B. Raveau, C. Martin, M. Nevriva, K. Knizek, P. Roussel, Chem. Mater. 18 (2006) 3225. [8] R.J. Goff, J.P. Attfield, J. Solid State Chem. 179 (2006) 1369. [9] R.P. Borges, F. Ott, R.M. Thomas, V. Skumryev, J.M.D. Coey, J.I. Arnaudas, L. Ranno, Phys. Rev. B 60 (1999) 12847; J. Nogues, Vassil Skumryev, J.S. Mu~noz, B. Martinez, J. Fontcuberta, L. Pinsard, A. Revcolevschi, Phys. Rev. B 64 (2001) 024434. [10] H. Woo, T.A. Tyson, M. Croft, S.-W. Cheong, J.C. Woicik, Phys. Rev. B 63 (2001) 134412. [11] S. Chaudhuri, R.C. Budhani, Phys. Rev. B 74 (2006) 054420. [12] A. Barnabe, A. Maignan, M. Hevieu, F. Damay, C. Martin, B. Raveau, Appl. Phys. Lett. 71 (1997) 3907. [13] C.M. Xiong, J.R. Sun, R.W. Li, S.Y. Zhang, T.Y. Zhao, B.G. Shen, J. Appl. Phys. 95 (2004) 1336. [14] O. Toulemonde, F. Studer, B. Raveau, Solid State Commun. 118 (2001) 107. [15] O. Toulemonde, F. Studer, A. Barnabe´, B. Raveau, J.B. Goedkoop, J. Appl. Phys. 86 (1999) 2616.
Solid State Sciences 10 (2008) 476e480 www.elsevier.com/locate/ssscie
Neither Goodenough ionic model nor Zener polaron model for Bi0.5Ca0.5Mn1xNixO3d system O. Toulemonde*, I. Skovsen, F. Mesguich, E. Gaudin ICMCB, CNRS, Universite´ Bordeaux I, 87, Av. Dr. A. Schweitzer, 33608 PESSAC Cedex, France Received 10 October 2007; received in revised form 7 February 2008; accepted 11 February 2008 Available online 21 February 2008
Abstract The magnetic susceptibilities of three Bi0.5Ca0.5MnO3d compounds synthesised by three different methods were characterised and analysed. Large magnetic Mnx clusters (x 4) were considered to explain the high value of the CurieeWeiss constant. Unlike previous studies on similar systems, Goodenough ionic model or Zener polaron model is not suitable. In all cases, cluster behaviour is observed at low field and at low temperature. The influence of the oxygen stoichiometry and the homogeneity of the cation distribution depending on the method of the synthesis used is discussed. Finally, the effects of nickel doping on the magnetic properties were studied and the cluster behaviour was confirmed. The distribution in size of the clusters depends on the amount of nickel and it induces a glassy magnetic behaviour. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Charge ordered manganites; Magnetic transition; Bismuth manganites
1. Introduction Mixed valent manganites T1xDxMnO3 with T trivalent and D divalent elements derived from perovskite structure are studied extensively for their fascinating properties like charge/orbital ordering, colossal magnetoresistance and magnetic phase separation. Among the charge ordered compounds that are often found near half doping x w 0.5 a certain controversy arises. At the so-called TCO temperature at which the charge/orbital ordering occurs does a Goodenough purely ionic model [1] or a Zener polaron model [2] describe the ordered state? From a structural point of view, Pbnm orthorhombic structures are observed above TCO. In contrast, far below TCO powder X-ray synchrotron diffraction pattern shows appearance of satellite peaks in the X-ray patterns [3]. This superstructure was correlated to an ordered distribution of Mn3þ and Mn4þ ions in the lattice which suggests a Goodenough model. However, atomic positions could also suggest a distribution of pairs
* Corresponding author. Tel.: þ33 5 40 00 22 82; fax: þ33 5 40 00 27 61. E-mail address: [email protected] (O. Toulemonde). 1293-2558/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.02.015
