The crucial role of mixed valence in the magnetoresistance properties of manganites and cobaltites

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1 366, doi: /rsta Published online 7 September 2007 The crucial role of mixed valence in the magnetoresistance properties of manganites and cobaltites BY B. RAVEAU* Laboratoire CRISMAT, UMR 6508, CNRS ENSICAEN, 6 Boulevard Maréchal Juin, Caen, Cedex 4, France The mixed valence Mn 3C /Mn 4C and Co 3C /Co 4C in manganites and cobaltites with the perovskite structure is absolutely necessary for the appearance of magnetotransport properties. It is shown that in these systems the Jahn Teller effect of the transition element, the charge and orbital ordering and the oxygen stoichiometry play a key role in the appearance of large and even colossal magnetoresistance. It has been discovered that these oxides exhibit a new phenomenon, the crystallographic and electronic phase separation. It is this phenomenon that is at the origin of the competition between ferromagnetism and antiferromagnetism or spin glass behaviour and which leads to the negative magnetoresistance (MR). The doping of these materials at different sites appears then to be a means of inducing large MR effects. Keywords: Jahn Teller effect; charge-orbital ordering; crystallographic and electronic phase separation; competition between ferromagnetism and antiferromagnetism or spin glass; colossal magnetoresistance manganites and cobaltites 1. Introduction Since the discovery of superconductivity at high temperature in cuprates, it appears that oxides involving strong electron correlations are of great interest for the generation of new physical properties and applications. In all these systems, the mixed valency of the transition element is of crucial importance for the appearance of such properties. It was the colossal magnetoresistance (CMR) effect that was found promising for applications in magnetic recording, detectors and sensors (Rao & Raveau 1998; Tokura 1999). In the present paper, we review the key factors that are at the origin of such unexpected properties in manganites and cobaltites with the perovskite structure. 2. Manganites The magnetic and transport properties of mixed valent manganites are mainly governed by the double exchange (DE) phenomena that take place between Mn 3C and Mn 4C, as described by Zener (1951). The DE mechanism explains *bernard.raveau@ensicaen.fr One contribution of 15 to a Discussion Meeting Issue Mixed valency. 83 This journal is q 2007 The Royal Society

2 84 B. Raveau (a) Pr 0.7 Ca 0.26 Sr 0.04 MnO T T T(K) r(w.cm) (b) Mn 3+ Mn 4+ Figure 1. Magnetoresistance manganites: (a) decrease of the resistivity by 15 orders of magnitude by applying a magnetic field of 5 T to the perovskite Pr 0.7 Ca 0.26 Sr 0.04 MnO 3 and (b) charge and orbital ordering in the form of Mn 3C and Mn 4C stripes in antiferromagnetic Pr 0.5 Ca 0.5 MnO 3. why such perovskites may exhibit ferromagnetism and metallic properties simultaneously. Thus, in order to induce particular properties in the perovskites LnMnO 3, the Ln 3C cation must be partly replaced by a divalent cation A 2C (AZBa, Sr, Ca, Pb), leading to the formulation Ln 1Kx A x MnO 3. Spectacular CMR effects appear in these compounds, the resistance of the latter decreasing by several orders of magnitude by applying a magnetic field of some Tesla, as illustrated for the perovskite Pr 0.7 Ca 0.26 Sr 0.04 MnO 3 (figure 1a; Maignan et al. 1995). This means that, in the present case, the structure is more distorted in the absence of a magnetic field, hindering a sufficient overlapping of the Mn 3d and O 2p orbitals to get DE, whereas by applying a magnetic field the structure is made more symmetric, allowing metallicity to appear. In other words, the application of a magnetic field involves a structural transition. This suggests that the Jahn Teller (JT) effect of trivalent manganese plays a key role in the appearance of the CMR effect. The magnetic phase diagrams of these manganites corroborate this viewpoint (Martin et al. 1999). For high Mn 3C contents (x%0.3), those perovskites are ferromagnetic but insulating (FMI), in agreement with the strong distortion induced by the JT effect of Mn 3C. For intermediate Mn 3C contents (0.35!x!0.85) one observes, as very early predicted by Goodenough (1955), charge-orbital ordering (CO) of the Mn 3C /Mn 4C species. The latter appear in the form of stripes (Jirak et al. 1985, 2000; Chen & Cheong 1996; Radaelli et al. 1997; Woodward et al. 1998; Van Tendeloo et al. 2004). For instance for xz0.50, electron microscopy observations have shown that one layer of strongly elongated octahedra ( Mn 3C ) alternates with one layer of much less distorted octahedra ( Mn 4C ; figure 1b). Thus, the JT-distorted octahedra are responsible for the antiferromagnetic and insulating nature of these charge-orbital ordered compounds. Nevertheless, by application of a very high magnetic field (O25 T), the structure can be made more symmetric so that a transition to a ferromagnetic and metallic state is observed. In fact, the best CMR effect can be obtained at the boundary between the FMI and the CO states, where the two states are less stable, so that a much smaller magnetic

