New Magnetic Transitions in the Ordered Oxygen-Deficient Perovskite

shows that a small deviation from the “O5.50” stoichiometry influences dramatically the magnetic ... the existence, in some of them, of several ma...
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New Magnetic Transitions in the Ordered Oxygen-Deficient Perovskite LnBaCo2O5.50+δ Md. Motin Seikh,† Ch. Simon, V. Caignaert, V. Pralong, M. B. Lepetit, S. Boudin, and B. Raveau* CRISMAT, ENSICAEN-CNRS UMR 6508, 6bd. Marechal Juin, 14050 Caen, France ReceiVed September 18, 2007. ReVised Manuscript ReceiVed October 24, 2007

The investigation of the ordered oxygen-deficient perovskites LnBaCo2O5.50+δ with Ln ) Eu and Sm shows that a small deviation from the “O5.50” stoichiometry influences dramatically the magnetic properties of this structural type. Three ferromagnetic (FM) states can be obtained, depending on the oxygen content, δ. For a cobalt valency close to +3 (δ ∼ 0) a ferromagnetic ordering FM1 takes place around 250 K, previously observed for Ln ) Gd, Tb cobaltites. For δ < -0.1 corresponding to the mixed valency Co2+/Co3+ a new ferromagnetic state FM2 is observed which can be interpreted either in the frame of phase separation scenario or as a ferrimagnetic state, Co2+ and Co3+ species sitting in different sites. For the mixed valency Co3+/Co4+ (δ > +0.10) a new ferromagnetic state FM3 appears, involving Co3+-O-Co4+ superexchange interactions.

I. Introduction The discovery of an exceptionally high magneto-resistance, associated with a metal-to-insulator transition around room temperature, has attracted the attention of researchers working in the field of strongly correlated electrons systems on the 112 ordered oxygen-deficient perovskites: LnBaCo2O5.50, with Ln ) Eu, Gd.1 These most intriguing magneto-transport properties were soon confirmed by Troyanchuk et al.2 Following, the fascinating behavior of these compounds was explored, taking into consideration the possibility of spin * Corresponding author: e-mail [email protected]; Fax +33-231951600. † Present address: Department of Chemistry, Visva-Bharati University, Santiniketan–731235, West Bengal, India.

(1) Martin, C.; Maignan, A.; Pelloquin, D.; Nguyen, N.; Raveau, B. Appl. Phys. Lett. 1997, 71, 1421. (2) Troyanchuk, I. O.; Kasper, N. V.; Khalyavin, D. D.; Szymczak, H.; Szymczak, R.; Baran, M. Phys. ReV. Lett. 1998, 80, 3380. (3) Maignan, A.; Martin, C.; Pelloquin, D.; Nguyen, N.; Raveau, B. J. Solid State Chem. 1999, 142, 247. (4) Respaud, M.; Frontera, C.; García-Muñoz, J. L.; Aranda, M. A. G.; Raquet, B.; Broto, J. M.; Rakoto, H.; Goiran, M.; Llobet, A.; Rodríguez-Carvajal, J. Phys. ReV. B 2001, 64, 214401. (5) Taskin, A. A.; Lavrov, A. N.; Ando, Y. Phys. ReV. Lett. 2003, 90, 227201. (6) Taskin, A. A.; Lavrov, A. N.; Ando, Y. Phys. ReV. B 2005, 71, 134414. (7) Zhou, Z. X.; Schlottmann, P. Phys. ReV. B 2005, 71, 174401. (8) Baran, M.; Gatalskaya, V. I.; Szymczak, R.; Shiryaev, S. V.; Barilo, S. N.; Bychkov, G. L.; Szymczak, H. J. Phys.: Condens. Matter 2005, 17, 5613. (9) Khalyavin, D. D.; Barilo, S. N.; Shiryaev, S. V.; Bychkov, G. L.; Troyanchuk, I. O.; Furrer, A.; Allenspach, P.; Szymczak, H.; Szymczak, R. Phys. ReV. B 2003, 67, 214421. (10) Soda, M.; Yasui, Y.; Fujita, T.; Miyashita, T.; Sato, M.; Kakurai, K. J. Phys. Soc. Jpn. 2003, 72, 1729. (11) Soda, M.; Yasui, Y.; Ito, M.; Iikubo, S.; Sato, M.; Kakurai, K. J. Phys. Soc. Jpn. 2004, 73, 464. (12) Chernenkov, Y. P.; Plakhty, V. P.; Fedorov, V. I.; Barilo, S. N.; Shiryaev, S. V.; Bychkov, G. L. Phys. ReV. B 2005, 71, 184105. (13) Maignan, A.; Caignaert, V.; Raveau, B.; Khomskii, D.; Sawatzky, G. Phys. ReV. Lett. 2004, 93, 26401. (14) Zhou, H. D.; Goodenough, J. B. J. Solid State Chem. 2004, 177, 3339. (15) Roy, S.; Dubenko, I. S.; Khan, M.; Condon, E. M.; Craig, J.; Ali, N.; Liu, W.; Mitchell, B. S. Phys. ReV. B 2005, 71, 024419.