of manganese with a mixed valence state of 3.5þ resulting in a so-called Zener polaron order (ZP) [2]. From a magnetic point of view a paramagnetic regime is observed far above TCO. Considering T0.5D0.5MnO3 with an ionic distribution of Mn3þ (S ¼ 3/2) and Mn4þ (S ¼ 2) ions, the effective paramagnetic moment mionic is estimated to eff 4.42 mB for a spin only value. If a ZP pair model is considered, an eg electron is shared by two Mn to form the MneOeMn unit called polaron. The (teg)3 core electrons of the polaron are parallel reflecting a local double exchange phenomena. It results in a total spin value ST ¼ S1 þ S2 ¼ 7/2 where S1 ¼ 1/2 is the spin value of the eg electron ferromagnetically coupled to the core spins with S2 ¼ 2 3/2. Assuming that the angular moment has been quenched the effective paramagnetic moment for a ZP pair is then mZP eff ¼ 7.94 mB. This analysis was recently used to interpret anomalies at the charge order temperature on the magnetic susceptibility of Y0.5Ca0.5MnO3 compound which shows a magnetic transition from ionic paramagnetic to pairs paramagnetic entities [2]. For Bi0.75Sr0.25MnO3 compound a change in meff at TCO was also explained by the presence of ZP pairs [4]. Such magnetic anomalies were already observed in Bi1/2(Ca1ySry)1/2MnO3 series (0 < y < 1) but not discussed [5]. It consequently becomes interesting to extract
O. Toulemonde et al. / Solid State Sciences 10 (2008) 476e480
meff values from magnetic measurements in the new series Bi0.5Ca0.5Mn1xNixO3 (0 x 0.10) to test the validity of the different models. For the first time, the consequences of the synthesis method on the magnetic properties are presented and allow us to conclude that the cluster transition observed at low temperature is depending on both the cationic distribution on the perovskite A site and the oxygen stoichiometry. 2. Experimental 2.1. Powder synthesis The samples Bi0.5Ca0.5Mn1xNixO3d (x ¼ 0, 0.03, 0.5 and 0.10) were prepared by solid state route using stoichiometric amounts of CaCO3, MnO2, NiO and Bi2O3 (CaCO3 was stored in an oven at 250 C to avoid its hydration). The samples were ground under acetone in an agate mortar and annealed for 8 h at 900 C, then heated for 8 h at 1100 C twice with intermediate grinding. All samples were fired in air except Bi0.5Ca0.5Mn0.9Ni0.1O3 which was fired under oxygen flow. The samples Bi0.5Ca0.5Mn1xNixO3d (x ¼ 0, 0.015) were prepared by a liquid route. Stoichiometric amounts of Bi2O3, CaCO3, Mn(CH3COO)2$4H2O and Ni(NO3)2$6H2O were dissolved in concentrated nitric acid. The nitric acid was evaporated on a hotplate and the residue was heated with a Bunsen burner until it turned black. The remains were heated in the furnace at 500 C for 5 h under oxygen flow to remove any remaining nitric gases. Thereafter the samples were annealed at 900 C for 8 h and fired twice at 1100 C for 8 h in air with intermediate grinding. Bi0.5Ca0.5MnO3d was synthesised using the Pechini method [6]. Stoichiometric amount of Bi2O3, CaCO3 and Mn(CH3COO)2$4H2O was dissolved in concentrated nitric acid. Thereafter citric acid and ethylene glycol were P added with the molar ratio ncitric acid ¼ nethylene glycol ¼ 5 nreactant. The liquid was evaporated on a hotplate and the residue was heated with a Bunsen burner. The remains were heated in an oven at 300 C for 5 h to remove nitric gasses and CO2 produced during the esterification. The sample was thereafter annealed at 800 C for 8 h under oxygen flux. The resulting mixture was reground, pressed into pellets and sintered at 1100 C for 8 h under oxygen flux. For clarity, the polycrystalline samples of Bi0.5Ca0.5MnO3d prepared by conventional ceramic route, autocombustion reaction and ‘‘Pechini’’ reaction will be denoted as BiCaMn-S, BiCaMn-L, BiCaMn-P, respectively.
