Cr enhanced ferromagnetism in La0.5Ba0.5CoO3 due to possible double-exchange interaction

Journal of Alloys and Compounds 628 (2015) 251–256

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Cr enhanced ferromagnetism in La0.5Ba0.5CoO3 due to possible double-exchange interaction Shile Zhang a,⇑, Li Pi a,b,*, Wei Tong a, Shun Tan b, Changjin Zhang a, Yuheng Zhang a,b a b

High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 3 November 2014 Received in revised form 16 December 2014 Accepted 17 December 2014 Available online 31 December 2014 Keywords: Phase separation Curie temperature Exchange interaction

a b s t r a c t The magnetic studies have been carried out on the R-doped La0.5Ba0.5Co0.9R0.1O3 (R = Cu, Zn, Cr, Ni, Ti and Ru) cobaltites. X-ray absorption near-edge structure (XANES) proves these ions are in the valence state of Cu2+, Zn2+, Cr3+, Ni3+, Ti4+ and Ru4+. Magnetic moment calculation shows both Co3+ and Co4+ hold in the intermediate spin state. Based on the ratio of Co3+/Co4+, the average B-site size, the tolerance factor and electron itinerancy, the different influence of these ions on the magnetism were discussed. Among all these ions, Cr3+ gives an extraordinary effect of enhancing TC a lot. It is attributed to the Co-O-Cr exchange interaction, which is confirmed by the electron spin resonance spectra. The typical magnetotransport behavior implies a double-exchange property of this exchange interaction. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Perovskite cobaltites with the general formula Ln1xAxCoO3 (Ln = trivalent rare earth cation and A = divalent alkaline earth cation) have been explored by many authors due to their unusual magnetic and transport properties [1–6]. Three alternative spin configurations, low-spin, intermediate-spin and high-spin are induced by the strong competition between the crystal-field splitting energy and the intra-atomic Hund coupling[7–15]. The La0.5Ba0.5CoO3, with a 1:1 mixture of Co3+ and Co4+, shows a paramagnetic–ferromagnetic transition at TC = 189 K and turns to a magnetic glassy state below 140 K [4,15]. Such a cluster glass behavior occurs as a result of the competition between randomly distributed ferromagnetic and antiferromagnetic interactions[16]. Fauth et al. [4] observed a semi-metallic behavior with a metal– metal transition at TC in this compound, whereas Nakajima et al. [15] observed a metallic behavior down to 140 K, with an abrupt increase in resistivity below this temperature. Fauth et al. also observed a structural change from cubic to tetragonal which is compatible only with static Jahn–Teller (JT) distortion of CoO6 octathedra. The origin of the ferromagnetic state in metallic cobaltites has been a subject of discussion for a long time. One knows that cobaltites are extremely sensitive to order–disorder, doping, temperature, and crystal structure [15–31]. The substitution of ⇑ Corresponding authors at: High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China. Tel./fax: +86 551 65595249 (S. Zhang). Tel./fax: +86 551 65595664 (L. Pi). E-mail addresses: zhangsl@hmfl.ac.cn (S. Zhang), [email protected] (L. Pi). http://dx.doi.org/10.1016/j.jallcom.2014.12.188 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

other ions for Co ions could directly modify the magnetic ordering and facilitate understanding the nature of the ferromagnetic state as well as clarifying the spin state of Co ions. Our previous work has researched the effect of the substitution of Ti ions for Co [22]. The magnetic measurements under external pressures suggest that both Co3+ and Co4+ ions are stabilized in the intermediate-spin (IS) state, and a detailed phase diagram has been constructed. In this paper, we performed a contrastive research on the effect of introducing foreign species Cu, Zn, Cr, Ni, Ti and Ru at the Co sites. All these ions stabilize the intermediate spin state of both Co3+ and Co4+. For Cu2+, Zn2+, Ni3+ and Ru4+, they decrease T C and induce a complex magnetism. Whereas Cr3+ increases T C a lot. Definitely, there is a Co-O-Cr exchange interaction existing in the system, which is confirmed by ESR experimental results as well. The magnetoresistance effect with a typical peak at TC implies a double-exchange property of this exchange interaction.

