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Advances in Condensed Matter Physics
Volume 2013 (2013), Article ID 714143, 6 pages
Magnetic and Electric Properties of , () Layered Perovskites
1Energy Harvest-Storage Research Center and Department of Physics, University of Ulsan, Ulsan 680-749, Republic of Korea
2Basic Science Department, Faculty of Industrial Education, Helwan University, Saray El-Quba, Cairo, Egypt
3Department of Physics, Faculty of Science, Fayoum University, El Fayoum 63514, Egypt
4Department of Physics, Faculty of Science and Education, Taif University, 21985 Al Khurma, Saudi Arabia
Received 27 January 2013; Accepted 20 March 2013
Academic Editor: Jan Alexander Jung
Copyright © 2013 A. I. Ali and A. Hassen. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The electric and magnetic properties of layered perovskites have been investigated systematically over the doping range . It was found that both Sr1.5Y0.5CoO4 and Sr1.4Y0.6CoO4 undergo ferromagnetic (FM) transition around 145 K and 120 K, respectively. On the other hand, Sr1.3Y0.7CoO4 and Sr1.2Y0.8CoO4 compounds showed paramagnetic behavior over a wide range of temperatures. In addition, spin-glass transition () was observed at 10 K for Sr1.3Y0.7CoO4. All investigated samples are semiconducting-like within the temperature range of 10–300 K. The temperature dependence of the electrical resistivity, , was described by two-dimensional variable range hopping (2D-VRH) model at 50 K < ≤ 300 K. Comparison with other layered perovskites was discussed in this work.
Cobalt oxides have particular interest, not only because of the unique features of Co ions, but also due to their technological applications, such as solid oxide fuel cells and membranes for gas separation [1–5]. Double-exchange interaction between is known to be ferromagnetism (FM) while superexchange (SE) interaction between Co ions with the same oxidation of states is antiferromagnetism (AFM) [6–10]. The spin states of Co ions exhibit several possible spin states: low-spin (), intermediate-spin (), or high-spin () for ions and (), (), or () for ions. All spin states of or ions are possible because the crystal-field splitting energy of -state electrons () and Hund energy () are comparable for perovskite cobaltates. This implies that the energy gap between the and states is small and the electrons in can be thermally exited into the state. As a result, it is difficult to determine the spin states of these ions in cobalt oxides. In other words, complex magnetic properties arise not only in the original perovskite compounds ( is the rare earth ions) which exhibit a rather isotropic 3D arrangement of magnetic ions, but also in layered perovskite systems, , , Nd or Pr.
The layered-type cobaltates with structure are characterized by two-dimensional confinement of the B-O-B network that significantly reduces the electron bandwidth. In turn, electron correlations are strong and can alter the interplay between the different microscopic degrees of freedom (lattice charge, orbital, and spin degrees of freedom). For instance, the magnetic state of the compounds transforms from AFM to FM upon doping for . On the other hand, the zero field susceptibility of () shows different magnetic transitions . The first is due to spin-glass (SG) transition at 18 K as reported for layered manganites . The magnetic frustration necessary for the formation of a spin-glass state is attributed to the competition between double-exchange () and superexchange ( or ) interactions. The second transition is ascribed to a Griffiths singularity around 210 K, that is, the formation of short-range FM clusters with large spins in the paramagnetic matrix.
Magnetic and transport properties of the layered perovskite system , , were studied by Wang and Takayama-Muromachi . It was found that is FM with a transition temperature, K [14, 15]. Substituting Sr by Y ions, decreases to 150 K for and the ferromagnetism does not exist when the doping level exceeds 0.67. All samples are semiconducting-like. Compared to other layered perovskites such as , a semiconducting behavior was also dominant and the resistivity () follows an Arrhenius law for , 0.33, and 0.60. This electrical behavior is due to hopping of small lattice polarons, as in the case of manganites. Doping-dependent charge and spin superstructures in and (, 0.61) were reported . It was noticed that the charge carriers in these doped cobaltates are strongly localized and so that thermal activated behavior was observed. A systematic increase of the resistivity with decreasing A-site rare earth ionic radius in cobaltates was suggested  implying that upon narrowing the bandwidth, the mobility of electrons is decreased.
