Investigation of Structural, Ferromagnetical, Electrical properties and semiconducting states results of polycrystalline Pr2€’xSrxCoO4 samples with (1.0 ‰¤ x ‰¤ 1.5)AbstractMagnetic, electrical and structural properties of Pr2€’xSrxCoO4 with (1.0 ‰¤ x ‰¤ 1.5) were investigated. Ferromagnetic/semiconducting states were observed in Pr2€’xSrxCoO4 with different values of compositions (1.2 ‰¤ x ‰¤ 1.5). The ferromagnetic transition near the high doping was accompanied by a semiconducting like behavior. M(H) indicated the transition from paramagnetic state to ferromagnetic state, also from M(T), the transition temperature increased to higher temperature with increasing the Sr-ratio.
The effective magnetic moment referred to that, the spin behaviors of the two cobalt ions near the high doping level to be an intermediate spin state. X-ray analysis of powder confirmed that these compounds crystallize in K2NiF4-type structure with space group I4/mmm. With addition Sr, the lattice parameter c increased, whilst, a decreased. The electrical resistivity scaled using Arrhenius law for compositions with low doping, and Variable Range Hopping Model (VRHM) for compositions with high doping of Sr.
The phase diagram of these system is deduced and the correlation among the experimental data was discussed. Keywords: Pr2€’xSrxCoO4 polycrystalline, electrical resistivity, magnetic properties, ferromagnetic/ semiconducting states and density of states__________________________________________________________________1. Introduction Recently, there has been much interest in perovskite cobalt oxide because of their spin state transitions and their technological applications . The main physical characteristics of perovskite cobalt oxide is governed by cobalt d electrons. There are 6 d electrons in Co3+ ions and 5 d electrons in Co4+ ions. The crystal-field splitting of Co d states (E€) and Hund energy (Eex) are comparable for perovskite cobalt oxide. This means that the energy gap between the t2g and eg states is rather small (10 m eV in LaCoO3), and t2g electron can be thermally exited into the eg state, resulting in the high spin states [2-8]. The perovskite La1€’xSrxCoO3 (LSCO) is regarded as a typical perovskite cobalt oxide. Because of its structural and chemical simplicity, in addition to the double exchange nature exhibited during the ferromagnetic transition (280 K), extensive studies have been conducted on the characteristics of LSCO [9″10]. The unique low resistivity of perovskite L0.5S0.5CO3, coupled with thin film processibility, makes it one of the most promising materials for modern conducting thin film applications. It is generally believed that divalent Sr ions substitute for trivalent La ions, requiring a charge compensation process in the LSCO based perovskite. One is mixed valence formation in cobalt ions, and the other is oxygen vacancy formation. If one can carefully prohibit oxygen vacancy formation, the substitution of Sr2+ for La3+ in LSCO converts an adapted number of trivalent Co3+ ion into the tetravalent Co4+ ions, which introduces a predominantly ferromagnetic order which is a result of interaction between Co4+ and Co3+ [11″14]. Other Ln1€’xSrxCoO3 systems are also show the same physical properties, which are influenced by the nature of the rare earth ion [15″17]. Structure, electrical and also magnetic properties Nd1€’xSr1+xCoO4 samples had been investigated . On the other hand many authors had studied the magnetic properties of Cobalt Oxides [19-20]. While the magnetic properties behavior with temperature for Cobalt oxides compounds were studied[21-22]. The magnetic properties and structure of La1€’xSr1+xMnO4 crystal has been studied . The doping ratio of Mn affected strongly on both of the structure and the magnetic properties of the studied samples [23-24]. Many authors had studied the magnetic properties rare earth oxides compound [25-28]. Thus far, various studies have been carried out on K2NiF4-type cobalt oxides [29″33], although the number of reports is relatively small compared with the 3D perovskite system[34-37]. In accordance with these results, they proposed a spin state transition of Co3+ ion from HS to IS. Al Daroukh et al reported that the thermo chemical stability of A2BO4 oxides with the perovskite-related K2NiF4-type structure is higher than the perovskite ABO3 oxides and the thermal expansion of A2BO4 oxides is generally lower than that of the ABO3 oxides with comparable cationic compositions . H. Zhong et al  had studied the temperature effect on Surface Properties of Perovskite-type Pr2€’xSrxCoO4±” mixed oxides. The magnetic structure of these samples were carried out with a temperature range (7″370 K) was carried out . The influence of doping on the physical properties of both LaCoO3 and HoCoO3 were studied. On the other hand the A thermo gravimetric study of The perovskite compounds (Ln0.33Sr0.67CoO3€’ґ) with (Ln=Y3+, Ho3+ and Dy3+) had been carried out . In this research, we studied the magnetic and electrical properties as a function of temperature (5K ‰¤ T ‰¤ 300K) as well as the concentration of Pr2-xSrxCoO4 (0.5 ‰¤ x ‰¤ 1.5). Using the experimental results, we successfully deduced the phase diagram of this system. Moreover the electronic and spin states of each phase was proposed. The correlations among the experimental measurements were discussed. 