Comparison of the magnetic and electrical transport properties of La 2/3 Ca 1/3 Mn 1 x Cr x O 3 and La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3 (x = 0 and 0.

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1 Solid State Communications 141 (2007) Comparison of the magnetic and electrical transport properties of La 2/3 Ca 1/3 Mn 1 x Cr x O 3 and La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3 (x = 0 and 0.06) X. Xiao, S.L. Yuan, Y.Q. Wang, G.M. Ren, J.H. Miao, G.Q. Yu, Z.M. Tian, L. Liu, L. Chen, S.Y. Yin Department of Physics, Huazhong University of Science and Technology, Wuhan , People s Republic of China Received 23 April 2006; received in revised form 2 November 2006; accepted 7 November 2006 by J. Fontcuberta Available online 27 November 2006 Abstract Polycrystalline samples with formulas La 2/3 Ca 1/3 Mn 1 x Cr x O 3 and La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3 (x = 0 and 0.06) have been prepared. The structural, magnetic, electrical transport and magnetoresistance properties have been investigated comparatively. Compared with La 2/3 Ca 1/3 Mn 1 x Cr x O 3, the magnetization and conductivity are enhanced in La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3 at low temperatures. It is found that there are two peaks in the resistivity versus temperature curve for La 2/3 Ca 1/3 Mn 1 x Cr x O 3 and the additional peak disappears for La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3 (x = 0.06). The experiment results are discussed by considering a variety of tolerance factors, the ratio of [Mn 3+ ]/([Mn 4+ ] + [Cr 3+ ]), the effects of A-site radius and the A-site mismatch effect. We suggest that these peculiar effects of Cr doping could be the consequence of a possible double exchange interaction between Mn 3+ and Cr 3+. c 2006 Elsevier Ltd. All rights reserved. PACS: h; Kz Keywords: A. Perovskite manganites; B. Metal insulator transition; D. Double exchange; D. Magnetization 1. Introduction The perovskite manganites such as Ln 1 x A x MnO 3 (Ln being rare-earth ions and A generally divalent alkaline earth elements) have been the subject of intense research because of their exotic magnetic and electrical properties [1 5]. The conventional understanding of electrical transport and magnetic properties is generally based on the double exchange (DE) mechanism [6], which considers the ferromagnetic (FM) coupling and the e g electrons hopping through Mn 3+ O Mn 4+ network. However, theoretical considerations indicate that the DE mechanism alone could not quantitatively account for all the physics in these compounds. The Jahn Teller distortions, lattice, spin and orbital degrees of freedom should be considered [7,8]. Many previous studies have been reported on the effects of Mn site substitution by foreign elements in La 2/3 (Ca, Sr) 1/3 MnO 3 such as Fe, Ga [9], Al [10], Co [11], Corresponding author. Tel.: ; fax: address: slyuan@hust.edu.cn (S.L. Yuan). Ti [12], Cr [13 18], etc. Generally, doping at Mn site decreases the Curie temperature (T c ) and the metal insulator (M I) transition temperature (T p ). However, since Mn acts as the heart of DE interaction, doping at Mn sites is the only way to experimentally test the possibility of DE hetero-ionic coupling between Mn and other ions. Among these doping elements, Cr 3+ has the same electronic configuration t 3 2g as and a similar ionic radius to that of Mn 3+, these features make Cr 3+ an interesting ion to study for the possible existence of DE between Cr 3+ and Mn 3+. More recently, several groups have made systematic investigations in Crdoped manganites [13 24]. Some of the literature argues that the magnetic coupling between Cr 3+ and Mn 3+ is FM due to superexchange (SE) and/or DE interaction [13 20]; others indicate that the antiferromagnetic (AFM) interaction must be an important factor in the interpretation of their results [21 24]. These results have not come to terms with the nature of the Mn 3+ O Cr 3+ interaction. For this reason, substituting Cr for Mn has not yet been conclusive, and remains an open question. Mn /$ - see front matter c 2006 Elsevier Ltd. All rights reserved. doi: /j.ssc

