Peculiarities in the electrical and magnetic properties of cobalt perovskites Ln1−xMxCoO3 (Ln3+: La3+, M2+: Ca2+, Sr2+, Ba2+; Ln3+: Nd3+, M2+: Sr2+)

Peculiarities in the electrical and magnetic properties of cobalt perovskites Ln1−xMxCoO3 (Ln3+: La3+, M2+: Ca2+, Sr2+, Ba2+; Ln3+: Nd3+, M2+: Sr2+) M.A. Señarís Rodrígueza, , M.P. Breijob, S. Castroa, C. Reya, M. Sáncheza, R.D. Sánchezb, J. Mirac, A. Fondadoc, J. Rivasc a Dpt. Química Fundamental e Industrial, Universidad de A Coruña, 15071 A Coruña, Spain b U.A. Tecnología de Materiales y Dispositivos, Centro Atómico de Bariloche, 8400 San Carlos de Bariloche, Argentina c Dpt. Física Aplicada, Universidad de Santiago de Compostela, 15706 Santiago de Compostela, Spain International Journal of Inorganic Materials Volume 1, Issues 3–4, September–October 1999, Pages 281–287 Available online 29 October 1999 doi:10.1016/S1466-6049(99)00042-2 Abstract We refer here to the electrical and magnetic properties of the Ln1−xMxCoO3 systems (Ln3+: La3+, M2+: Ca2+, Sr2+, Ba2+; Ln3+: Nd3+, M2+: Sr2+), paying special attention to those ferromagnetic compounds that display M–I transitions as temperature rises: La1−xMxCoO3 (M2+: Ca2+, Sr2+, Ba2+) in the compositional interval x=0.2–0.3, and Nd1−xSrxCoO3, with x=0.40. The magneto-transport properties of such materials are peculiar and interesting: they show diodic behavior and large relaxation effects — these latter being specially important in the Nd compound — they display © 1999 Elsevier Science Ltd. This manuscript version is made available under the CC-BY-NC-ND 4.0 license (https:// creativecommons.org/licenses/by-nc-nd/4.0/) magnetoresistive effects specially at the M–I transition temperatures, and they age with time. All these results are discussed on the basis of the inhomogeneous electronic structure of these doped cobalt perovskites and taking into account the influence of the lanthanide ion on their magnetic and electrical properties. Keywords A. oxides; D. electrical properties; D. magnetic properties 1. Introduction The interplay between magnetism and electrical conductivity in perovskite-type oxides has been a matter of study in the last decades [1] and with renewed interest since the discovery of colossal magnetoresistance (CMR) in doped manganese-based compounds [2] and [3]. Within this family of compounds, cobaltites constitute another specially interesting system in view of the peculiar way their magnetic and electrical properties change with temperature and upon doping [4], [5], [6],[7] and [8], and more recently in view of the magnetoresistive effects found in La1−xMxCoO3 (M=Sr, Ba) systems [9], [10] and [11]. Early work on the undoped LaCoO3 compound [4] established the existence of a thermally induced spin transition at the Co ions responsible for its peculiar behavior [5]. Since then many studies have tried to elucidate in more detail the characteristics of such transition, not only in LaCoO3[12], [13], [14] and [15] but also in other LnCoO3 (Ln=rare earths) compounds [13], [16], [17] and [18]. Those early studies also showed that while LaCoO3 shows high resistivity and antiferromagnetic exchange interactions, upon doping the La1−xSrxCoO3 materials evolve towards a ferromagnetic metallic behavior[6]. Since then, the magnetic and electrical properties of the La1−xSrxCoO3 system have been repeatedly investigated, finding that the evolution takes place smoothly, and that a number of different magnetic and electrical behaviors are present for the different degrees of doping: superparamagnetism, spin-glass/cluster-glass behaviors, etc., semiconducting/metallic behaviors, and even metal–insulator transitions as a function of temperature [7] and [19]. Those studies have also been extended to Ca- and Ba-doped