477
1000 Oe. The samples were first heated to 350 K and then cooled to 5 K in zero field at 5 K/min before the magnetic field was applied. Measurements using field cool cooling have also been already performed. 3. Results 3.1. Bi0.5Ca0.5MnO3d compounds Fig. 1 shows the room temperature XRD patterns for sample BiCaMn-P. The powder patterns were indexed using an orthorhombic cell within the space group Pbnm. It leads to the lattice parameters a ¼ 5.4163(2), b ¼ 5.4692(2) and ˚ . Almost the same cell parameters were c ¼ 7.5333(2) A obtained for BiCaMn-S and BiCaMn-L samples within the standard uncertainties. Such cell parameters are not expected around room temperature since the charge ordering is already established as seen in Fig. 2 that shows a discontinuity of the magnetic susceptibility measurements around 330 K. As previously discussed by Giot et al. [7], a doubling of the b cell parameter is observed below TCO. However, this doubling is not clearly evidenced for all crystallites. At room temperature, an average structure in between ordered (at low temperature) and disordered (at high temperature) structures would be observed. X-ray diffraction measurements far below TCO are needed to get better accuracy of the cell symmetry. As explained, the magnetic anomalies around room temperature are related to the charge ordering temperature. On the inverse magnetic susceptibility versus temperature curve (Fig. 3), when temperature decreases, they are followed up by a linear domain characteristic of a paramagnetic regime at lower temperature. A CurieeWeiss analysis of such linear domain (c ¼ C/(T qCW)) allows the determination of the critical paramagnetic temperature qCW and the Curie constant C as shown in the inset of Fig. 3. The decrease of qCW indicates an increase of the antiferromagnetic interactions. Such behaviour was already observed when qCW goes from 30 K to 10 K when Mn3þ/Mn4þ ratio decreases in the series
2.2. Characterisations High resolution X-ray diffraction. The instrument used was an X’Pert MPD PANalytical diffractometer with q 2q BraggeBrentano geometry with a primary monochromator and Cu Ka1 radiation (45 kV, 40 mA). The divergence and anti-scatter slits were both equal to 1 . Magnetic properties. Magnetisation measurements were made with a superconducting quantum interference device SQUID (MPMS, quantum design) using a dc current under
Fig. 1. Observed (crosses) and indexed (solid lines) XRD patterns for BiCaMn-P. The row of the reflection markers shows the positions of the allowed reflections for the space group Pbnm.
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Fig. 2. Magnetic susceptibility of BiCaMn-P compounds deduced from ‘‘zero field cooled warming’’ and ‘‘field cooled cooling’’ measurements under low and high applied magnetic fields. For clarity, the high field measurement was shifted up by 2.25 103 emu/mol.
Bi1xCaxMnO3 (0.4 x 0.6) [7]. Since the estimated qCW is the lowest in the case of BiCaMn-P compound and since an optimised oxidising process (lowest annealed and sintered temperature, oxygen flux) was indeed used for this compound, one can conclude that the antiferromagnetic interactions are weakened when oxygen content decreases. If one now considers the Curie constant C, it increases from BiCaMn-L to BiCaMn-P. The ideal Curie constant for an ionic model is given by C ¼ N m2eff/3kB (N is the number of magnetic entities which is equal in the ionic case to the Avogadro number NA, mionic eff ¼ 4.42 mB and kB is the Boltzman constant) and is estimated to be about 2.44 emu K/mol. The ideal Curie constant for ZP pairs model is estimated to be about 3.94 emu K/mol (N ¼ NA/2, mZP eff ¼ 7.94 mB). The experimental values of C are much larger than the theoretically predicted ones. It suggests the existence of a stronger coupling between
Fig. 3. Temperature dependence of the inverse magnetic susceptibility of the samples Bi0.5Ca0.5MnO3d depicted as BiCaMn-P, BiCaMn-S, BiCaMn-L (see text). The results of the CurieeWeiss analysis (c ¼ C/(T qCW)) obtained in between 160 K and 230 K in the temperature range for which the first derivative is constant are given in the inset.