The number of reports of two-dimensional layered cobaltates (A2CoO4) is still relatively small compared with the isotropic three-dimensional perovskite cobaltate (ACoO3) although they are quite interesting. Therefore, an attempt has been made in this work to throw light on the electrical properties of , , and to examine the gradual disappearance of the ferromagnetic property in this range of doping. In addition, a comparison of the investigated system with similar layered perovskites was given. The systematic study of the physical properties of layered perovskites as a function of doping gives an interplay between spin, charge, and lattice/orbital degrees of freedom at the ion-rich side of the phase diagram.
Polycrystalline samples () were prepared by the solid state method. Stoichiometric mixtures of (4N, MTI), (5N, Cerac), and (4N, Aldrich) were well ground and palletized. Then the specimens were calcined at 1100°C and sintered in oxygen at 1050~1100°C for 48 h with intermediate grindings. The sintered samples were annealed in oxygen at 1150°C for 48 h. A polycrystalline sample was prepared as in . All investigated samples were confirmed to be single phase by x-ray diffraction (XRD) using CuK radiation (Bruker D8).
DC magnetization measurements were performed in a SQUID magnetometer (Quantum Design MPMS-5S). Temperature-dependent magnetization curves were measured in field cooled (FC) mode between 5 and 300 K with an applied magnetic field of 0.20 kOe. Zero-field cooled (ZEC) magnetization of and were also performed at 0.2 kOe applied field. The field-dependent magnetization was measured within kOe at 5 K. The temperature dependence of the electrical resistivity was measured both on cooling and on heating (from 10 K to 300 K) by the standard dc four-probe method using a CCR type refrigerator.
3. Results and Discussion
3.1. Magnetic Properties
The temperature dependence of the field cooled (FC) magnetic susceptibility, , of , , at 0.20 kOe is shown in Figure 1. of both and (the main frame of Figure 1) show ferromagnetic transitions, and 120 K, respectively. Increasing of (), the compounds exhibited PM behavior in a wide range of temperatures as seen in the inset of Figure 1. In addition to the PM behavior of , spin-glass (SG) transition was observed at 10 K in agreement with other cobaltates [19, 20] and manganites . No traces of a Griffith phase were identified within this range of doping compared with that in similar layered perovskites . It can be suggested that the substitution of Sr by Y ions weakens the ferromagnetic exchange because no long range ordered FM phase was found for the compounds of . The disappearance of FM in by increasing () can be attributed to two major parts compensated by a decrease of both the magnetic moments in the Co sublattices and the tolerance factor (). It was suggested previously  that both and are in intermediate spin states when .
The verification of Curie-Weiss (CW) law was examined for the investigated samples. For and 0.60 compounds, CW was verified at K as shown in the inset and the main frame of Figure 2, respectively. Further increasing (), CW behavior was observed over a wide range of temperatures (see the inset of Figure 2). A small deviation from CW was found at low temperature for this compound due to SG transition. In contrast, CW of the sample of (not shown) was verified over the entire range of temperatures. Based on the CW law, Curie-Weiss temperatures, , and the effective paramagnetic moments, , were calculated and listed in Table 1. It was noted that alters from positive to negative values for the compounds of . Similar behavior was also observed with the change of in both  and . The change of the sign of with showed that the substitution of by ions changes the magnetic transition from predominantly FM () to Curie-Weiss PM (). The effective paramagnetic moment shows moderate changes upon increasing the content of Y ions. Because the value of depends on the temperature range over which we estimated it, there is a difference between and that in previous report .