La1−xMxCoO3 (M=Ca, Ba) materials[8] and [11], and to other Ln1−xMxCoO3 systems (Ln=rare earths) [8] and [20] that similarly show interesting electrical and magnetic properties. Although the general trend in these Ln1−xMxCoO3 series of compounds (Ln=La, rare earth, M=Ca, Sr, Ba) is again that the materials tend to metallic and ferromagnetic behavior as x increases, the characteristics of that evolution depends on the nature of the lanthanide ion [8] and [20] and the nature of the dopant [8]. And while the properties of the end members of the systems seem to be well established, different magnetic and electrical behaviors have been reported for the intermediate members of the series [7], [8] and [19]. It is in that intermediate compositional range that we have focused our studies. And in this paper we report peculiar magneto-transport properties exhibited by certain Ln1−xMxCoO3 (Ln3+: La3+, M2+: Ca2+, Sr2+, Ba2+; Ln3+: Nd3+, M2+: Sr2+) samples with x in the range 0.2–0.4 for which we find ferromagnetic behavior and metal-insulator transitions as temperature rises. 2. Experimental La1−xMxCoO3 (M=Ca, Sr, Ba) and Nd1−xSrxCoO3 were prepared by decomposition of the corresponding mixture of nitrates. Stoichiometric amounts of dry La2O3/Nd2O3, CaCO3/SrCO3/BaCO3 and Co(NO3)2 were dissolved in nitric acid. The resulting solution was gently warmed up so as to slowly evaporate the solvent. The so-obtained mixture of nitrates was decomposed at 600°C. The resulting products were pressed into pellets and annealed at temperatures ranging from 970°C, for the Nd1−xSrxCoO3 series, up to 1175°C, for the La1−xCaxCoO3 samples. In all cases, after the thermal treatment at high temperatures, the samples were cooled slowly to room temperature (0.7°/min). The product materials were examined by X-ray powder diffraction with a Siemens D- 5000 diffractometer and Cu Kα=1.5418 Å radiation. The morphology and size of the particles were studied in a scanning electron microscope (SEM) Jeol 6400. Thermogravimetric analyses (TGA) and iodometric titrations under an argon atmosphere were carried out to study the oxygen content of the samples. Magnetic properties were studied in a DMS-1660 Vibrating-Sample Magnetometer. Zero-field cooled (ZFC) and field cooled (FC) magnetic susceptibility data were obtained in a field of 1000 Oe from 77 to 330 K. ZFC magnetization curves M(H) were obtained with fields±10 kOe at 77 K. The electrical resistivity ρ was measured as a function of temperature in the range 77<T≤300 K in a zero magnetic field (H=0) and with a constant field (H=5 kOe) with a dc four-probe method using silver paint contacts. These contacts were renewed or even replaced by four new indium contacts or gold sputtered on to the samples, and the measurements repeated when detecting anomalous changes in the resistivity of the samples, in order to discard effects between sample and electrical contacts. The resistivity data were obtained, as commonly done, by calculating the averages ρ+ and ρ− (+ and− stand for the directions of the current) out of 10 values, respectively, and getting an average resistance ρ=(ρ++ρ−)/2. But in some cases, as it will be shown below, only one polarity of the current was used for the experiments, thus measuring either ρ+ or ρ−. The magnetoresistance of the samples was measured between 0 and 9 kOe, specially at those temperatures at which the ρ(H) vs. T curves, corresponding to H=0 and H=5 kOe, showed bigger separation. 