transition, charge and orbital ordering phenomena, and phase separation.3–18 The crystal structure of the LnBaCo2O5.50 oxides was first determined by X-ray diffraction and electron microscopy.3 It was later confirmed and refined for all the Ln members of the series, using neutron diffraction.16,19–23 These compounds can be described (see Figure 1) as ordered oxygen-deficient perovskites, characterized by 1:1 ordering of the Ba2+ and Ln3+ cations in the form of alternating planes. Oxygen vacancies are located at the level of the “Ln3+” layers and distributed in an ordered way. As a consequence, the ideal crystallographic description consists of layers of CoO6 octahedra along the (a,c)planes. These layers are interconnected by two-leg ladders—along the a-direction of CoO5 pyramids. In between these ladders, the six-sided tunnels are occupied by Ln3+ cations. One of the most intriguing properties of these systems is the existence, in some of them, of several magnetic transitions in the insulating phase. For instance, a clear appearance of a ferromagnetic moment between 260 and 300 K has been observed in the GdBaCo2O5.50 compound.6 Let us note that the metal–insulator transition in this system is at TIM ) 360 K. One thus observes with decreasing temperature successive (16) Moritomo, Y.; Akimoto, T.; Takeo, M.; Machida, A.; Nishibori, E.; Takata, M.; Sakata, M.; Ohoyama, K.; Nakamura, A. Phys. ReV. B 2000, 61, R13325. (17) Burley, J. C.; Mitchell, J. F.; Short, S.; Miller, D.; Tang, Y. J. Solid State Chem. 2003, 170, 339. (18) Plakhty, V. P.; Chernenkov, Y. P.; Barilo, S. N.; Podlesnyak, A.; Pomjakushina, E.; Moskvin, E. V.; Gavrilov, S. V. Phys. ReV. B 2005, 71, 214407. (19) Kusuya, H.; Machida, A.; Moritomo, Y.; Kato, K.; Nishibori, E.; Takata, M.; Sakata, M.; Nakamura, A. J. Phys. Soc. Jpn. 2001, 70, 3577. (20) Frontera, C.; Garcia-Munoz, J. L.; Llobet, A.; Aranda, M. A. G. Phys. ReV. B 2002, 65, R180405. (21) Roy, S.; Khan, M.; Guo, Y. Q.; Craig, J.; Ali, N. Phys. ReV. B 2002, 65, 64437. (22) Akahoshi, D.; Ueda, Y. J. Solid State Chem. 2001, 156, 355. (23) Fauth, F.; Suard, E.; Caignaert, V.; Mirebeau, I. Phys. ReV. B 2002, 66, 184421.

10.1021/cm7026652 CCC: $40.75  2008 American Chemical Society Published on Web 12/05/2007

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In order to better understand these complex materials, we have systematically investigated the magnetic and transport properties as a function of the oxygen content for two series of LnBaCo2O5.50+δ oxides that were the object of very few studies so far,7,27 namely the Ln ) Sm and Eu. We have observed two new ferromagnetic states, namely FM2 and FM3, for δ < -0.10 and δ > 0.10, respectively, which are not observed in the Gd system.6 II. Experimental Details

Figure 1. Structure of LnBaCo2O5.50 with layers of CoO6 octahedra interconnected with rows of CoO5 pyramids indicating the doubling of unit cell along b- and c-axes.