2. Experiment La0.5Ba0.5Co0.9R0.1O3 (R = Cu, Zn, Cr, Ni, Ti and Ru) samples and the parent compound were prepared by solid state reaction. Dry pure La2O3, BaCO3, Co3O4, CuO, ZnO, Cr2O3, Ni2O3, TiO2 and RuO2 powders were well mixed in the stoichiometric amounts and initially decarbonated in the air at 1170 K for 20 h. After regrinding, the samples were fired in the air at 1470 K for 15 h and slowly cooled for several times [4,23]. The cooling rate is 30 K/h. The same method as described in Ref. [32] to produce Pr0.5Sr0.5Co1xMnxO3 was used to maintain oxygen stoichiometry. Ni-doped sample was selected to do the iodometric titration. Oxygen content was determined at 2.99, leading within the limits of experimental error to the formulation of stoichiometric oxygen.

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The phase structure and purity have been checked by the X-ray diffraction (XRD) on a Rigaku-D/max-cA diffractometer using high intensity Cu Ka radiation at room temperature. The crystal structure refinement has been performed by the Rietveld method with GSAS software package [33]. The K-edge X-ray absorption near-edge structure (XANES) spectra of the powder samples were measured at the U7C beamline of the National Synchrotron Radiation Laboratory (NSRL), China. The XANES signals were collected in fluorescence mode with seven-element high purity-Ge solid detector. The magnetic properties were measured by using a Quantum Design MPMS XL7. Electron spin resonance (ESR) spectra were recorded on the powder sample by a Bruker EMXplus 10/12 spectrometer at 9.40 GHz.

3. Results and discussion XRD patterns at room temperature shown in Fig. 1 confirms that all La0.5Ba0.5Co0.9R0.1O3 (R = Cu, Zn, Cr, Ni, Ti and Ru) samples and the parent compound are in pure single phase. All the compositions can be refined in perovskite structure with cubic symmetry  (space group Pm3m). For the parent one, the unit cell parameter (a = 3.8865 Å) is consistent with stoichiometric La0.5Ba0.5CoO3 compound (a = 3.885 Å) [4,15]. The cell parameter are shown in Table 1. Fig. 2(a-g) plots the magnetization as a function of temperature for La0.5Ba0.5Co0.9R0.1O3 (R = Cu, Zn, Cr, Ni, Ti and Ru) and the parent compound. The data were obtained on warming and under 1 kOe after zero-field cooling (ZFC) and field cooling (FC), respectively. T C is defined as the temperature corresponding to the crossing point of the PM line and tangent line on the paramagnetic (PM)ferromagnetic (FM) transition region. The parent shows Tc at 180 K and turns to a magnetic glassy state below around 140 K, which is consistent with stoichiometric La0.5Ba0.5CoO3 compound [4,15]. Doping with Cu, Zn and Ni which similar to Ti significantly reduces the Curie temperature T C of La0.5Ba0.5CoO3 from 189 K down to 154–142 K [22]. Fig. 2(a)–(c) for Cu, Zn and Ti doping, show a peak and difference between the ZFC and FC curves, which indicates an antiferromagnetic transition and a reentrant spin glass state, respectively. The linear frequency dependence of the ac susceptibility in Ti doping compound proves the reentrant spin glass state, which are shown in Fig. 2(h). Ru obscures the ferromagnetism and the peak on the M–T curve is suppressed to a bump, while remarkably Cr enhances the T C up to 205 K. In order to determine the valence state of R ions in our samples, XANES measurements were performed. The K-edge XANES spectra of R ions for both our doping systems and reference samples (CuO, ZnO, Ni2O3 and Cr2O3) are shown in Fig. 3. One can see that the absorption thresholds (defined the position the first inflection point as the absorption thresholds, listed in Table 2) in the K-edge XANES spectra of R ions are the same for our doping systems and reference samples, indicating identical valence state of R ions in