3. Results 3.1. Sample characterization According to their X-ray diffraction patterns, the following compounds were obtained as single-phase crystalline materials: La1−xCaxCoO3 and La1−xBaxCoO3 for 0≤x≤0.30; La1−xSrxCoO3 for 0≤x≤0.50; Nd1−xSrxCoO3 for 0≤x≤0.40. These results also show that in the La1−xMxCoO3 samples, the introduction of the divalent ions progressively reduces the rhombohedral distortion present in the parent LaCoO3 compound and increases the volume of the unit cell, in good agreement with data reported in the literature [6] and [8]. In the case of the Nd1−xSrxCoO3 system, for 0≤x≤0.40 the compounds have all cubic perovskite structures with a≈2ac, the cell parameter increasing with x. As for the particle size and morphology of these samples, SEM micrographs show that, while La1−xSrxCoO3 and Nd1−xSrxCoO3 consist of small spherical particles of average size d≈0.5 μm, in the Ca- and Ba-doped compounds, due to the higher temperatures used in their synthesis, sintering between particles has already taken place, resulting in plaquets 3.5 and 2 μm long, respectively. 3.2. Magnetic results Upon substitution of the trivalent La3+ or Nd3+ ions by the divalent M=Sr2+, Ca2+, Ba2+ ions, the La1−xMxCoO3 and Nd1−xSrxCoO3 materials evolve towards ferromagnetic behavior. The critical x to achieve magnetic percolation is x=0.20 in the case of the La1−xSrxCoO3, La1−xBaxCoO3and Nd1−xSrxCoO3 systems, while it increases to x=0.30 in the La1−xCaxCoO3 series. The Curie temperatures vary from system to system, although within each series Tc increases only very slowly with x, so that: Tc(La1−xCaxCoO3)≈145 K, Tc(La1−xBaxCoO3)≈175 K, Tc(La1−xSrxCoO3)≈250 K,Tc(Nd1−xSrxCoO3)≈225 K ( Fig. 1a–d). Fig. 1. Temperature dependence of the ZFC and FC molar magnetic susceptibility and the electrical resistivity of (a) La0.75Sr0.25CoO3, (b) La0.70Ca0.30CoO3, (c) La0.80Ba0.20CoO3, and (d) Nd0.70 Sr0.30CoO3 samples . The χ(T) data follow a Curie–Weiss law for≈250 K<T<330 K. From the corresponding χ−1(T) fittings we can obtain information about how the effective magnetic moment per cobalt ion (μeff-Co) and the Weiss constant (θ) are evolving with x in the different Ln1−xMxCoO3 series. In the case of the La1−xMxCoO3 (M=Sr, Ca, Ba) systems, μeff-Co is directly obtained as μeff-Co=μeff= , while in the case of the Nd1−xSrxCoO3 series the rare earth contribution to the obtained μeff has to be considered and substracted (μeff(Nd3+)=3.8 μB). The results obtained for the different series show that μeff-Co are in general rather similar for the La1−xSrxCoO3 and La1−xCaxCoO3 series, while they are higher in the case of the La1−xBaxCoO3compounds and smaller in the Nd1−xSrxCoO3 system. ( Fig. 2a). Fig. 2. (a) Effective magnetic moment per cobalt ion, and (b) Weiss constant corresponding to the La1−xMxCoO3 (M=Ca, Ba) and Nd1−xSrxCoO3 systems in the compositional interval 0.20≤x≤0.40. As for θ, it is seen to increase with x in all four series ( Fig. 2b) so that the negative values found for lower doping degrees change to higher positive values as x gets higher, in good agreement with literature data[6] and [8]. As for the M(H) curves corresponding to the ferromagnetic samples, they show that at 77 K none of them reaches saturation under a field of H=10 kOe ( Fig. 3), even if the values of maximum magnetization increase with x in all the studied series. Fig. 3. ZFC magnetization M versus applied field H of x=0.30 La0.70 M0.30CoO3 (M: Ca, Sr, Ba) and Nd0.70 Sr0.30CoO3 samples, measured at 77 K. If we compare M(H) loops corresponding to the different Ln1−xMxCoO3 systems ( Fig. 3), we find that the values of maximum magnetization, extrapolated magnetization (result of the linear extrapolation of M(H) for higher fields back to H=0) and remnant magnetization follow the sequence: La1−xBaxCoO3>La1−xSrxCoO3≫La1−xCaxCoO3>Nd1−xSrxCoO3. As for the coercive field, it is remarkably higher in the Nd1−xSrxCoO3 system, where it achieves a maximum value of ≈3800 Oe for x=0.30, and then decreases to 3200 and 2400 Oe for x=0.40 and for x=0.20, respectively. It should be noted here that the magnetic properties of these samples are not seen to experience temporal evolution, at difference with what occurs with their electrical behavior (see below). 