magnetic transitions usually interpreted as paramagneticferromagnetic-antiferromagnetic (PM-FM-AF) transitions. This particular behavior raised a large controversy between authors,1,3,6,12,16,18,23–25 and many different models of the magnetic structure for these phases were proposed in the literature. The controversy was partly due to the fact that the physical properties of these systems are strongly dependent on the exact oxygen stoichiometry. Beside the ideal “O5.50” stoichiometry involving only Co3+ species, a tiny variation of the oxygen content may induce dramatic variations in the magnetic properties.17,21,26 In fact, these materials are very sensitive to the method of synthesis (oxygen pressure, temperature, etc.), leading to a more general formula LnBaCo2O5.50+δ. In addition, the oxygen content does also depend on the size of the Ln3+ cations.3,15 It is thus quite difficult to understand the influence of the different factors on the observed magneto-transport properties: structural distortion, cobalt valency, spin state of cobalt ions, etc. Unfortunately, up to now very few systematic studies concerning the effects of the oxygen content were conducted. In fact, the GdBaCo2O5.50+δ system is the only one for which a magnetic phase diagram has been established versus the oxygen content.6 The existence of a nanoscopic phase separation was seen at low temperature; for δ < 0 between two antiferromagnetic insulating phases (AFI) and for δ > 0 between an antiferromagnetic phase (AFI) and a ferromagnetic metallic phase (FMM). For δ ) 0 the abovementioned insulating antiferromagnetic and insulating ferromagnetic phases were observed. (24) Flawell, W. R.; Thomas, A. G.; Tsoutsou, D.; Mallick, A. K.; North, M.; Seddon, E. A.; Cacho, C.; Malins, A. E. R.; Patel, S.; Stockbauer, R. L.; Kurtz, R. L.; Sprunger, P. T.; Barilo, S. N.; Shiryaev, S. V.; Bychkov, G. L. Phys. ReV. B 2004, 70, 224427. (25) Conder, K.; Pomjakushina, E.; Pomjakustin, V.; Stingaciu, M.; Streule, S.; Podlesnyak, A. J. Phys.: Condens. Matter 2005, 17, 5813. (26) Kim, W. S.; Chi, E. O.; Choi, H. S.; Hur, N. H.; Oh, S. J.; Ri, H. C. Solid State Commun. 2000, 116, 609.

Polycrystalline samples of LnBaCo2O5.5+δ (Ln ) Sm and Eu) were prepared by the standard ceramic method. Ln2O3 was dried at 900 °C prior to use, and BaCO3 and Co3O4 were used as received. The appropriate proportions of the staring materials were weighed and thoroughly mixed by mortar pestle, adding ethanol for homogeneous mixing. The precursor was initially fired at 900 °C for 24 h for decarbonation, followed by successive heatings at 1000 and 1100 °C with intermediate grindings in air. The process was continued until there were no further changes and traces of the starting materials in the X-ray powder diffraction patterns. In the final sintering step, the samples were prepared in the bar shape, and the cooling rate was kept as low as 0.5 °C /min. We call them the slowly cooled samples. The phase purity of the samples was determined by X-ray diffraction (XRD) patterns recorded by using a PHILIPS X’pertPro diffractometer with Cu KR radiation. The estimation of cationic ratio Ln:Ba:Co 1:1:2 was carried out by the energy-dispersive analysis (EDAX) using a Kevex analyzer mounted on a JEOL 200CX electron microscope. The oxygen content was determined by iodometric titration performed in argon atmosphere within an accuracy of (0.02 oxygen per formula LnBaCo2O5.50+δ. On the basis of these analyses, the determined stoichiometry of the slowly cooled compounds are SmBaCo2O5.53 and EuBaCo2O5.52. The oxygen content of this family of materials is dependent on the synthesis conditions such as cooling rate and atmosphere. We have treated the slowly cooled samples at various conditions to vary the oxygen content. Though the change in oxygen stoichiometry is reversible in these cobaltites, we used different bars, treated under different conditions, to get different oxygen content samples. The annealing of the samples at 400 °C in the flow of argon for 12 h results to the oxygen stoichiometry SmBaCo2O5.31 and EuBaCo2O5.33. The quenching of the samples from 1000 °C in air leads to the compositions SmBaCo2O5.49 and EuBaCo2O5.48. To achieve higher oxygen content, we have annealed the samples at 600 °C for 12 h in an oxygen atmosphere at 1 and 100 bar pressure. The 1 bar oxygen treated samples have the compositions SmBaCo2O5.59 and EuBaCo2O5.57. The composition of the 100 bar oxygen annealed samples is LnBaCo2O5.65 (Ln ) Sm and Eu). Differential scanning calorimetry (DSC) was carried out using a TA 2920 instrument with a heat flow of 10 °C /min. The transport properties were measured by standard four-probe method using a physical property measuring system (PPMS) within the temperature range of 5-400 K. The magnetic measurements were carried out with a SQUID magnetometer (MPMS, Quantum Design). For each sample the zero-field-cooled (ZFC) and field-cooled (FC) data were collected with an applied field of 100 Oe. The magnetization vs applied field (M-H) measurements were performed within the applied field of (5 T. (27) Zhou, Z. X.; McCall, S.; Alexander, C. S.; Crow, J. E.; Schlottmann, P.; Barilo, S. N.; Shiryaev, S. V.; Bychkov, G. L.; Guertin, R. P. Phys. ReV. B 2004, 70, 24425.