(a)

our doping system and reference samples. The result shows our samples can be divided into three groups among the doping ions, i.e., divalent ions (Cu,Zn), trivalent ions (Cr,Ni) and tetravalent ions(Ti,Ru). The pre edge peak and the edge for the doped cobaltites and reference also listed in Table 2. Here, we define the pre edge peak and the edge as the position of the top of the pre edge peak and the half maximum intensity of the peak, respectively. Although the IS state can be expected for Co3+ due to the suppression of the low-spin (LS) state by very small amount Ba doping[34], the spin state of the Co4+ ion is still an open issue. One should bear in mind that intermediate or high-spin states for Co4+ ions are somewhat surprising as in other compounds Co4+ favors the low-spin state [11]. For clarifying the different effects of these doping ions, one must make sure the spin state of Co ions in different doping system first. From Fig. 4(a), one can see that the 1=v-T curves of both La0.5Ba0.5Co0.9R0.1O3 and the parent compound above T C well fit into a straight line, which means a PM state at high temperature with Curie–Weiss law. Then the effective moment of ions (Co + R) is obtained and plotted in Fig. 4(b). Based on charge conservation and the hypothesis of simple spin configurations as usually being used in previous works, the theoretical effective magneton numbers can be calculated by u2eff ¼ 0:3u2Co3þ þ 0:6u2Co4þ þ 0:1u2R2þ {1} for Zn and Cu, u2eff ¼ 0:4u2Co3þ þ 0:5u2Co4þ þ 0:1u2R3þ {2} for Cr and Ni, u2eff ¼ 0:5u2Co3þ þ 0:4u2Co4þ þ 0:1u2R4þ {3} for Ru and Ti, u2eff ¼ 0:5u2Co3þ þ 0:5u2Co4þ {4} for matrix. Following these formulas, we can obtain the total effective magneton numbers with IS Co3+ and assuming that the spin state of Co4+ ions are in LS state, IS and HS state respectively. The results are shown in Fig. 4(b). It is clear that the double IS case is the most accordant to the experimental data. Therefore we may claim that the intermediate-spin state is adopted by both Co3+ and Co4+ ions in all samples at high temperature. The observation of an IS spin state on sample x = 0 is also consistent with the results of the neutron scattering measurement [4]. Structural aspects are always important factors to the material’s properties. Usually, the average cationic radius in the B site P hr B i= X i r i (X i is the fractional occupancy of B-site ions, and ri is the corresponding pffiffiffi ionic radius), as well as the tolerance factor f = (hrA i+hrO i)/ 2(hrB i+hr O i) is taken into account to evaluate the effects. The calculated hrB i and f for all samples are shown in Table 1. Obviously, the f values are around 0.985, very close to 1. Thereby a stable cubic perovskite phase is favored, which is consistent with the XRD results. Structurally, the B-O-B’ bond angle is always 180 degree with cubic phase, and the bond-length variation should be considered. From Table 1, we can see that the average cationic radius hrB i in all doping system is large than the parent

(b)

Fig. 1. (a) Room-temperature XRD of La0.5Ba0.5CoO3 and La0.5Ba0.5Co0.9R0.15O3 (R = Cu, Zn, Cr, Ni, Ti, Ru). (b) Room-temperature XRD patterns for La0.5Ba0.5CoO3. The red lines are the Rietveld fittings for the data. The fork and the blue line refer to measurements and background, the bottom line represents the difference. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 1 The comparison of Valence, ratio of Co3+/Co4+, Ions radius,average B-site radius hr B i, tolerance factor f, ferromagnetic temperature (T C /K), Curie–Weiss temperature (hK /K), antiferromagnetic temperature (T P /K), reentrant spin glass (T RSG /K), cell parameter (a). Ions (samples) The parent Cu Zn Cr Ni Ti Ru