To give more evidence about fully developed FM phase, the magnetic hysteresis of all samples at K is displayed in Figures 3(a) and 3(b). As seen from Figure 3(a), the samples of and 0.60 are FM and the area of the hysteresis loop of the first compound is larger than that of the second one. A paramagnetic behavior of the samples of is evidenced by negligible hysteresis although shows slight deviations from a strict linear behavior. A very narrow hysteresis with no tendency of to saturate was found for due to SG phase (see Figure 3(b)). Remnant magnetization (), maximum magnetization at 5 K, (), and coercive field () for the investigated samples were tabulated in Table 1. The values of and of are consistent with previous report  and is comparable with that of similar perovskites .
It is better to compare ZFC- of with that of at 0.20 kOe as shown in Figure 4. One noticed that the ZFC- of is larger than that of because of the large magnetic moment of ions. More than one transition can be seen in . One transition is due to Griffiths phase () around 190 K, where there are magnetic clusters with large spins in the paramagnetic matrix. The second transition was observed around 160 K that can be speculated to short-range FM. Similar transitions were also observed in . The interplay between these different magnetic transitions provides the basis of the complex magnetic behaviors in layered perovskites. The third transition was identified around 70 K for both and and probably is due to SG transition, . This means that transition has the same origin in both systems and is probably due to the transition states of Co ions.
3.2. Electrical Properties
The normalized resistivity values of some selected samples of system are presented in Figure 5. Due to microcracks in the ceramic samples, the absolute values of resistivities may not be very reliable and the room-temperature normalization was used to show the different temperature dependencies of the resistivity of , 0.70, and 0.80. Since the resistivity of the sample of is close to that of , the resistivity of the first compound was not shown here. The temperature dependence of the resistivity exhibits semiconducting characteristics (), consistent with previous report  and the results of similar perovskites [18, 19]. The values of the room temperature resistivity () with increasing Y content were given in Table 1. One notes that there is no systematic change of with increasing .
To describe the resistivity of these compounds, different models were checked. One of them is the Mott’s two-dimensional variable range hopping (2D-VRH) model [19, 22]: where is constant and is the variable-range hopping parameter described by Mott and Davis . As shown in Figure 6(a), the temperature range over which 2D-VRH model was verified depends on . Within the temperature range of 50 K ≤ ≤ 300 K, for the compounds of , 0.6, and 0.80 obeys well 2D-VRH model. The range of the fitting ln versus of is narrow (150 K ≤ ≤ 300 K) compared with that of other studied compounds. The values of the fitting parameters were listed in Table 1.
It was found that the temperature-dependent resistivity of the investigated samples is thermally activated within the range 10 K < ≤ 50 K; that is, Arrhenius law was verified: where , , and are a material constant, the activation energy, and Boltzmann’s constant, respectively. The variation of as a function of is shown for , 0.70, and 0.80 in Figure 6(b). The calculated values of were given in Table 1. One noticed from Table 1 that the values of increase with increasing and still exhibited higher , , , and compared to that of other investigated samples because this content of Y is the boundary of vanishing ferromagnetism in the studied system.
Both magnetic and electrical properties of system have been investigated as a function of doping . The FM double-exchange disappears with increasing Y content () maybe due to the mismatch between Sr and Y ions and the decrease of the oxidation of to ions. A spin-glass ground state was observed for followed by a PM phase at high temperatures. The compounds of show PM behavior in the entire range of temperatures. With the dilution of by ions in compound, the Curie-Weiss temperature alters from positive to negative while the effective paramagnetic magnetic moment increases. The electrical conductivities of all studied samples are semiconducting-like. Based on the fitting of data, a change from thermally activated conduction to 2D-VRH one was identified with increasing temperature. Finally, preparation of single crystalline samples with performing an additional experiment was suggested to emphasize the gradual disappearance of FM phase with in this system.
Ahmed Ali was supported by the priority research centers program (Grant no. 2009-0093818) through the National Research Foundation of Korea and received funding from the Ministry of Education, Science and Technology of Korea.
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