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Figure 2. (a) Variation of subcell parameters and subcell volume as a function of oxygen content δ in LnBaCo2O5.5+δ (Ln ) Sm and Eu). The open (solid) symbols correspond to Sm (Eu). (b) DSC measurement of EuBaCo2O5.33. The sign of the heat flow values have been changed for the endothermic curve.

III. Results and Discussion A. Structural Characterization of LnBaCo2O5.5+δ (Ln ) Sm and Eu). All the XRD patterns for both the Eu and Sm compounds can be indexed in an orthorhombic Pmmm space group. The doubling of the cell parameters takes place along the b and c directions, giving rise to the so-called ap × 2ap × 2ap unit cell, characteristic of the “112” type structure (ap is the perovskite cell parameter3). The doubling along the c direction is clearly observed for all compositions, whereas along the b direction it appears clearly only for -0.05 < δ < 0.1. There are significant changes in the lattice parameters with the variation in oxygen content, as shown in Figure 2a. The a and b parameters show little contraction with the increase in oxygen content, whereas c is increased in both Sm and Eu series. However, as expected, the cell volume decreases with the increase in oxygen content. The DSC measurements show a discontinuity in the heat flow at TIM. This discontinuity is typical for this family of compounds and suggests the occurrence of a structural change at TIM. Figure 2b displays a typical set of DSC data for the δ ) -0.17 composition of Eu systems. B. Air-Synthesized Trivalent Cobaltites LnBaCo2O5.5+δ: -0.02 e δ e 0.03. The magnetization curves, registered in an applied field of 100 Oe, are very similar for the Eu phases O5.48 (Figure 3a) and O5.52 (Figure 3b) and for the Sm phases O5.49 (Figure 3c) and O5.53 (Figure 3d), in agreement with the results previously observed for the Gd phases for similar oxygen contents, i.e., O5.45 to O5.50.6 Indeed, one observes with decreasing temperature a magnetization peak centered at ∼250–240 K, which corresponds to the appearance of a ferromagnetic moment at ∼280–260 K and its disappearance at ∼220–200 K. These results confirm the existence of successive paramagnetic-ferromagnetic-antiferromagnetic

Figure 3. M(T) curves measured at 100 Oe for (a) EuBaCo2O5.48, (b) EuBaCo2O5.52, (c) SmBaCo2O5.49, and (d) SmBaCo2O5.53. The solid (open) symbols correspond to ZFC (FC) data. The corresponding right panels show H-T diagram generated from M(H) curves for each composition. C-AF in right panels represents canted-antiferromagnetic phase.

(PM-FM-AF) transitions below 280–300 K, as previously observed for GdBaCo2O5.5+δ.6 We also confirm that this ferromagnetic state, which will be further labeled FM1, takes place in a rather narrow temperature window, i.e., smaller than ∼60 K. The complex nature deals with the fact that the FM-AF transition tends to take place in multiple steps, which can be visualized clearly from the double humps, as shown for example for SmBaCo2O5.53 (Figure 3d). Since it has been observed by many authors in different samples, with different lanthanides and even in small single crystals, this multistep transition can be considered as a very stable effect. The M(H) curves registered at selective temperatures for LnBaCo2O5.5+δ (-0.02 e δ e 0.03) are shown in Figure 4. Figure 4a shows the M(H) curves at 5 K of EuBaCo2O5.48, revealing a typical feature of an antiferromagnetic material as seen for other members of this series at δ ∼ 0. In contrast, at 150 K (Figure 4b) and 200 K (Figure 4c), hysteresis loops with a particular shape, suggesting the steplike behavior, are observed. Similar hysteretic behavior is also observed for EuBaCo2O5.52 (Figure 4d), SmBaCo2O5.49 (Figure 4e), and SmBaCo2O5.53 (Figure 4f) in the temperature range of 150–250 K. Such a steplike feature in the M(H) curves indicates the existence of a temperature-dependent spontaneous magnetization at zero field as well as at some critical field. Above 250 K the steplike behavior of the M(H) curves ceases to exist, and finally above TC, e.g. 300 K, the system