2 X. Xiao et al. / Solid State Communications 141 (2007) Table 1 Experimental values on structural, magnetic and electrical transport properties of LCMO, LCMCr and L + CMCr Sample a (Å) b (Å) c (Å) V (Å 3 ) T p (K) T c (K) θ (K) LCMO LCMCr L + CMCr Fig. 1. XRD patterns for La 2/3 Ca 1/3 Mn 1 x Cr x O 3 and La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3 (x = 0 and 0.06) at room temperature. In order to scrutinize the magnetic exchange interaction in Cr doping manganites further, compared with the reported La 2/3 Ca 1/3 Mn 1 x Cr x O 3, where the La/Ca ratio is maintained at 2:1 and the [Mn 3+ ]/[Mn 4+ ] ratio decreased, it is meaningful to investigate the electrical transport and magnetic properties of La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3 with the [Mn 3+ ]/([Mn 4+ ] + [Cr 3+ ]) ratio fixed at 2:1 by assuming that Cr 3+ plays the role of Mn 4+ in the Mn 3+ O Cr 3+ interaction, just like in that of the optimal doped system La 2/3 Ca 1/3 MnO 3 with the [Mn 3+ ]/[Mn 4+ ] ratio fixed at 2:1. In this paper, we report a comparative study of electrical transport and magnetic properties of polycrystalline samples La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3 and La 2/3 Ca 1/3 Mn 1 x Cr x O 3. In order to avoid promoting the AFM SE interaction of Cr 3+ O Cr 3+, we choose a lower Cr doping level with x = 0.06 to investigate comparatively. 2. Experiments Polycrystalline samples of La 2/3 Ca 1/3 Mn 1 x Cr x O 3 and La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3 (x = 0 and 0.06) were prepared by the conventional solid-state reaction method. Stoichiometric amounts of high purity La 2 O 3, CaCO 3, MnO 2 and Cr 2 O 3 powders were thoroughly mixed and heated in air to 800 C for 24 h to achieve decarbonization. Then, the materials were annealed at a temperature of 1000 C for 24 h and 1200 C for another 24 h with intermediate grinding, respectively. After being reground to a fine powder and pressed into pellets, the samples were sintered in air at 1450 C for 30 h followed by slow cooling to room temperature. The structural characterization of samples was examined by using X-ray diffraction (XRD) (X Pert PRO, PANalytical B.V.) at room temperature. The temperature dependence of dc magnetization under field cooling was taken in magnetic fields of 0.1 and 3 T. The real (χ ) and imaginary (χ ) components of ac susceptibility (χ) versus temperature were obtained in an ac magnetic field of 10 Oe with a frequency of 77 Hz. The electrical properties is experimentally studied by measuring resistance as a function of temperature by means of the standard four probe method without an applied magnetic field and with 3 T. The magnetoresistance (MR) ratio is defined as MR 0 (%) = [(ρ(0) ρ(h))/ρ(0)] 100%, where ρ(0) is the zero field resistivity and ρ(h) is the resistivity with the applied magnetic field of H = 3 T. All electrical transport and magnetic properties were performed by a commercial physical property measurement system (PPMS, Quantum Design) from 10 to 300 K. 3. Results and discussion XRD patterns are shown in Fig. 1 for La 2/3 Ca 1/3 MnO 3 (LCMO), La 2/3 Ca 1/3 Mn 1 x Cr x O 3 (LCMCr) and La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3 (L + CMCr) (x = 0.06) polycrystalline samples. The XRD analysis indicates that all samples are single phase with perovskite orthorhombic structure. The similarity between the crystal structures of the pure and Cr doped samples suggests that the doped Cr 3+ does not obviously change the crystalline structure of pure LCMO, which may mainly result from the similar ionic size of Mn 3+ and Cr 3+. The lattice parameters of the samples are calculated and the results are summarized in Table 1. Compared with pure LCMO, the cell volume of the sample LCMCr decreases, which is consistent with the smaller radius of Cr 3+ to that of Mn 3+. However, the cell volume of the L + CMCr sample increases, which can be attributed to the increase of the average radius of the A-site cation. This fact suggests that the doped Cr 3+ ions substitute mainly for the Mn 3+ ions in LCMO. That is, the doped Cr 3+ ions are located at the B-sites of ABO 3 perovskite. Then we perform a comparative study of dc magnetization as a function of temperature (M T ) for the samples. The experimental curves measured at H = 0.1 and 3 T are plotted in Fig. 2. All samples show a steep paramagnetic ferromagnetic (PM FM) phase transition around T c. The T c is defined as the inflection point of the M T (H = 0.1 T) curve and is listed in Table 1, where T p and the PM Curie temperature θ are also displayed. Compared with other elements doped in LCMO at the same impurity content [9 12], the decrease of T c is suppressed a little for LCMCr and L + CMCr. For the present samples, at a high magnetic field 3 T and low temperature 10 K, the magnetization of LCMO is nearly equal to the theoretical value 98 (emu/g) as shown in Fig. 2(a).