3 Mixed valence and magnetoresistance property 85 (a) Mn 4+ Mn 3+ Mn 4+ (b) : Ca 2+ : Ba 2+ Mn 4+ Mn 3+ Mn 4+ Cr 3+ or Ga 3+ Figure 2. Creation of more symmetric perovskite domains by introducing (a) larger Ba 2C cations on the Ca 2C sites and (b) Ga 3C cations on the JT-distorted Mn 3C sites. field is required to reach the ferromagnetic metallic (FMM) state. These few examples demonstrate that the JT effect of Mn 3C is absolutely necessary for the appearance of the CO state since it hinders a perfect overlapping of the Mn and O orbitals; but the latter should be made metastable in order to lower the critical field that can be applied to reach more easily the more symmetric FMM phase. Bearing in mind this capital role of the JT effect of Mn 3C in the CMR effect, two methods can be proposed to induce the CMR effect in those manganites, both of them based on local suppression of the JT effect. The first one consists in doping the Mn 3C sites by a non-jt trivalent cation, such as Ga 3C (Hardy et al. 2003). The latter, due to its regular coordination, makes the structure more symmetric around it, as schematized in figure 2a; it results in small domains, more symmetric around Ga 3C, which then favour ferromagnetism and electron delocalization, in spite of the non-magnetic character of this cation. Such an effect is illustrated by the evolution of the magnetization versus the applied magnetic field for the Ga-doped phase Pr 0.5 Ca 0.5 Mn 0.95 Ga 0.05 O 3 (figure 3a), where it can be seen that ferromagnetism is induced in this phase even at 5 T, in contrast to the pristine phase Pr 0.5 Ca 0.5 MnO 3 which remains antiferromagnetic up to 25 T. A second method for suppressing locally the JT distortion consists in the doping of the calcium sites by a bigger cation such as barium (Raveau et al. 2003). The latter makes the perovskite cages more symmetric as schematized in figure 2b and weakens the JT distortion of the surrounding octahedra; consequently, it develops around Ba 2C more symmetric domains which are then favourable to the overlapping of the Mn 3d and O 2p orbitals and, therefore, to the formation of ferromagnetic small domains. Thus, similarly to Ga, Ba-doping induces ferromagnetism (figure 3b). Remarkably, in all these systems, one observes that the magnetization curves are characterized by the presence of multisteps (figure 3). The latter originate from phase separation phenomena: FMM domains of higher symmetry are first formed in the antiferromagnetic insulating (AFMI) matrix whose crystal symmetry is lower. Then, strains are developed at the interface between the domains and the

4 86 B. Raveau (a) (b) ii M (µ B / f.u.) i T = 2.5 K x = H (T) H (T) Figure 3. Magnetization versus magnetic field, showing multisteps in (a) Ga-doped manganite Pr 0.5 Ca 0.5 Mn 0.95 Ga 0.05 O 3 and (b) Ba-doped manganite Pr 0.5 Ca 0.96 Ba 0.04 MnO 3. f.u., formula units. matrix, due to the different symmetry between the two perovskite structures, resulting in a martensitic-like phenomenon (Hardy et al. 2004). Thus, this mechanism is at the origin of the multiple steps that are observed in the magnetization curves. In other words, the crystallographic phase separation that appears in these systems induces an electronic phase separation and is of primordial importance for the appearance of the CMR effect. In this way, one can understand that, by changing the doping level or by increasing the magnetic field, percolation of the domains will appear rapidly, explaining the CMR effect. Coming back to the doping of the octahedral sites by non-jt cations, it is worth emphasizing that the local substitution of a magnetic cation for manganese reinforces considerably the CMR effect. For instance, in the CE-type AFM structure of Pr 0.5 Ca 0.5 MnO 3, the Cr 3C and Ru 4C species couple, respectively, antiferromagnetically and ferromagnetically with their manganese neighbours and develop, in this way, ferromagnetic domains around them by domino effect (Martin et al. 2001). As a result, the doping by these cations induces a transition from an insulating antiferromagnetic to a metallic ferromagnetic state. In summary, the JT effect of trivalent manganese and phase separation are closely related in the crystal chemistry of mixed valent manganites and are the key factors for the control of their magnetotransport properties. 3. Cobaltites Similar to manganites, cobaltites exhibit also a great ability to form mixed valent perovskites, involving the Co 3C and Co 4C species. However, it must be emphasized that differently from manganites the cobaltites may, in many cases, exhibit a large oxygen deficiency. This is the case for the strontium-rich perovskites Ln 0.1 Sr 0.9 CoO 3Kd and Ln 0.2 Sr 0.8 CoO 3Kd (Raveau et al. 2005), for which it was observed that the oxygen deficiency can be large, d ranging from 0.10 to 0.40 and increases as the size of the lanthanide cation decreases. As a consequence, two sorts of behaviours are observed for these oxygen-deficient perovskites, depending on the cobalt valency V Co. For higher cobalt valencies, one obtains metallic ferromagnets that exhibit a metal metal transition, whereas for smaller cobalt valencies one obtains weak ferromagnetic semiconductors.

