Effect of sol-gel method on structural and electron magnetic resonance properties of Pr0.6Sr0.4MnO3 manganite

Structural and electron magnetic resonance studies in a broad temperature range are reported for Pr0.6Sr0.4MnO3 manganite synthesized by sol-gel method. Temperature dependence of magnetic resonance spectra is analyzed in the paramagnetic state and compared to similar systems. Using the temperature variation of signal intensity the activation energy is calculated.


Introduction
Perovskite manganites (ABO 3 ) with general formula R 1-x A x MnO 3 (R=La,Pr etc. and A=Ca, Sr etc.,) have attracted much attention for interesting magnetic refrigeration based on the magnetocaloric effect [1]. Therefore, the materials with large magnetocaloric effect are actively searched and examined [2] on their higher energy efficiency and environmental friendliness superior to the conventional vapor-compression-based refrigeration. The discovery of the large magnetocaloric effect in the hole-doped perovskite manganites [3,4] as well as the observation of colossal magnetoresistance effect [5] in these materials, have attracted intensive interest [6][7][8][9]. The origin of the ferromagnetic state and metallicity has been explained on the basis of the double exchange (DE) and superexchange (SE) mechanisms. In many studies, it has been shown that the strength of DE and SE interactions in manganites is very sensitive to the variation of the average ionic radius of the Asite or Mn-site ions and the carrier density controlled by the Mn 3+ /Mn 4+ ratio [10] . In previous work we have reported the electron magnetic resonance of Pr 0.6 Sr 0.4 MnO 3 synthesized by conventional solid state method [11]. In order to understand the effect of the elaboration method on physical properties, the present work reports the properties of Pr 0. 6  Suitable amount of citric acid (CA) was employed as complexation agent (1.5 mol of citric acid corresponds to 1mol of metal cation). The resulting mixture was well magnetically stirred for 2 h in order to ensure complete dissolution of citric acid and to obtain stable metal-CA complexes. At this stage, the ethylene glycol (EG) was added to the solution as chelating agent. Then, the temperature was slowly increased to 90°C under constant stirring for about 4h to evaporate the excess solvent and to accelerate the poly-etherification reaction between CA and EG. When the solvent in the system was completely evaporated, a homogeneous red resin with a glassy appearance was obtained. This resin was then dried at 150°C for about 6h to evaporate the nitrogen and obtain a black fine powder which was then crushed and heated in air at 300°C before sintering at various temperatures (400°C-800°C) for 6h hours. After cooling, the sample was pressed into a pellet (of about 1mm thickness and 13mm diameter), and then sintered at 900 and 1000°C for 12h in air with intermediate regrinding and repelleting. Phase purity, homogeneity and cell dimensions were determined by powder X-ray diffraction at room temperature. Structural analysis was made using the standard Rielveld technique [12,13]. Electron magnetic resonance measurements were performed with a Bruker spectrometer, operating at 9.44 GHz (Xband).

Structural studies
The room-temperature X-ray diffraction pattern of Pr 0.6 Sr 0.4 MnO 3 shown in Figure 1 indicates that our sample has a single phase and can be indexed in the orthorhombic structure with Pnma space group. The structural parameters were refined by the standard Rietveld technique. The quality of the refinement is evaluated through the indicator χ 2 equal to 1.59. Lattice parameters are found to be equal to a=5.4156(1) Å, b=7.6540(7) Å and c=5.4709(5) Å, the unit cell volume is equal to 227.436(4) Å 3 . The unit cell parameters and volume are relatively reduced as compared to the values observed for polycrystalline Pr 0.6 Sr 0.4 MnO 3 manganite elaborated by the solid state reaction [11].

EMM-FM2011
The average crystallite size was evaluated from a width of diffraction peaks using Sherrer formula [14] and is found to be equal to 26 nm.

EMR results
In Figure 2, we plot the electron magnetic resonance (EMR) spectra of the Pr 0.6 Sr 0.4 MnO 3 recorded in the temperature range 200 -370 K. The maximum of intensity I(T) is observed at T= 250 K. This temperature is lower than 315 K found for the Pr 0.6 Sr 0.4 MnO 3 manganite elaborated by solid state method. In the paramagnetic phase the EMR spectrum consists of a single approximately symmetrical line with a shape close to a Lorentzian derivative. Using the EMR measurements the line width ∆H, the asymmetry, the intensity and the g eff values were calculated. ∆H is deduced from the peak-to-peak distance between the maximum and the minimum of the derivative of the EMR absorption, the asymmetry is defined by the maximum to minimum ratio. The intensity of the response is defined by β(∆H) 2 , where β is the peak amplitude. The g eff value is determined via the following equation: g eff = hυ/µ B H res (1) where h is the Planck constant, µ B is the Bohr magneton, υ is the frequency and H res is the resonance field. We should note that the resonance field diminishes slowly in the paramagnetic phase from 3366 Oe at 370 K down to 3348 Oe at 200 K. The similar behavior was reported by R. Thaljaoui et al. [11] for Pr 0. 6  We plot in Figure 3 the temperature dependence of g eff . One may see that at T=370 K the corresponding value of g eff is equal to 2.00054, which falls into a range characteristic for doped manganites in the paramagnetic state [15,16]. We should also note that the line width ∆H is a non monotonic function of temperature (not plotted) for a broad temperature interval. The ∆H exhibits a minimum at 310K. One may conclude that a transition temperature is located somewhat below 310K as it is observed in manganites [11,17].  Figure 4 shows the temperature variation of signal intensity I(T). In the ferromagnetic phase the signal intensity is an abruptly diminishing function of temperature. In the paramagnetic phase I(T) increases slowly. A similar behavior was observed for Pr 0.6 Sr 0.4 MnO 3 manganite prepared by solid state reaction [11]. Above 250 K the intensity diminishes abruptly with temperature rising up to 310 K, reaches a minimum and then increases slowly. One should notice that the accuracy of applied I (T) calculation is high enough only in paramagnetic phase, where signals are more symmetric ( Figure 5). Therefore the I(T) dependence below T C offers only a rough approximation. In the paramagnetic phase the evolution of the signal intensity versus temperature can be expressed as follows: where k B is the Boltzmann constant and E 1 is the activation energy. The activation energy E 1 equal to 67.6 meV was deduced from Ln(I) versus 1/T dependence. Such activation energy is somewhat smaller than values reported for similar manganites [11].

Conclusions
The structural study shows that our present sample crystallizes in the orthorhombic structure with Pnma space group and the mean crystallite size is about 26 nm. Rietveld refinement indicates a decrease in the lattice volume of our sample compared to the manganite Pr 0.6 Sr 0.4 MnO 3 prepared by solid state reaction. The transition temperature is somewhat lower as compared to the Pr 0.6 Sr 0.4 MnO 3 manganite prepared by the solid state method. It can be related to the influence of sol-gel method on grain size and its distribution. In the paramagnetic phase the EMR spectra consists of a single line with a shape close to a Lorentzian one and the resonance field corresponds to a geff factor close to 2 and is temperature independent.