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Figure 5. Magnetization as a function of temperature measured at 100 Oe applied field for (a) EuBaCo2O5.33 and (b) SmBaCo2O5.31. The solid (open) symbols correspond to ZFC (FC) data. Figure 4. M(H) curves of LnBaCo2O5.5+δ at a few selected temperatures. Parts b-f show the steplike feature of the M(H) curve whose origins are explained in the text.

exhibits a paramagnetic behavior. The origin of such steps in the M(H) curves could be associated either with the reversal of the canted-antiferromagnetic (C-AF) phase at a critical field or the domain wall motion of ferromagnetic phase. Both could be the possible reason. In the right panels of Figure 3a-d we plotted the H-T diagram by extracting data from the M-H curve for both Eu and Sm phases. This diagram shows that below TC to 250 K there is only one spontaneous magnetization at H ) 0. In this temperature range the phase is mainly ferromagnetic. Below 250 K the H-T curve diverges symmetrically on both sides of the H ) 0 line. The region below H ) 0 line consists of C-AF phase, as shown in the figures. Finally the system becomes antiferromagnetic. C. Deviation from the O5.50 Stoichiometry: Generation of Two New Ferromagnetic States. (i) Argon-Annealed Co2+/Co3+ Cobaltites LnBaCo2O5.5+δ, δ ) -0.19 and -0.17. For the argon-annealed samples, a large oxygen deficiency is observed, leading to the formulation EuBaCo2O5.33 and SmBaCo2O5.31, respectively. For both compounds the M(T) curves (Figure 5) show, as the temperature is decreased, a steep increase of magnetization around 290 K followed by a small kink at ∼280 K. The FC curve of EuBaCo2O5.33 (Figure 5a) suggests a paramagnetic to ferromagnetic transition, whereas SmBaCo2O5.31 exhibits also a ferromagnetic component, but one observes a broad hump (Figure 5b) suggesting a strong competition between ferromagnetic and antiferromagnetic interactions. The kink

at 280 K could be associated with an antiferromagnetic ordering, as observed for δ ∼ 0 compositions. Moreover, the ZFC data show a broad antiferromagnetic transition expanding from 280 K to the lowest temperature. It must also be emphasized that though the system becomes ferromagnetic at lower temperature the moment is very small in magnitude (Figure 5a,b). In order to check the appearance of ferromagnetism in these materials, M(H) curves were measured at different temperatures. In the case of EuBaCo2O5.33 a large hysteresis loop is observed at 2 K (Figure 6a), whereas for SmBaCo2O5.31, the loop is rather narrower at 5 K (Figure 6c). The hysteretic behavior persists up to 200 K (see Figure 6b,d), though the coercive field decreases with the increase in temperature. From 200 to 290 K though it shows ferromagnetic like M(H) curves (not shown here), the coercive field is vanishingly smaller, and above 290 K the oxides become paramagnetic. Thus, these results clearly show that a new ferromagnetic phase labeled FM2 is generated with TC ) 290 K, independently on the nature of the lanthanide (Eu3+ or Sm3+). This new type of ferromagnetism FM2, observed for the first time at low temperature in the “112” cobaltites, can be explained by the mixed valence Co3+/Co2+, since a Co3+:Co2+ ratio 83:17 (81:19) is observed for Eu (Sm) phases. In order to explain the low value of the magnetic moment, different hypotheses can be considered. The first one deals with the fact that the majority of the Co3+ species would be in the low-spin configuration (S ) 0) at low temperature. However, recent optical study in Eu system reveals that such low-spin

Oxygen-Deficient PeroVskite LnBaCo2O5.50+δ

Figure 6. M(H) curves of EuBaCo2O5.33 at (a) 2 and (b) 200 K, whereas (c) and (d) show the same curve of SmBaCo2O5.31 at 5 and 200 K, respectively.