Valence

Co3+/Co4+

2+ 2+ 3+ 3+ 4+ 4+

1/1 1/2 1/2 4/5 4/5 5/4 5/4

Ions radius/Å

hrB i/Å

f

T C /K

hK /K

0.73 0.74 0.615 0.6 0.605 0.62

0.54 0.544 0.545 0.5425 0.541 0.5515 0.553

0.989747 0.987781 0.987291 0.988518 0.989255 0.984117 0.983387

189 142 145 205 152 154

206.3 160.3 172.3 220.5 186 187.1 132.4

(a)

T RSG /K

95 97

55 60

110 170

67 76

a/Å 3.8865(2) 3.8939(6) 3.8927(5) 3.8867(1) 3.8846(9) 3.8921(8) 3.8929(8)

(e)

(b)

(f)

(c)

(g)

(d)

T P /K

(h)

Fig. 2. (a)–(g) The M–T curves of La0.5Ba0.5Co0.9R0.1O3 (R = Cu, Zn, Cr, Ni, Ti, Ru) and the parent compound. (h) Ac susceptibility curves for Ti-doped compound.

compound. As we discussed in Ref [22] Ti enlarges the Co–O bond length resulting in weakening the double exchange (DE) interactions and reducing the Curie temperature and magnetization. So it is understandable that the T C and magnetization decrease with large size of R = Cu, Zn, Ni and Ru doping. We notice that both the Curie temperature and magnetization for tetravalent Ti doping are much larger than for divalent Zn and Cu doping while the hrB i for Ti doping is bigger. It is quite clear that the valence state of the

doping ions plays an important role. For divalent Zn and Cu, the number ratio of Co3+/Co4+ is 1/2 and for tetravalent Ti it is 5/4 according to formula {1},{3}. This implies that the ferromagnetic exchange interaction is suppressed by either raising or lowering the ratio of Co3+/Co4+ form 1:1 and the suppression effect is more sensitive to lowering than raising the proportion. The Curie temperature for Ni3+ doping is comparable to Ti doping but the hr B i is smaller for the former, which is a good illustration for the above

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Fig. 3. The R K-edge XANES spectra for the La0.5Ba0.5Co0.9R0.1O3, and reference samples (CuO, ZnO, Ni2O3 and Cr2O3). The black and the green line refer to doped cobaltites and reference, respectively. The red arrow denote the absorption thresholds. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 2 The comparison of the absorption thresholds, the pre edge peak and the edge for the doped cobaltites and reference. Here, we define the position of the first inflection point, the top of the pre edge peak and the half maximum intensity of the peak as the absorption thresholds, the pre edge peak and the edge, respectively. The doped cobaltites and reference

Zn doped

ZnO

Cu doped

CuO

Cr doped

Cr2O3

Ni doped

Ni2O3

The absorption thresholds (eV) The pre edge peak (eV) The edge (eV)