3 350 X. Xiao et al. / Solid State Communications 141 (2007) Fig. 2. Temperature dependence of magnetization of La 2/3 Ca 1/3 Mn 1 x Cr x O 3 and La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3 (x = 0 and 0.06), measured in magnetic fields H = 0.1 T (bottom) and H = 3 T (top). Fig. 3. Temperature dependence of real (χ ) and imaginary (χ ) parts of the ac susceptibility of La 2/3 Ca 1/3 Mn 1 x Cr x O 3 and La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3 (x = 0 and 0.06) in ac field of H = 10 Oe at 77 Hz. In Fig. 2(b), in a low magnetic field 0.1 T and low temperature 10 K, the magnetization of LCMO is not saturated. The magnetization values of LCMCr and L + CMCr are smaller than those of LCMO due to the substitution Cr 3+ for Mn 3+. However, an unusual magnetization behavior is observed in Fig. 2. The T c of L + CMCr is lower than that of LCMCr, and their M T curves show a point of intersection at temperatures of 201 K, in Fig. 2(a), and 191 K, in Fig. 2(b), respectively. The temperature dependence of ac susceptibility (χ T ) obtained at a frequency of 77 Hz and 10 Oe field is shown in Fig. 3. All samples exhibit a steep PM FM transition, and the χ T dependence does not display obvious differences. A maximum at high temperature in the χ T curves should be the PM FM transition observed in dc M T curves. Simultaneously, there is a peak in the χ T curves and the peak appears at the corresponding temperature related to the PM FM transition. The PM Curie temperature θ is obtained by extrapolating the high temperature data of 1/χ to zero and is shown in Table 1. One can see that θ has the same behavior as T c, that is, θ is suppressed slightly by Cr doping. The resistivity versus temperature (ρ T ) and corresponding MR ratio for LCMO, LCMCr and L + CMCr are presented in Fig. 4, respectively. All samples show an insulating behavior above the temperature T p and enter a metallic state below T p. Compared with LCMO, it can be seen that T p decreases and the resistivity increases with Cr doping, which can be attributed to the weakening of the DE interaction. In Fig. 4(b), the ρ T curves of LCMCr show more complicated and interesting electrical properties, which are consistent with previous studies [14 18]. The M I transition shifts to lower temperature, meanwhile, an additional broad peak grows up after it. Under 3 T field, the resistivity is suppressed drastically for both the peaks, and while the original peak near T c shifts to higher temperature, the additional peak seems to be independent of the field. Fig. 4(c) displays the transport behavior of L + CMCr. One can see that the ρ T curves of L + CMCr resemble those of LCMO, which is different from LCMCr exhibiting double peaks. Corresponding to the zero field resistivity, the ρ T curve of L + CMCr also presents a single peak in 3 T field. The temperature dependence of colossal magnetoresistance (CMR) is shown in Fig. 4 also. The application of the 3 T field causes a substantial decrease in peak resistivity and a shift of T p to higher temperature, which are characteristic features for intrinsic CMR. For L + CMCr, there is a visible enhancement of CMR effect over that of LCMO and LCMCr, meanwhile, the temperature range of CMR response is enormously broadened simultaneously. The comparative study of ρ T curves for LCMCr and L + CMCr is presented in Fig. 5. One can see that the T p of L + CMCr is lower than that of LCMCr. Meanwhile, their ρ T curves show a point of intersection at a temperature of 192 K without an applied field. With an applied field of 3 T, the crossing of the curves occurs at a temperature of 214 K and the unusual electrical transport behavior remains. A similar phenomenon appears in the dc magnetization measurement in Fig. 2.