3.3. Electrical properties When measuring the resistivity as a function of temperature ρ(T), the first general observation is that the resistivity of the samples decreases as the doping degree increases, in full agreement with results reported in the literature [8] and [20] ( Fig. 4), and that a smooth transition from semiconducting to metallic behavior takes place. The doping degree at which the electrical percolation is achieved depends on the system: it is,x=0.30 for La1−xBaxCoO3 and La1−xSrxCoO3; x>0.30 for La1−xCaxCoO3; and x>0.40 for Nd1−xSrxCoO3. Fig. 4. Electrical resistivity versus temperature of Nd1−xSrxCoO3 samples (0≤x≤0.40). Very interestingly, for compositions very close to the electrical percolation threshold, we find samples that experience M–I transitions as temperature rises. Among them, the compounds La0.70 Ca0.30CoO3, La0.80Sr0.20CoO3, La0.75Sr0.25CoO3 and La0.80Ba0.20CoO3, that show I–M–I transitions, also known as a ‘reentrant semiconducting behavior’ (RSB) [7], i.e., the samples are semiconducting below a TMI1 and above a TMI2, and metallic for TMI1<T<TMI2 ( Fig. 1a–c); and the Nd0.60Sr0.40CoO3 sample, which is semiconducting for T<TMI=160 K and metallic above it ( Fig. 1d). In this case, only a small change in the positive slope of the ρ(T) curve takes place at Tc and the sample remains metallic up to room temperature. These samples with M–I transitions are very sensitive to external influences. Although from the crystallographic and magnetic point of view the samples seem to be stable and are not seen to experience temporal evolution, their electrical resistivity does indeed change with time. Depending on the nature and characteristics of the sample this aging takes place at a different speed, being slower for the La0.80Ba0.20CoO3 and La0.70 Ca0.30CoO3 samples. In these cases it is possible to see the resistivity of the samples progressively increase in the interval of several weeks, until the RSB disappears giving rise to a purely semiconducting behavior in the whole temperature range [21]. La1−xSrxCoO3 (x=0.20 and 0.25) samples also arrive to this final situation, but much faster. It is also interesting to note that electrical current is seen to help to that ‘aging’: after making several runs ofρ(T) measurements the resistivity of the samples increases ( Fig. 5), specially if the electrical current is applied only in one direction. This effect is specially important in the Nd1−xSrxCoO3 samples. Fig. 5. Evolution of the electrical resistivity of La0.80Sr0.20CoO3 when making several runs of ρ(T) measurements. Also, very surprisingly, the value and even temperature dependence of the resistivity are greatly affected by the polarity of the applied electrical current. For example, for La0.80Ba0.20CoO3: ρ+(T) shows a semiconducting behavior, ρ−(T) is smaller and shows a M–I transition, ρav, measured inverting the polarity of the current, gives intermediate values and a RSB ( Fig. 6). Solid State Phys, 46 (1992), p. 147 23. E.L. Nagaev Phys Stat Sol (B), 186 (1994), p. 9 24. C.H. Ahn, R.H. Hammond, T.H. Geballe, M.R. Beasley, J.M. Triscone, M. Decroux, Fisher, L. Antognazza, K. Char Appl Phys Lett, 70 (1997), p. 206 25. R.D. Sánchez, J. Mira, J. Rivas, M.P. Breijo, M.A. Señarı́s-Rodrı́guez J Mater Res., 14 (1999), p. 2533 26. R. Ramesh, A. Krishnan, D. Keeble, E. Poindexter J Appl Phys, 81 (1997), p. 3543 27. S. Chikazumi Physics of magnetism, Krieger, Malabar, FL (1986) chs. 7 and 10 Paper presented at the First International Conference on Inorganic Materials, Versailles, France, 16–19 September, 1998.