configuration of Co3+ is unlikely.28 A second hypothesis is that the resulting small total magnetization observed at low temperature for the mixed valency Co2+/Co3+ ions could be recognized as ferrimagnetic ordering or mutually orientated moments in the nonequivalent pyramidal and octahedral sites, similar to the proposed ferrimagnetic state of Tb system for δ ) 0 by Plakhty et al.18 A third hypothesis is that the FM2 phase coexists with an antiferromagnetic phase or is segregated in the antiferromagnetic matrix in the form of FM2 domains. This phase separation scenario, also observed for GdBaCo2O5.5+δ,6 is another possibility. In this case, the application of the magnetic field continuously transforms the antiferromagnetic state into ferromagnetism. Such a mechanism also supports the fact that the critical field is strongly temperature dependent and that the virgin curve appears outside the hysteresis loop (Figure 6), suggesting that the coercive field of the ferromagnetic component is smaller than the field of the metamagnetic transition which transforms the antiferromagnetic state into ferromagnetism. The complexity of these mechanisms makes very difficult the direct interpretation of the magnetization curves. In particular, the spectacular decrease of the coercive field above 200 K is very puzzling. (ii) Oxygen-Annealed Co3+/Co4+ Cobaltites, LnBaCo2O5.5+δ, δ ) 0.07 and 0.15. The annealing of the samples in oxygen flow allows the oxygen content to be increased, leading to an excess of oxygen with respect to the ideal O5.50 formula, so that the mixed valence Co3+/Co4+ is involved. The samples annealed at 1 bar oxygen pressure and 600 °C are formulated as SmBaCo2O5.59 and EuBaCo2O5.57, whereas higher oxygen content, i.e., LnBaCo2O5.65 (Ln ) Sm and Eu), is reached by annealing at 100 bar and 600 °C. As soon as the oxygen content is increased beyond O5.50, the FM1 phase tends to disappear. This is illustrated by the M(T) curves of EuBaCo2O5.57 (Figure 7a) which still exhibit peaks around 250 K, very similar to EuBaCo2O5.48 (Figure 3a) and (28) Makhnev, A. A.; Nomerovannaya, L. V.; Tashlykov, A. O.; Barilo, S. N.; Shiryaev, S. V. Phys. Solid State 2007, 49, 894.

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Figure 7. Magnetization curves measured at 100 Oe for (a) EuBaCo2O5.57, (b) EuBaCo2O5.65, (c) SmBaCo2O5.59, and (d) SmBaCo2O5.65. The solid (open) symbols correspond to ZFC (FC) data. The inset in (a) displays an H-T diagram generated from M(H) curves for EuBaCo2O5.57. C-AF in insets represents the canted-antiferromagnetic phase.

EuBaCo2O5.52 (Figure 3b), but whose maximum magnetic moment is 1 order in magnitude smaller than for the latter. In EuBaCo2O5.57, steplike features also appear in the M(H) curves. Like EuBaCo2O5.48 and EuBaCo2O5.52, we have generated the H-T diagram for the EuBaCo2O5.57 composition (inset of Figure 7a), which signifies the existence of the C-AF phase. Further increase of oxygen content leads to the disappearance of the peak at 250 K, as shown for SmBaCo2O5.59 (Figure 7c), which exhibits new PM-FM-AF transitions at much lower temperature, i.e., below 140 K. A similar transition is observed below 130 K for EuBaCo2O5.65 (Figure 7b), whereas this new ferromagnetic state labeled FM3 persists down to the lowest temperature, i.e., 5 K for SmBaCo2O5.65 (Figure 7d). The corresponding M(H) curves registered at different temperatures show that there remains only a small hysteresis loop characteristic of FM1 for EuBaCo2O5.57 at 250 K (Figure 8a). Moreover, the narrow hysteresis loops at around 100 K are observed for EuBaCo2O5.65 (Figure 8b) and SmBaCo2O5.59 (Figure 8c), which correspond to the FM3 state. The FM3 state exists below 140 K in these oxides. It is then definitely confirmed by the hysteresis loop of SmBaCo2O5.65 at 5 K (Figure 8d) which shows a rather large coercive field. These results show that the FM1 state characteristic of “pure Co3+” is completely destroyed as Co4+ is introduced into the matrix. Thus, for a sufficiently high Co3+:Co4+ ratio 85:15 (δ ) 0.15), a new ferromagnetic state FM3 is installed. The FM3 state appears due to Co3+-O-Co4+ superexchange interactions, in agreement with the Goodenough-Kanamori rule.29 It must be emphasized that, though the oxygen stoichiometry (over doping) is a prominent factor for the appearance of the longrange FM3 ordering, it is not the only parameter which influences the FM3 state. One indeed observes that, for a (29) Goodenough, J. B. Magnetism and the Chemical Bond; WileyInterscience: New York, 1963.