4 X. Xiao et al. / Solid State Communications 141 (2007) Fig. 4. Temperature dependence of resistivity (under magnetic field of H = 0 and 3 T) and corresponding MR ratio for La 2/3 Ca 1/3 Mn 1 x Cr x O 3 and La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3 (x = 0 and 0.06). Fig. 5. Temperature dependence of resistivity (under magnetic field of H = 0 and 3 T) of La 2/3 Ca 1/3 Mn 1 x Cr x O 3 and La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3 (x = 0.06). On basis of the experimental results, we assume that these peculiar effects of Cr doping could be the consequence of a possible DE interaction between Mn 3+ and Cr 3+. In Cr doping manganites, the inefficiency in driving T c and θ suggests that Cr doping does not destroy DE ferromagnetism remarkably, which is different from doping with other elements. This feature implies that Cr may take part in the FM exchange interaction [17,18]. Many authors have observed the presence of double peaks in resistivity measurements in the La 2/3 Ca 1/3 Mn 1 x Cr x O 3 system. However, there is no agreement between the different studies on the origin of the second peak. Sun et al. [17] reported double peaks in the same system and ascribed the additional peak to a possible occurrence of DE interactions through Mn 3+ O Cr 3+. On the other hand, Roy et al. [16] argued that the presence of Cr 3+ on the Mn 3+ sites gives rise to a FM interaction through Mn 3+ O Cr 3+ and the occurrence of the additional peak is due to charge ordering and the AFM nature of Cr 3+. Other authors gave different views about this phenomenon also. From these consistent results, the double peak feature of the resistivity can be regarded as an intrinsic characteristic and ascribed to the crucial role of Cr 3+ in lower Cr doping manganites La 2/3 Ca 1/3 Mn 1 x Cr x O 3. Rao et al. [26] related the double peaks to the ratio of Mn 4+ in a certain range, < [Mn 4+ ] < 0.5, in La 1 x Ca x MnO 3 compounds. One possible explanation for the appearance of double peaks in LCMCr is to assume that Cr 3+ acts in the role of Mn 4+. Thus, the ratio of ([Mn 4+ ] + [Cr 3+ ]) is 0.39 and is just in the range of < [Mn 4+ ] < 0.5, which can explain the appearance of double peaks in LCMCr. The original peak near T c, which is related to the M I transition, originates from DE interactions in both Mn 3+ O Mn 4+ and possible Mn 3+ O Cr 3+ couples, so it shifts to higher temperature under a 3 T field. The additional peak almost does not shift in magnetic field, which implies that this resistivity peak originates from a delicate balance between AFM insulating and FM metallic phases apart from the DE interaction of Mn 3+ O Cr 3+. However, the ρ T curves of L + CMCr display a single M I transition peak that originates from DE interactions without an additional peak in the low temperature range. Such a disappearance of the additional peak in L + CMCr may be interpreted in terms of the ratio of [Mn 3+ ]/([Mn 4+ ] + [Cr 3+ ]) fixed at the optimal value 2:1. In L + CMCr, except for the DE interactions of Mn 3+ O Mn 4+ and Mn 3+ O Cr 3+, which mediate FM and metallic conduction, there are AFM SE interactions such as Mn 4+ O Cr 3+ and Cr 3+ O Cr 3+, but the former are overwhelming in the dual ratio of [Mn 3+ ]/([Mn 4+ ] + [Cr 3+ ]), so its ρ T curves show a single peak. The absence of an additional peak in L + CMCr is a signature of the FM DE Cr 3+ O Mn 3+ interaction since the ratio [Mn 3+ ]/([Cr 3+ ] + [Mn 4+ ]) corresponds to the optimal value 2:1. This is supported by the fact that the second peak is absent for the LCMO. There exists a point of intersection in the M T curves of the two Cr-doped samples, as well as in the ρ T curves. As seen in Fig. 2, for L + CMCr, the [Mn 3+ ]/([Mn 4+ ] + [Cr 3+ ]) ratio is fixed at 2:1, which should have good ferromagnetism, but the T c of L + CMCr is lower than that of LCMCr. The tolerance factor t is defined as ( r A + r O )/ 2( r B + r O ) [7] and is listed in Table 2. Here, r A is the average ionic radii of A-sites ions, r O is the oxide ion radius and r B is the average ionic radii of the B-sites ions, respectively. The t becomes small for L + CMCr, meaning that the Mn O Mn angle deviates from the ideal value of 180 and the MnO 6 octahedron tilts more, which lead to a reduction of DE interactions. In contrast, the t increases for