9648.5

9648.5

8974.5

8974.5

9664

9658.1

8989.7

8983.1

5989.3 5994.7 6009.2

5989.3 5991.9 6001.5

8333 8336 8349.6

8333 8335.3 8342.1

opinion. With regard to the identical valence and hr B i for Zn and Cu, the electronic itinerant character weakens the localized magnetism, causing T C a little lower for the Cu2+ doping. In contrast to Ti4+, the radius of Ru4+ is bigger and the hr B i is larger. Furthermore, 4d electrons of Ru4+ have strong itinerant character. So the FM state is completely suppressed for the Ru4+ doping. Additionally, One may notice from Table 1 that both the antiferromagnetic temperature (Tp) and reentrant spin glass (TRSG) temperature decrease with increasing f. When f excesses a threshold value, Tp and TRSG disappear. This implies that the antiferromagnetic (AFM) and reentrant spin glass (RSG) state are strongly correlated with f. In a summary of the above analysis, we get following results. The increasing of hr B i would suppress the FM interaction. Both raising and lowering the ratio of Co3+/Co4+ from 1:1 would destroy the FM interaction and this effect is more sensitive to lowering the ratio. In addition, the electronic itinerant character of doping ion would reduce the FM interaction, too. When f is below a threshold value, AFM and RSG occur in the system. The surprising phenomenon occurs for Cr doping. It is the only one among all ions that enhances the TC. As we know from Table 1, the hr B i for Cr doping is bigger than the parent compound and Ni doping. And this doping lowers the proportion of Co3+/Co4+. All these effects suppress the FM interaction. Why does the TC increase? The only reply to this question is the FM exchange interaction between Co and Cr. This FM exchange interaction is also

confirmed experimentally by ESR spectra. Fig. 5 shows the ESR spectra for La0.5Ba0.5CoO3 and La0.5Ba0.5Co0.9Cr0.1O3 with the temperature range between 200 and 5 K. For the parent compound, paramagnetic resonance line with g  2 occurs at 200 K. Below this temperature,the PM line disappears and the ferromagnetic resonance lines (FL1) emerge, which are indicated by red arrows in the Fig. 5. With decreasing temperature, FL1 shifts toward low field since the FM interaction is developing and magnetization is increasing. Below 40 K, there is a second very low field resonance line (FL2) appears, which is related to ferromagnetism/domains, too. For the Cr-doped compound, the resonance field of FL1 starts from 200 K, and is a little higher than the parent compound at the same temperatures from 160 K to 100 K. Generally, lower ferromagnetic resonance field means larger magnetization. So the ESR data are coincident with the M-T results. Secondly, one may notice the FL2 happens at a much higher temperature of 100 K for Cr compound than 30 K for the parent one. This implies the strong FM clusters are formed earlier in Cr-doped compound than in parent one. By the FL2 appearance, the FL1 starts shifting toward high field and becomes weak with further decreasing temperature. It is because the antiferromagnetic interaction develops among those FM clusters. The M-T curves did prove this signature with cluster glass behavior. In one word, ESR results demonstrate again the Cr induces strong FM interactions at higher temperature in La0.5Ba0.5Co0.9Cr0.1O3 system, which means Cr plays not an

S. Zhang et al. / Journal of Alloys and Compounds 628 (2015) 251–256

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(a) Fig. 6. Temperature dependence of resistivity under zero and 14T magnetic field and the corresponding MR ratio for La0.5Ba0.5Co0.9Cr0.1O3. Inset: ln(q) versus 1000/T for La0.5Ba0.5Co0.9Cr0.1O3 under zero magnetic field.

(b) Fig. 4. (a) The 1=v-T curves of La0.5Ba0.5Co0.9R0.1O3 and the parent compound above 260 K; (b) The calculated effective moment according to different combinations of spin-state configurations compared with the experiment data.

impurity role but the one of enhancing Co-O-Co/Cr network FM interaction. Though we have recognized that the exchange coupling between Co and Cr can bring about ferromagnetism, whether its nature is double exchange or superexchange cannot be concluded merely from the magnetization measurments. As is well known, a prominent feature of double exchange is the strong correlation