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Figure 8. M(H) curves of (a) EuBaCo2O5.57 at 250 K, (b) EuBaCo2O5.65 at 90 and 110 K, (c) SmBaCo2O5.59 at 110 and 120 K, and (d) SmBaCo2O5.65 at 5 K.

same oxygen content, δ ) 0.15, the FM3 state is completely established below 140 K down to 5 K in the Sm phase (Figures 7d and 8d), whereas FM3 corresponds only to a narrow window between 130 and 80 K in the Eu phase (Figures 7b and 8b). Thus, there exists a second factor, the effect of the size of the A-site cations, which influences to a lesser degree the appearance of FM3. In other words, this result suggests that the increase of the size of the Ln3+ cations favors the appearance of the FM3 phase. D. “High-Temperature” Magnetic Properties. The evolution of the inverse susceptibility vs temperature is very similar for both Eu and Sm “112” phases. The χ-1(T) curves of the Eu phase (Figure 9) allow the following features to be emphasized: (i) For the “Co2+/Co3+” or “pure Co3+” cobaltites, the -1 χ (T) curves (Figure 9a-c) exhibit two discontinuities: an abrupt slope change at ∼340 K corresponding to the metal–insulator transition, TIM, and a second rupture at 260–280 K, which corresponds to the appearance of the FM1 state. The discontinuity observed at TIM is often interpreted as a change in the spin state, since there is a decrease in paramagnetic moment below TIM.6,13,19,20 Indeed, the Curie constant, which is directly related to the effective atomic moments, may not produce exact moment in a complex system like the present one. However, many authors devoted their investigations to claim that the TIM is driven by the structural transition rather than spin transition.24,25,28–31 Our present interpretation is at the verge of the latter scenario: the cobalt ions are always in the high-spin state, and the change in the couplings, driven by structural transition, is probably at the origin of this discontinuity. Similar interpretation was proposed in man(30) Wu, H. Phys. ReV. B 2001, 64, 92413. (31) Wu, H. J. Phys.: Condens. Matter 2003, 15, 503. (32) Hidaka, M.; Soejima, M.; Wijesundera, R. P.; Soda, M.; Sato, M.; Choi, S.-H.; Sung, N. E.; Kim, M. G.; Lee, J. M. Phys. Status Solidi B 2006, 243, 1813.

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Figure 9. Inverse susceptibility (χ-1) as a function of temperature for EuBaCo2O5.50+δ at different δ values. The solid line in the curve represents the Curie–Weiss fit to the data in the high-temperature range.

ganites33 in which a structural dimerization creates such a discontinuity. In the present case, the observed doubling of the cell along the c axis could be sufficient. (ii) For the “Co3+/Co4+” cobaltites, both transitions tend to disappear (Figure 9d,e). (iii) In a general way, one observes that the slope of the χ-1(T) curves in the high-temperature region (above TIM ∼ 340 K) increases continuously as the oxygen content increases for the whole series from EuBaCo2O5.33 to EuBaCo2O5.65. By fitting the corresponding data with the Curie–Weiss law, one observes that the system exhibits antiferromagnetic interactions at high temperature for the “Co2+/Co3+” and “pure Co3+” cobaltites but that the latter decreases gradually as the oxygen content increases, and finally the interaction becomes ferromagnetic for the “Co3+/ Co4+” cobaltites (see Figure 10). The variation of θp with the oxygen content for both the Eu and Sm series (Figure 10a) shows that the crossover of the AF to FM interactions in the high-temperature regime appears clearly for the “pure Co3+” cobaltites. These results are of interest for the understanding of the particular magnetic properties of these materials. The decrease of the effective moment µeff as a function of δ is very difficult to interpret since it results mainly from the magnetic coupling rather than the atomic spin state itself. E. Electrical Properties of LnBaCo2O5.5+δ (Ln ) Sm and Eu). In Figure 11 we have shown the resistivity of EuBaCo2O5.5+δ samples as a function of temperature. For δ ) -0.17, -0.02, and 0.02 compositions the characteristic metal–insulator transition is observed in the neighborhood of 350 K (Figure 11). This transition could be ascribed to the structural transition in this family of compounds. There (33) Douad-Aladine, A.; Rodriguez-Carvajal, J.; Pinsard-Gaudart, L.; Fernandez-Diaz, M. T.; Revcolevschi, A. Phys. ReV. Lett. 2002, 89, 97205.

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Figure 12. Temperature dependence of resistivity (F) of SmBaCo2O5.50+δ for different δ values (heating run data). From top to bottom at 150 K, δ ) -0.19, -0.01, 0.03, 0.09, and 0.15. Inset: enlargement of the δ ) 0.15 curve showing the positive slope above 250 K.