5 Mixed valence and magnetoresistance property 87 (a) Tb Gd Ln 0.1 Sr 0.9 CoO 3 d (b) Tb Ln 0.2 Sr 0.8 CoO 3 d MR 5 K (%) Eu Gd 2 0 Sm Nd Pr cobalt valence 10 Sm Nd Pr cobalt valence Figure 4. Evolution of the MR at 5 K versus cobalt valency in the cobaltites (a) Ln 0.1 Sr 0.9 CoO 3Kd and (b) Ln 0.2 Sr 0.8 CoO 3Kd. Thus, for the two series of oxides, diagrams can be established plotting the Curie temperature T C versus the cobalt valence V Co. The latter can be divided into two regions, with a crossover at V Co zc3.43. For V Co OC3.43, corresponding to larger lanthanide cations, the compounds are ferromagnetic and metallic, whereas for V Co!C3.43, involving smaller cations, the perovskites are weakly ferromagnetic and become semiconductors. In a general way, the Curie temperature T C decreases with V Co, but we note that this decrease is abrupt in the region where V Co!3.43. These results show that the magnetotransport properties of these phases are very sensitive to the cobalt valency and that they can be tuned by a very tiny variation of the oxygen content. These cobaltites are also magnetoresistive, though the magnetoresistance (MR) is much smaller than in the manganites. The magnitude of the MR at a given temperature is closely related to the cobalt valency, as illustrated for the two series of oxides at 5 K (figure 4). One indeed observes that at this temperature the MR value is very small, less than 2%, for the metallic ferromagnets, i.e. for the larger V Co values, then as soon as V Co decreases below 3.4, the MR value increases abruptly reaching values between 10% and 60%. Like in manganites, the possibility of modifying the magnetotransport properties of these cobaltites by doping at the cobalt sites has been considered. In this respect, the perovskites of the Sr Co O system are of interest. It was indeed found that the stoichiometric perovskite SrCoO 3 prepared under high oxygen pressure or by electrochemical oxidation is a metallic ferromagnet and consequently does not exhibit any MR (Bezdicka et al. 1993; Kawasaki et al. 1996). In contrast, the oxygen-deficient perovskite SrCoO 2.75 prepared at normal pressure by a two-step method was found to be antiferromagnetic and a semiconductor with a negligible MR ( Takeda et al. 1986; Maignan et al. 2001). The introduction of a pentavalent cation on the cobalt sites allows the perovskite structure to be stabilized for the compounds