5 352 X. Xiao et al. / Solid State Communications 141 (2007) Table 2 The tolerance factor (t), A-site cation size r A, A-site mismatch effect (σ 2 ) and the ratio of [Mn 3+ ]/[Mn 4+ ] and [Mn 3+ ]/([Mn 4+ ] + [Cr 3+ ]) of LCMO, LCMCr and L + CMCr Sample t r A (Å) σ 2 (10 4 Å 2 ) [Mn 3+ ]/[Mn 4+ ] [Mn 3+ ]/([Mn 4+ ] + [Cr 3+ ]) LCMO :1 2:1 LCMCr : :0.39 L + CMCr :0.27 2:1 According to the Shannon radii [25]: r 3+ La = 1.22 Å, r2+ Ca = 1.18 Å, r3+ Mn = Å, r4+ Mn = 0.53 Å, r3+ Cr = Å, ro 2 = 1.28 Å. LCMCr, which will be of benefit to DE interactions to a certain extent. Obviously, the influence of t on magnetic properties may be the main factor for the present samples, which leads to T c of L + CMCr being lower than that of LCMCr. For the same reason, the magnetization of L + CMCr is lower than that of LCMCr. But below the corresponding temperature of the point of intersection, the magnetization of L + CMCr becomes larger than that of LCMCr. Thus, there might exist a poor DE between Mn 3+ and Cr 3+ coupling at high temperature, this FM interaction leads to a parallel alignment of Cr 3+ and Mn 3+, which becomes strong with a decrease in temperature. Thus, the [Mn 3+ ]/([Mn 4+ ] + [Cr 3+ ]) ratio is 2:1 for L + CMCr, but is 0.61:0.39, which deviates from the optimal proportion of 2:1, for LCMCr, and then the magnetization of L + CMCr becomes larger than that of LCMCr in the low temperature range. In this scenario, we suggest that there exists a poor DE between Mn 3+ and Cr 3+ coupling at high temperature so that Cr 3+ cannot play the role of Mn 4+ in the Mn 3+ O Cr 3+ interaction. Correspondingly, Cr 3+ partially plays the role of Mn 4+ in the Mn 3+ O Cr 3+ interaction in the low temperature range. Now we discuss the point of intersection in Fig. 5. Generally, a stronger DE always corresponds to a higher T p. As we know, there are three factors which have been shown to strongly affect T p [27], i.e. the hole carrier density controlled by the [Mn 3+ ]/[Mn 4+ ] ratio, r A, and σ 2 defined by σ 2 = yi ri 2 r A 2. These are shown in Table 2. As a rule, increasing r A would strengthen the DE mainly due to the increase of the one-electron e g band, while decreasing σ 2 would prevent the localization of e g electrons consequently strengthening the DE. Considering the factors of r A and σ 2, the T p of L + CMCr should be higher than that of LCMCr, however, one can see that this is contrary to Fig. 5. In the high temperature region, Cr 3+ cannot play the role of Mn 4+, so the [Mn 3+ ]/[Mn 4+ ] ratio is considered. This ratio is 0.67:0.27 for L + CMCr, and is 0.61:0.33 for LCMCr, respectively. Compared with L + CMCr, there is a suitable ratio that approaches the optimal proportion 2:1 for LCMCr. Thus, the influence induced by the [Mn 3+ ]/[Mn 4+ ] ratio plays a dominant role for the variation of T p. For the same reason, the resistivity of L + CMCr is larger than that of LCMCr. With a decrease in temperature, Cr 3+ partially plays the role of Mn 4+ in the Mn 3+ O Cr 3+ interaction, Mn 3+ and Cr 3+ ions turn to favor FM ordering, and the probability of the electron transfer between Mn 3+ and Cr 3+ ions increases, which would promote the conductivity. Then, the [Mn 3+ ]/([Mn 4+ ]+[Cr 3+ ]) ratio must be considered. This ratio for L + CMCr is 2:1, but for LCMCr is 0.61:0.39, which deviates from the optimal proportion of 2:1. Thus, below the corresponding temperature of the point of intersection, the resistivity of L + CMCr becomes smaller than that of LCMCr. Certainly, the DE interaction of Mn 3+ O Cr 3+ is weaker than that of Mn 3+ O Mn 4+. The different effect of the crystal field over Mn 4+ and Cr 3+ causes an energetic difference between the e g orbital levels of Mn 3+ and Cr 3+, which becomes strong enough to result in an electronic exchange between them that is not equivalent to that between Mn 3+ and Mn 4+. Hence, the DE interaction between Mn 3+ and Cr 3+ is similar to the DE interaction between Mn 3+ and Mn 4+ but the former would be weaker than the latter. 4. Conclusions In summary, we have presented a comparative study on the electrical transport and magnetic properties of polycrystalline samples of La 2/3 Ca 1/3 Mn 1 x Cr x O 3 and La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3 with x = 0 and Compared with the double peaks in ρ T curves and CMR behavior for La 2/3 Ca 1/3 Mn 1 x Cr x O 3, a single M I transition peak and an extraordinary CMR effect have been observed in La 2/3+x Ca 1/3 x Mn 1 x Cr x O 3. 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