manganese within larger clusters than pairs. If Mn4 clusters are now considered with a total spin value of ST ¼ 7 and assuming that the angular moment is quenched, an effective paramag4 netic moment mMn ¼ 14:97 mB is expected for a Curie eff constant C ¼ 7 emu K/mol. In contrast to the previous predictions the experimental value of 6.80 emu K/mol for BiCaMn-P which again may present the highest oxygen content, is now close to the theoretically expected value for Mn4 cluster model. In fact, on one hand recent detailed analysis of Bi0.6Ca0.4MnO3 crystals suggests that the ordered state consists of alternating layers of two kinds of MnO6 octahedra in an Mn(1)eMn(2)eMn(1)eMn(2) sequence [7]. On the other hand, in the Bi0.75Sr0.25MnO3 compound, an ordered intergrowth of double stripes of antiferromagnetic orbital ordering LaMnO3-type and ferromagnetic orbital ordering YBaMn2O6type is observed [8]. The hypothesis is that Mn4 clusters may also originate from an intergrowth mode. Finally, the existence of large magnetic clusters was already reported in manganites [9]. Now let us discuss the sensitivity of the Mn4 cluster model to the oxygen stoichiometry. Indeed, for Bi0.5Ca0.5MnO3d the oxygen off-stoichiometry d will drastically enhance the number of free Mn3þ ions and consequently the Curie constant will vary for example from 7 to 4.33 emu K/mol for d ¼ 0 to d ¼ 0.05, respectively. Preliminary thermogravimetric and Inductively Coupled Plasma analysis performed on BiCaMnS estimate d to be 0.023 and Bi/Ca ratio equals close to 1, respectively. It results on a mixed valence state of 3.44þ for manganese to respect the charge balance. One mole of hypothetical Bi0.55Ca0.45MnO3 will give 0.45/4 ¼ 0.1125 mole of Mn4 clusters and 0.1 mol of free Mn3þ ions. As a result, a theoretical CurieeWeiss constant of 5.12 emu K/mol can be calculated which is close to the experimental value for the BiCaMn-S compound. Consequently, a lower content of oxygen will increase the number of free Mn3þ ions and decrease the CurieeWeiss constant. Based on the Goff model [8], a loss of the oxygen stoichiometry would induce disorder in the intergrowth of the distinctive blocks. Several experiments such as X-ray Photoelectron Spectroscopy, microprobe, ICP and thermogravimetric analysis are under progress to determine the Bi/Ca ratio and the d value for BiCaMn-L and BiCaMnP samples. The magnetic anomalies observed at 130 K are ascribed to the Ne´el temperatures (TN) as shown in Fig. 3. For the three compounds no significant evolution of TN is evidenced as already observed for TCO. Our results are in the range of the already published Ne´el temperature in between 120 K and 150 K for Bi1xCaxMnO3 (0.4 x 0.6) samples [7,10]. Finally, the minima at low temperature correspond to cluster transitions. They are displaced to lower temperature for higher applied magnetic field until vanishing under 5 T (Fig. 2). It is a clear indication that they are not due to a true magnetic phase transition. At low applied magnetic field the net difference in field cooled and zero field cooled measurements increases significantly at the cusp (Fig. 2). The cluster transition is less pronounced for BiCaMn-L than for BiCaMn-P that it is likely due to the change in oxygen stoichiometry. In fact, such magnetic anomalies and the existence of
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ferromagnetically ordered clusters were already pointed out when Bi0.6Ca0.4MnO3 thin films were deposited at a reduced O2 partial pressure [11]. In addition, the cluster behaviour could also come from an inhomogeneous distribution of the perovskite A site cations. Macroscopic Bi3þ rich region may enhance ferromagnetic domains in contrast to macroscopic Ca2þ rich region which would enhance antiferromagnetic domains. The distribution of the Bi3þ and Ca2þ perovskite A cations is in fact improved using a Pechini method. Both variations of the amplitude of cluster anomalies and of the critical paramagnetic temperature qCW suggest the existence of ferromagnetic coupling between clusters. It reveals that the long-range antiferromagnetic ordering in between clusters is decreased by the free Mn3þ ions and the macroscopic inhomogeneities of the cationic distribution.
3.2. Bi0.5Ca0.5Mn1xNixO3d When nickel is introduced on the perovskite B site of the BiCaMn-S compounds, the charge ordering state is expected to become unstable [12]. First, Fig. 4 shows the cell parameter evolution with Ni2þ ion content at room temperature for powder patterns indexed using an orthorhombic cell. When Ni2þ content increases, the convergence of the cell parameters suggests a continuous melting of the charge ordering. However this convergence is not associated with a structural transition from orthorhombic to tetragonal unit cell in contrast to what was proposed by Xiong et al. [13]. Indeed, in our case some peaks in the X-ray diffraction pattern cannot be indexed using a tetragonal setting. The melting of the charge ordering by nickel doping is further supported by the magnetisation measurements. First, a continuous reduction of TCO from w330 K for BiCaMn-S to w260 K for Bi0.5Ca0.5Mn0.9Ni0.1O3 is observed in Fig. 5. The almost linear decrease of TCO as a function of doping level agrees with the results obtained with chromium doping of the same compound [13]. However, TCO decreases for nickel doping by about 6 K per % Ni, whereas 1% of Cr doping decreases TCO by about 8 K. In fact Cr3þ and Ni2þ
Fig. 4. Lattice parameters as a function of nickel content obtained at room temperature. Solid lines are guides for the eyes.