6 88 B. Raveau (a) M(m B /f.u.) as-synthesized x = (b) as-synthesized x = (c) M(m B /f.u.) O 2 -annealed x = (d) O 2 -annealed y = T (K) T (K) Figure 5. Magnetization versus temperature for the cobaltites SrCo 1Kx M x O 3Kd, with (a,c) MZNb and (b,d ) MZRu. f.u., formula unit. SrCo 1Kx Nb x O 3Kd and SrCo 1Kx Ru x O 3Kd (Motohashi et al. 2005). Again, the chemical analysis clearly shows that those perovskites are oxygen deficient with cobalt valencies ranging from 3.26 to 3.39 for compounds prepared in an oxygen flow, whereas the annealing at low temperature, 4008C, in an oxygen flow allows the cobalt valency to be increased, the materials still remaining oxygen deficient. The magnetization curves of both series of oxides Nb and Ru (figure 5) show the existence of ferromagnetism but the latter is much weaker than reported for SrCoO 3. Moreover, the smoothness of the M(T ) curves does not allow an accurate determination of the transition temperature, which ranges from 130 to 180 K, against 280 K for SrCoO 3. In both series, magnetization is significantly increased by O 2 annealing, showing that the increase of Co 4C content plays an important role in the enhancement of this ferromagnetism. The particular nature of ferromagnetism in these cobaltites is confirmed in the M(H ) curves registered at 5 K, as shown by the SrCo 1Kx Nb x O 3Kd series (figure 6). Though the magnetization loops are characteristic of a ferromagnetic behaviour, the value of the magnetic moment at 5 K remains low, i.e m B /formula unit (f.u.) for xz0.05, showing that saturation is far from being reached. This suggests that another state competes with the ferromagnetic phase in those materials. This viewpoint is confirmed by the AC-susceptibility curve c 0 (T ) (inset figure 6), which shows a bump at approximately K. The latter is frequency dependent and shifts towards higher temperature as the frequency d increases. The frequency shift close to the value reported for typical spin glass oxides (Mydosh 1993) suggests strongly that this behaviour is that of a spin glass or a

7 Mixed valence and magnetoresistance property c' (10 2 emu/g Oe) M = Nb f =1Hz 10Hz 100Hz M(m B / f.u.) , as-synthesized 0.10, as-synthesized 0.15, as-synthesized 0.20, as-synthesized 0.10, O 2 -annealed m 0 H (T) Figure 6. Magnetization versus magnetic field registered at 5 K for the cobaltites SrCo 1Kx Nb x O 3Kd showing ferromagnetic hysteresis loops. The inset shows, as an example, the frequency-dependent AC-susceptibility c 0 (T ) curve recorded under 30e for SrCo 0.9 Nb 0.1 O f.u., formula unit. cluster glass. Such a phenomenon can be explained by the disordering introduced by niobium or ruthenium on the cobalt sites, leading to a disordered magnetic state, similar to that previously observed in numerous manganites, such as Pr 0.5 Ca 0.5 Mn 1Kx M x O 3 doped with various cations (MZAl, Ti, Fe, In, Sn, Ga; Hébert et al. 2002). Thus, this unusual ferromagnetic behaviour of the Nb- and Ru-doped cobaltites results from a competition between ferromagnetism induced by the presence of Co 4C and spin glass or cluster glass behaviour induced by the disordering on cationic sites. In other words, these cobaltites exhibit phase separation leading to a glassy ferromagnetic behaviour. In fact, these results together with those observed for La 1Kx Sr x CoO 3 (Wu et al. 2005) clearly demonstrate that phase separation is a common feature in cobaltites and plays a very important role in their particular magnetic properties. Like the manganites, these oxides also exhibit magnetoresistive properties. A large negative MR effect is thus observed, whose onset coincides with the magnetic transition. The MR value increases with decreasing temperature and the highest magnitude at 5 K reaches 30% for O 2 -annealed Nb-doped samples with xz , to be compared with Sr 2 FeMoO 6 (Kobayashi et al. 1998). The niobium-doped compounds show a much higher MR than the ruthenium-doped phases, as illustrated in figure 7 for the large MR effect observed for the O 2 -annealed samples of SrCo 0.85 Nb 0.15 O 3 and SrCo 0.85 Ru 0.15 O 3Kd. Note that the

8 90 B. Raveau K Ru-substituted rh r 0 / r Nb-substituted m 0 H (T) Figure 7. MR versus magnetic field registered at 5 K for O 2 -annealed cobaltites SrCo 0.85 Nb 0.15 O 2.78 and SrCo 0.85 Ru 0.15 O MR magnitude tends to increase with the Nb content, whereas it tends to decrease as the Ru content decreases. This MR effect is easily explained in the frame of the phase separation phenomenon. The strong competition that exists between the ferromagnetic phase and the spin glass or cluster glass state is most probably at the origin of the MR effect in those cobaltites, similar to La 1Kx Sr x CoO 3. Thus, it is reasonable to admit that the negative MR of the Nb- and Ru-substituted cobaltites stems from the spin-dependent scattering of carriers. For this reason, the existence of ferromagnetism is absolutely necessary for the appearance of the MR effect. However, a second factor is needed to induce large MR, which is the magnetic disorder due to the spin glass state. The latter increases the spin-dependent scattering of the carriers and thus decreases the electrical conductivity in the absence of magnetic field. At temperatures smaller than T f w80 90 K, the degree of FM spin alignment gets increased by applying a magnetic field and accordingly the spin-dependent scattering is significantly reduced, leading to a high MR effect. The difference between the Ru and Nb phases from the viewpoint of MR is easily explained by the fact that Nb 5C has a d8 electronic configuration and, in contrast to Ru 4C or Ru 5C, does not participate in the carrier transport. In the ruthenium-doped samples, a Ru-derived impurity band is formed due to the strong hybridization of the Ru-4d orbital. Such an impurity band makes an additional contribution to the electrical conductivity, leading to lower resistivity than for the Nb-substituted samples. Moreover, it determinates the spindependent scattering of the carriers, due to its less correlated character, resulting in the suppression of the MR effect in Ru-rich samples. The cationic ordering of the A site cations may also influence the magnetotransport properties of the cobaltites as shown in the ordered oxygendeficient perovskites LnBaCo 2 O 5.5Cd (Martin et al. 1997; Respaud et al. 2001; Maignan et al. 2004). In summary, cobaltites, like manganites, exhibit a complex crystal chemistry, where the mixed valence Co 3C /Co 4C is dominant, but differ from the manganites by the possibility of large oxygen deficiencies and different spin states that are to