Figure 10. Variation of (a) paramagnetic Curie temperature (θP) and (b) effective moment (µeff) per formula unit with oxygen content, δ, in LnBaCo2O5.50+δ (Ln ) Sm and Eu) above TIM. The open (solid) symbols correspond to Sm (Eu) compounds.

Figure 13. Phase diagram of (a) SmBaCo2O5.50+δ and (b) EuBaCo2O5.50+δ showing different regions including FM1, FM2, and FM3, paramagnetic insulator (PMI), paramagnetic metal (PMM), canted antiferromagnetic (CAF), and antiferromagnetic insulator (AFI).

Figure 11. Temperature dependence of resistivity (F) of EuBaCo2O5.50+δ for different δ values (heating run data, for δ ) -0.17, -0.02, 0.02, 0.07, and 0.15 from top to bottom at 150 K). The hump is shown by a downward arrow. The inset shows the temperature derivative of resistivity (dF/dT) as a function of temperature. One can see the discontinuity at TC. The solid circles in the same inset are the plot of TC as a function of δ.

is a little change in resistivity with the temperature below 350 to 100 K for all the compositions except δ ) -0.17. Below ∼100 K the resistivity steeply increases down to the lowest temperature. One more curious observation is that we have seen a clear hump in the resistivity data in heating run for δ ) -0.17, -0.02, 0.02, and 0.07 compositions. Interestingly, this hump corresponds to the magnetic transition temperature TC. In the inset of the figure, we plotted TC vs δ, showing that TC matches with the discontinuity in dF/ dT.

The temperature dependence of the resistivity of both SmBaCo2O5.5+δ and EuBaCo2O5.5+δ series exhibits a similar behavior. Both series present a metallic behavior for δ ) 0.15, as shown in Figure 12 for the SmBaCo2O5.5+δ compounds. The plateau (∼340 to ∼100 K) in the resistivity data of LnBaCo2O5.5+δ (Ln ) Sm and Eu) is the region where different magnetic phases coexist as discussed above. The region below 100 K, where the resistivity steeply increases, could be associated with the complete antiferromagnetic ordering of the magnetic moments. Suard et al.34 showed that in the high-temperature range the resistivity follows a simple activation behavior and at lower temperature a T–1/4 law. At intermediate temperature a slow crossover appears in between the two behaviors. (34) Suard, E.; Fauth, F.; Caignaert, V.; Mirebeau, I.; Baldinozzi, G. Phys. ReV. B 2000, 61, R11871.

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F. Phase Diagram of LnBaCo2O5.5+δ (Ln ) Sm and Eu). On the basis of magnetic and electrical properties measurements, we have sketched an emperical phase diagram of LnBaCo2O5.5+δ (Ln ) Sm and Eu) as a function of oxygen content, δ which is shown in Figure 13. The phase diagram for both the Sm and Eu systems are qualitatively the same. For δ > 0.05 the paramagnetic insulating phase (PMI) ceases to exist, and consequently the paramagentic metallic (PMM) region expands. The FM1 phase appears in the region ∼-0.13 < δ < ∼0.06, and the C-AF phase exists in the similar δ range which are also shown in the H-T diagram in Figures 3 and 7a. For both systems the FM2 phase extends until the lowest temperature. The FM3 phase evolves for δ > 0.07. Above the δ values of 0.15, unlike the Eu system, the FM3 phase extends to lowest temperatures for the Sm system.

IV. Conclusions The investigation of the oxygen-deficient ordered perovskites LnBaCo2O5.50+δ with Ln ) Eu and Sm shows that beside the already known ferromagnetic FM1 state around

Seikh et al.

250 K, we have indeed observed two new ferromagnetic states at lower temperatures due to a deviation of the oxygen stoichiometry: the FM2 state which takes place for deficient phases (δ < -0.10) involving Co2+/Co3+ mixed valency and the ferromagnetic FM3 state which appears for oxygen excess (δ > 0.1) involving Co3+/Co4+ mixed valency Co3+-O-Co4+ superexchange interactions. Moreover, at higher temperature, 350–400 K, it appears a transition in transport properties (drop of resistivity), which also corresponds to a drastic change in the magnetic coupling, is associated with the structural changes rather than spin state spin transition of cobalt ions. Finally, on the basis of the magnetic and transport properties measurements, we have presented an emperical phase diagram for both the Sm and Eu phases. Acknowledgment. The authors gratefully acknowledge the CNRS and the French Minister of Education and Research for financial support through their Research, Strategic, and Scholarship programs and the European Union for support through the network of excellence FAME. CM7026652