Fig. 5. Temperature dependence of the magnetic susceptibility of the samples Bi0.5Ca0.5Mn1xNixO3d. The results of the CurieeWeiss analysis (c ¼ C/ (T qCW)) obtained below TCO are given in the inset.
destroy the charge ordering by two different mechanisms on Ln0.5Ca0.5MnO3 [14,15]. Second, it can be seen in Fig. 5 that the cluster behaviour observed in the Bi0.5Ca0.5MnO3 compounds vanishes whereas a broader glass transition appears at slightly higher temperature. The glass-like transition is evidenced by the difference between field cooled and zero field cooled measurements (not shown). The inter-clusters Mnx (x 4) magnetic exchange is essentially antiferromagnetic in the undoped phase as shown by the negative sign of qCW. In contrast ferromagnetic superexchange Niþ2eO(2p)2eMn4þ is observed in the doped phases which is enhanced with doping. qCW increases and even becomes positive when Ni content increases (inset Fig. 5). It confirms previous X-ray magnetic circular dichroism study showing an increase of the XMCD signal with chromium doping [15]. In the doped compounds, the cluster glass transition is consequently due to a change of the nature of the inter-cluster magnetic exchange interactions. From the linear parts of the inverse magnetisation graph the CurieeWeiss constants of the samples were estimated. Both theoretical and experimental values are presented in the inset of Fig. 5. The experimental values for Curie constants decrease continuously and are higher than the theoretically expected values considering an ionic model. Note that the temperature range of the paramagnetic behaviour increases with Ni doping. It supports a disordered insertion of the nickel ions on the perovskite B site. Secondly, the anomaly on the magnetic susceptibility at TCO becomes nearly suppressed for Bi0.5Ca0.5Mn0.9Ni0.1O3d. A magnetic phase segregation is then observed below TCO when nickel cations are introduced. However, the Curie Constants are still large compared to the ionic model reflecting the existence of magnetic clusters MnxNiy around Ni2þ. It indicates that no long-range ferromagnetic ordering can be induced by doping the Bi0.5Ca0.5MnO3d compound in contrast to Ln0.5Ca0.5MnO3d (Ln ¼ rare earth) [12]. Indeed, our results suggest a competition between ferromagnetic and antiferromagnetic inter-cluster interaction and support the intergrowth block mode which is destroyed by the doping giving a glassy magnetic behaviour. The intrinsic
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nature of Bi0.5Ca0.5MnO3d compound avoids the opportunity to induce long-range ferromagnetism by doping.
size of the clusters magnetic time relaxation measurements are needed.
4. Concluding remarks
References
Bi0.5Ca0.5Mn1xNixO3d compounds were synthesised following different methods in order to point out the literature incoherencies in the series Bi0.5Ca0.5MnO3d and to test the validity of the Zener polaron pairs theory. X-ray diffraction powder patterns collected at room temperature were indexed using an average orthorhombic cell. A preliminary Curiee Weiss analysis of the paramagnetic regime that is observed in between TN w 130 K and TCO w 330 K was performed. Both Curie constant and qCW parameters behave linearly with the relative expected oxygen content which depends on the way of synthesis. Disordered Mn4 clusters are proposed to explain the paramagnetic domain. A competition between ferromagnetic and antiferromagnetic inter-cluster interactions is clearly established in Bi0.5Ca0.5MnO3d compounds depending on the distribution of the A site cations and the oxygen stoichiometry. Our work supports an intergrowth distinctive block mode as proposed by Goff and Attfield [8]. When nickel is introduced at the perovskite B site, a convergence of the cell parameters corroborated with DC magnetic susceptibility measurements suggests a continuous and disordered melting of the charge ordering. In contrast to Ln0.5Ca0.5MnO3d (Ln ¼ rare earth) no long-range ferromagnetic ordering is observed when nickel doping is introduced but glassy magnetic behaviour of MnxNiy clusters is supported. In order to obtain the (in)homogeneity in
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