9 Mixed valence and magnetoresistance property 91 date not elucidated. Moreover, it is remarkable that the cobaltites, like manganites, exhibit phase separation, and it must be emphasized that this phenomenon also plays a key role in the control of the magnetotransport properties of these oxides. 4. Conclusion Manganese and cobalt oxides represent a great potential for the discovery of new magnetotransport properties. In these systems, the mixed valence of the transition element is of capital importance for the generation of MR properties, but the key factors that must be taken into consideration are the JT effect of the transition element and the oxygen vacancies that may appear at high concentration, especially in cobaltites. It must be emphasized that the study of the magnetotransport properties of these strongly correlated systems has allowed a new phenomenon to be discovered, the electronic phase separation and concomitantly the crystallographic separation. There is now no doubt that this phenomenon will be generalized to many of these oxides and is primordial since it governs the physics of these materials. Based on these results, we believe that a new strategy to search new magnetoresistive materials should be developed in which the doping of oxides at different sites is an important ingredient to induce large MR effects. Many of the results described in this review paper have been obtained thanks to the collaboration of V. Caignaert, V. Hardy, S. Hébert, M. Hervieu, A. Maignan, C. Martin, N. Nguyen, D. Pelloquin, V. Pralong and Ch. Simon from CRISMAT. We also thank collaborators from other laboratories, particularly F. Bourée, Z. Jirak, D. Khomskii, T. Motohashi, G. Sawatzky and G. Van Tendeloo. References Bezdicka, P., Wattiaux, A., Grenier, J. C., Pouchard, M. & Hagenmuller, P Preparation and characterization of fully stoichiometry SrCoO 3 by electrochemical oxidation. Z. Anorg. Allg. Chem. 619, (doi: /zaac ) Chen, C. H. & Cheong, S. W Commensurate to incommensurate charge ordering and its real space images in La 0.5 Ca 0.5 MnO 3. Phys. Rev. Lett. 76, (doi: /physrevlett ) Goodenough, J. B Theory of the role of covalence in the perovskite-type manganites [La, M(II)]MnO 3. Phys. Rev. 100, (doi: /physrev ) Hardy, V., Hébert, S., Maignan, A., Martin, C., Hervieu, M. & Raveau, B Staircase effect in metamagnetic transitions of charge and orbitallly ordered manganites. J. Magn. Magn. Mater. 264, (doi: /s (03) ) Hardy, V. et al Field-induced magnetization steps in intermetallic compounds and manganites: the martensitic scenario. Phys. Rev. B 69, (doi: /physrevb ) Hébert, S., Maignan, A., Martin, C. & Raveau, B Important role of impurity e g levels on the ground state of Mn-site doped-manganites. Solid State Commun. 121, (doi: / S (01) ) Jirak, Z., Krupicka, S., Simsa, Z., Dlouha, M. & Vratislav, S Neutron diffraction study of Pr 1Kx Ca x MnO 3 perovskites. J. Magn. Magn. Mater. 53, (doi: / (85) ) Jirak, Z., Damay, F., Hervieu, M., Martin, C., Raveau, B. & Bourée, F Magnetism and charge ordering in Pr 0.5 Ca x Sr 0.5Kx MnO 3. Phys. Rev. B 61, (doi: /physrevb ) Kawasaki, S., Takano, M. & Takeda, Y Ferromagnetic properties of SrFe 1Kx Co x O 3 synthesized under high pressure. J. Solid State Chem. 121, (doi: /jssc )

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