between the electrical transport behavior and the magnetic state [4,35]. So a study of the transport and magnetotransport properties is necessary to get further insight. Fig. 6 presents the temperature dependence of resistivity for Cr-doped sample at different magnetic fields (0 T/14 T). It shows semiconductor behavior in the temperature range studied and no semiconducting-metallic transition was observed at TC. However, it is clear that there is an apparent change in the slope of resistivity near the magnetic ordering temperature TC, which indicates that the onset of ferromagnetic exchange interaction promotes the transfer of carriers. Moreover, a decrease of resistivity is detected under an applied magnetic field of 14T. The corresponding magnetoresistance (MR) ratio is plotted in Fig. 6, where MR = ½qð0 TÞ  qð14 TÞ=qð14 TÞ. A large MR was observed and a peak locates near TC, a very similar behavior to that in the manganese oxide. This allows one to argue that a doubleexchange-like ferromagnetic interaction between different transition-metal elements Co and Cr could occur in La0.5Ba0.5Co0.9Cr0.1O3. However, the DE-like FM ordering in manganese oxide often accompanies with metallic conduction which is different from this Cr-doped semiconductive compound. The semiconducting behavior was caused by magnetic disorder, i.e., randomly distributed

Fig. 5. The ESR spectra for La0.5Ba0.5CoO3 and La0.5Ba0.5Co0.9Cr0.1O3 with the temperature range between 210 K and 5 K. Dash line indicate g = 2 position, Red arrow (high field) and blue arrow (low field) denote the resonance field of ðFL1Þ and ðFL2Þ, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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ferromagnetic and antiferromagnetic interactions in system[4,15,16]. Inside the double exchange ferromagnetic clusters, the electrical conductivity is metallic. Inside the antiferromagnetic cluster and among all clusters, the magnetic scattering and disorder effects of doped ions cause insulating conductivity. The whole resistance is determined by serial connection of these two parts, thus it presents a semiconductor behavior. Under the magnetic Field, the resistance inside the ferromagnetic clusters changes significantly near the Tp. So there is MR peak at this temperature. The big magnetoresistance at low temperature is because of the reducing of magnetic scattering among clusters since the magnetic field forces those clusters moments to be aligned. We notice that both IS Co ions contain an eg electron while the Cr3+ only has t2g electrons, no eg electron. Thereby it can be deduced that the FM double exchange interaction comes from Cr3+(t 32g e0g )-O-Co3+(t52g e1g ) and Cr3+(t32g e0g )-O-Co4+(t 42g e1g ). Indeed, it is this kind of FM double exchange interaction besides Co3+-O-Co4+ that results in the raise of the Curie temperature. 4. Conclusion The magnetism of La0.5Ba0.5Co0.9R0.1O3 (R = Cu, Zn, Cr, Ni, Ti and Ru) cobaltites are studied. The intermediate spin state of Co3+ and Co4+ is more stabilized with ions doping. For Cu2+, Zu2+, Ni3+, Ti4+ and Ru4+ doping, T C decreases and a complex magnetism presents at low temperature. Increasing of hrB i, raising or lowering the ratio of Co3+/Co4+, electronic itinerant of doping ion would suppress the FM interaction. In contrast, doping with Cr3+ increases T C . We presume that the Co-O-Cr exchange interaction exists in system and enhances the ordering temperature. ESR results confirmed this FM exchange interaction between Cr ions and Co ions. A close correlation between magnetic state and transport behavior as well as a large magnetoresistance was observed, which suggests a doubleexchange interaction. It can be deduced that this FM doubleexchange interaction comes from Cr3+(t 32g e0g )-O-Co3+(t52g e1g ) and Cr3+(t 32g e0g )-O-Co4+(t42g e1g ). Acknowledgements This work was supported by the State Key Project of Fundamental Research, China (2010CB923403), Natural Science Foundation of China (11304323/A0402) and (11174262/A0402) and Natural Science Foundation of Anhui (1208085MA10). Wei tong acknowledges the support from Youth Innovation Promotion Association of CAS and All authors would like to gratefully acknowledge the National Synchrotron Radiation Laboratory (NSRL). References [1] P.G. Radaelli, S.-W. Cheong, Phys. Rev. B 66 (2002) 094408. [2] M. Kriener, C. Zobel, A. Reichl, J. Baier, M. Cwik, K. Berggold, H. Kierspel, O. Zabara, A. Freimuth, T. Lorenz, Phys. Rev. B 69 (2004) 094417.

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