3. Results and Discussion 3.1 structureFig 1.a shows the XRD diffraction patterns of the Pr2-xSrxCoO4 compositions (1.0 ‰¤ x ‰¤ 1.5). These patterns correspond mainly to perovskite-type structures. As it is well known, A2BO4 mixed oxides of K2NiF4 structure can be classified into two forms, i.e. F/mmm orthorhombic phase and I4/mmm tetragonal phase. The lattices constants, a and c, of the compositions were calculated using the I4/mmm notation are shown in Fig. 1.b. With increasing the Sr2+ content the lattice constant a decreased, whilst c increased linearly. Since the atomic radius of Sr2+ ion is smaller than that of Pr3+ ion, it is expected that the increase of the lattice constant a resulting from the increase of the Sr2+ concentration (x). The experimental results confirmed that the lattice constant a increased linearly with an increase in x, confirming a uniform substitution of Pr3+ ion by Sr2+ ion. The change of the magnetic and electric properties is expected because the exchange interaction between cobalt ions as well as the crystal field splitting ґ can be strongly modified by lattice constant variation .The crystallite size (Cs) of these films has been calculated from the full width at halfmaximum (FWHM) of the most intense peak of these crystals using Sherrer’s formula  (1)where both ” is the wavelength of the used X-ray, is the Bragg’s angle and І (the FWHM of the peak) is expressed in radians and corrected for the instrumental broadening by measuring the width of a standard reference. While the dislocation density (), which refers to the number of defects in the films, is as : (2)The lattice strain (Ls) , which affects on the broaden of the X-ray peaks and also affects on the optical and dielectric results, which was determined using  (3)Another important factor for these studied samples was determined , this factor is the number of crystallites per unit area (N), which has been determined using the following equation  (4)where t is the crystal thickness. The calculate values for these studied crystals are shown in table 1 3.2. Ferromagnetic and electrical resultsThe Zero-Field-cooled magnetization of studied samples in the temperature range of (2 K < T < 300 K) is illustrated in Fig. 2a. From this figure the data of some concentrations are not included to show the difference in the compositions. One can conclude that the samples with 1.3 < x ‰¤ 1.5 are in ferromagnetic state and sample of 1.5 are the highest value for the magnetization value (~1.05 emu/mole/Oe). Also, sample with Sr = 1.5 is the higher value of transition temperature (TC). Whilst, the samples for different Sr-doping are lower magnetization and the behavior seems to be paramagnetic state.Magnetization susceptibility dependence of these samples with ferromagnetic regime were studied with a temperature range of (2 K < T < 300 K) is presented in Fig. 2b. This figure shows the transition from paramagnetic state for lower doping to ferromagnetic state for higher doping of Sr and Tc around 200 K. The isothermal magnetic behavior of the system at low temperature (5 K) with an applied magnetic field up to 50000 Oe. For samples with x = 1.0 and x = 1.1 is shown in Fig. 3a. For samples with other values of compositions (x = 1.2-1.5), ferromagnetic hysteresis was observed. The M(H) curves corresponding to the ferromagnetic samples. First of all, they showed that, the values of maximum magnetization increased with x and that at 5K none of them reached saturation under the maximum field used of 50 kOe. They also indicated that, the samples with x ‰Ґ 1.3 were hard ferromagnetic materials; their coercive fields were very high, the maximum was achieved for x = 1.4 and then decreased for sample with x= 1.5. They also showed high remnant magnetization values (MRem = 2јB/mol of Co for x = 1.4). Both magnitudes were much higher the corresponding R2-xSrxCoO4 compounds. On the other hand Fig. 3.b. presented the magnetic moment as functions of Sr-doping, as we see the magnetic moment has a constant value around 0.5 јB/mol of Co up to x = 1.3 then the magnetic moment increased rapidly for x ‰Ґ 1.3 to become 1.3 јB/mol of Co. For higher x = 1.5 doping the magnetic moment decreased into 0.8 јB/mol of Co. From the magnetic behavior we can conclude that the samples is transferred from paramagnetic state to ferromagnetic state with Sr = 1.0 ~1.5 and the transition temperature around 200 K. The electrical resistivity as a function of temperature has been summarized in Fig. 4. The resistivity of x=1 and 1.1 samples showed insulating like behavior, increasing resistivity with decreasing temperature, from room temperature to 100K. While other sample of the Sr = 1.2 showed semiconducting from 300K down to 100K then increased to become insulating like behavior. With increasing Sr (1.3‰¤ x‰¤ 1.5) the compositions showed pure semiconducting like behavior. Mott variable range hopping theory (VRH) describes the low temperature behavior of the resistivity in an extremely disordered system where the states are localized . The data from Pr2-xSrxCoO4 is best fitted with T1/4. This means that the Sr2+ doping causes strong potential fluctuation which is favorable to the VRH. The resistivity is described by; (1)where To is the reduced temperature, and it is related to the localization length by, (2)where І is the numerical factor, kB is the Boltzmann’s constant, g(ј) is the electronic density of states, and ao is the Bohr radius . We modified the VRH model equation to T1/2(n) (n=126).By studding and drawing the relation between ln () versus T1/2(n) , we have estimated the fitting factor (To) as shown with Sr-content in Table 1. To has acceptable values which are around 8.1012 K. The values of g (ј) versus Sr values of composition (x) is also shown in Table 1. It is clear that for the low doping compositions (x = 1) the density of state has nearly value ~ 1.34 — 1017 cm€’3.eV€’1. However, its value increased with increasing (Sr) reach to 1.5 to become 1.15 — 1021 cm€’3.eV€’1). The values of density of states have been estimated here roughly for the first time. To confirm the fitting for all compositions, we drew the master plot for ((ln () + Ln (o))/To) Vs T€’1/256 as shown in Fig. 4b, providing an excellent fit to Eq. (1) . We can conclude that, since Sr2+ contents can change the Co3+/Co4+ ratio, it is reasonable to assume that g(ј) is nearly depends on Sr-content. Therefore, the variation of To and g (ј) with Sr-content reflects the change of localization length. From Table (2), it is clear that, To changed with Sr-content. This implies that the disorder in the lattice decreases with Sr-doping. As the extent of disorder decreases with Sr-doping, it is evident from Fig. 4, that the conduction mechanisms in this material are not thermally activated but are related to the hopping between Co3+ and Co4+ ions.The electrical and magnetic data indicates transition temperature (TS) increased with Sr-doping from around 50K to 200K. The XRD studies show the lattice parameter a also increased with the Sr contents. Moreover, the previous studies using XRD and neutron scattering confirmed that the cell volume of Ln2-xSrxCoO4 increased with x (Sr2+). This is because the ionic radius of Sr2+ is larger than the ionic radius of Pr3+ . The unit cell volume can enlarge or the volume of CoO6 octahedral can increase, this can affect the crystal field splitting €c [1,4,8]. The increase of the transition temperature and magnetization with replacement of Pr3+ by larger Sr2+ ion can be interpreted as the flowing; the larger average A-site ionic radius causes a smaller distortion of the Co-O-Co bond and consequently, the double exchange interaction become stronger. This was reflected in the increase of ferromagnetic cluster, which exists at high doping (x=1.3~1.5) with an anti ferromagnetic phase at low temperature. While, exhibits insulator/anti ferromagnetic phase with the values of x (x=1~1.2). The transition temperature Ts dependence on x (Sr concentration) is illustrated in Fig. 5, from this figure we study the proposed phase diagram of Pr2-xSrxCoO4 by electrical and magnetization measurements, where (PM, FM, AFM, I, M, IS, LS) represent Paramagnetic, Ferromagnetic, Anti-ferromagnetic, Insulator, Metal, Intermediate Spin, and Low Spin, respectively. As shown in this figure that, the transition temperature as a function of for Pr2-xSrxCoO4 system (1.0 ‰¤ x ‰¤1.5). The transition temperatures of the these samples are changed from 50 K to 200 K depending on the values of x. However TS increased systematically with x reaching a maximum point for samples with x =1.5. It should be noted that the phase diagram is divided into two regions with respect to x. Firstly, at a low doping level (1.0 ‰¤ x ‰¤ 1.2), the samples exhibit a paramagnetic/insulator phase at Ts was around 50 K, at this temperature, the samples are transferred into an anti ferromagnetic/insulator phase. The second region with a higher doping level (1.2 ‰¤ x ‰¤ 1.5), these samples here seem to be in two phases, the first phase appear at Ts ~150k, while the second phase appeared at Ts ~200 K). All these samples with (1.2 ‰¤ x ‰¤ 1.5) are in a ferromagnetic/metallic state which transited from paramagnetic/insulator phase.3.3. semiconducting states resultsThe density of state for both the valence and conduction band were calculated using the following equations :- (3) (4)Where Nv, Nc were the density of states for both valence and conduction bans respectively, as a function of effective masses for both of electrons and holes respectively. The determined values for both Nv, Nc were shown in table 2 . From this table it is clear that the both values of Nv, Nc increase with the values of x, because the mobility of both electrons and holes increase with increasing x values.ConclusionTo conclude, we have presented a systematic study of the structural, magnetic, and electrical measurements of a series of Pr2-xSrxCoO4 samples (1.0 ‰¤ x ‰¤ 1.5). The structural data revealed that the samples were in single phase and the lattice constant increased with an increasing of the Sr2+ content. The magnetic phase transition temperatures depends on the Sr doping. The magnetic properties e of the Pr2-xSrxCoO4 system (1.3 ‰¤ x ‰¤ 1.5) seems to be in a ferromagnetic with metallic phase . At low temperature (5K) the system was confirmed to be in an anti ferromagnetic/ferromagnetic phases depending on doping of Sr. This behavior due to the double exchange between Co ions, producing ferromagnetic clusters, while the Co3+/Co4+ exchange can be an anti ferromagnetic correlation. The electrical resistivity for Pr2-xSrxCoO4 system fits well with the modified VRH model. The density of state near Fermi level energy were deduced and demonstrated that there is an increase with Sr-doping. We have also constructed the magnetic phase diagram of the Pr2-xSrxCoO4 samples. Presently, the DOS is being studied by using optical measurements, this is an opportunity to study the electronic states close to the Fermi level which are thought to be responsible for the electronic conduction process.