Magnetocaloric effect of monovalent K doped manganites

Magnetic and magnetocaloric properties are reported for polycrystalline monovalent potassium doped manganites Pr0.6Sr0. 4-xKxMnO3 (x = 0, 0.05, 0.1, 0.15 and 0.2) crystallized in orthorhombic structure with Pnma space group. The increasing K content shifts the paramagnetic to ferromagnetic transition temperature from 310 K for x = 0 to 269 K for x = 0.2. The magnetic entropy change under magnetic field variation of 2T is found to be 1.95, 3.09, 2.89, 3.05 and 3.2 Jkg−1K−1for x varying from 0 to 0.2, respectively. The highest relative cooling power of 102 J/kg is observed for the undoped sample. The sensitivity of magnetic entropy change to magnetic field is estimated by a local N(T) exponent exhibiting the characteristic temperature variation. Phenomenological universal curves of entropy change and Arrott plots confirm the second order phase transition. Introduction The magnetocaloric effect, known for more than 130 years, has been used for a long time to achieve the lowest temperatures. Fifteen years ago the discovery of giant magnetocaloric effect near room temperature has launched considerable interest and hope for practical applications in refrigerating systems competitive to traditional gas compression / expansion refrigerators [1]. However, the giant magnetocaloric effect (MCE) is observed only in a few materials with strong coupling between magnetic and lattice systems leading to first order magnetic phase transition entangled with structural changeover occurring in relatively narrow temperature interval. Since these systems contain expensive elements, like Gd, there is a search for alternative materials. It was found that several mixed valence manganites substituted on A site, exhibit the weaker magnetocaloric effect spread over a broader temperature interval, which may offer enough cooling power. Moreover, the manganites can be easy manufactured, are stable and cheap. Besides the possible technological applications the magnetocaloric studies enable the basic insight into the nature of the phase transitions. The selected manganites revealing a strong MCE, still remain the promising candidates for magnetic refrigerating systems and require additional complementary characterization. The manganites are well known to exhibit the transition from the paramagnetic insulator to ferromagnetic metallic with colossal magnetoresistive (CMR) behavior. This phenomenon has been explained on the basis of Zener double exchange theory involving the mobile electrons eg traveling between the Mn3+ and Mn4+ cations [2, 3]. Further investigations have confirmed that other factors such as ionic size mismatch, bond length Mn-O and bond Author postprint of the paper published in Journal of Magnetism and Magnetic Materials 352 (2014) 6–12 angle Mn-O-Mn, strongly affect physical properties [4, 5]. Several authors have reported that the substitution in the A sites by divalent elements (Ba, Ca) can improve physical properties [6, 7], since the addition of the divalent e.g. Sr2+ converts the antiferromagnetic insulating PrMnO3 to the metallic Pr0.6Sr0. 4MnO3 ferromagnet. Recently the new investigation field is focused on the manganites doped with monovalent elements, like Na, K, Li and Ag [8-10]. Monovalent elements are known to convert twice more Mn3+ to Mn4+ ions as compared to divalent ones [11]. Moreover, the crystal structure may be modified depending on the radius of monovalent ions. This in turn affects the effective magnetic interaction. It was also observed that the charge ordering appears for some doping limit, e.g. in Pr0.8Na0.2MnO3 manganites [12]. Previously we have studied the effect of the reduction of the mean ionic radius of A site in Pr0.6Sr0. 4MnO3 system with Na doping, on the magnetocaloric effect [13, 14]. In the present paper we extend the previous studies and report the structural and magnetocaloric properties of Pr0.6Sr0. 4-xKxMnO3 (x= 0, 0.05, 0.1, 0.15 and 0.2) samples for which the Sr2+ ions are partially replaced with larger K+ ions [15]. This in turn changes the Mn3+ / Mn4+ ratio and enhances the mean ionic radius of A-site. Temperature dependence of magnetic entropy change is analyzed by means of the so called universal curve [16-18]. An influence of applied magnetic field on the magnetic entropy change and relative cooling power are also discussed in order to assess potential application in magnetic refrigerating systems. Experimental details Powder samples Pr0.6Sr0. 4-xKxMnO3 (x = 0, 0.05, 0.1, 0.15 and 0.2) were elaborated by the standard solid state reaction at high temperature. Stoichiometric ratio of Pr6O11, SrCO3, K2CO3 and MnO2 (99.9%) was mixed in an agate mortar and then heated in air to 1000°C for 60 h with intermediate grinding. The obtained mixtures were then pressed into pellets and sintered at 1100 °C in air for 60 h with intermediate grinding. Phase purity, homogeneity and cell parameters were studied by the X-ray diffraction with Cu-Kα radiation (1.54 Å) in the 2Ɵ range of 10 100 degrees. Structural analysis was carried out using the standard Rietveld technique [19, 20]. The morphology and grain size distribution were studied by SEM. The magnetic measurements of K doped manganites were carried out using ppms system in applied magnetic field up to 2 T and the undoped manganite was measured with vibrating sample magnetometer up to 7 T. Structure The room temperature X-ray diffraction patterns for x = 0.15 and 0.2 are shown in Fig. 1A-B. The remaining samples diffractograms (x = 0, 0.05 and 0.1) were reported separately [13, 21]. The diffractograms confirm that all Pr0.6Sr0. 4-xKxMnO3 (x = 0 to 0.2) magnanites studied are a single phase with orthorhombic structure corresponding to the Pnma space group. No traces of any other secondary or impurity phase were detected. Structural parameters obtained by Rietveld analysis of the powder XRD data are listed in Table 1. The unit cell parameters vary smoothly with potassium content x, as shown in Fig. 2A. It is worth to notice that the so called O’ phase exists for all x values, as indicated by the relation 2  a b [22]. This is a signature of Jahn – Teller effect causing a distortion of MnO6 octahedra, which removes the remaining degeneration. Potassium is observed to slightly increase the unit cell volume from 229.108 Å3 for x = 0 to 229.295 Å3 for x = 0.2 (Fig. 2B). Such a volume increase is caused by the considerable difference between the ionic radii of Sr2+ and K+. For the ideal perovskite structure with 12-fold coordination of the A sites the ionic radii of Sr2+ and K+ are equal to 1.44 Å and 1.64 Å, respectively [15]. On the other hand, the ionic radii are known to shrink gradually down to 1.18 Å and 1.38 Å for Sr2+ and K+, Author postprint of the paper published in Journal of Magnetism and Magnetic Materials 352 (2014) 6–12 respectively, corresponding to the lowest six-fold coordination [15]. The more precise determination of ionic radii from the present structural data is not possible since the raising K doping level can modify both the lattice distortion and the local coordination. One may also notice the correlation between the mean ionic radius of the A site and the tolerance factor t, determined according to Shannon values [15] and with coordination 9, is varying from 0.9292 up to 0.9544, as seen in Tab. 1. The average crystallite sizes of the prepared manganites are estimated by the DebyeScherrer method, which considers a full width at half maximum of the most intense XRD peak. Tab. 1 shows the fine microstructure with average crystallite sizes (Cs) close to 50 nm for the studied K doped manganites. The SEM images (Fig. 3A-D) show that the grains of manganites studied are irregularly spherical like and the mean sizes GSEM are spread between 600 and 900 nm, which are slightly larger than those reported previously for Pr0.6Sr0. 4-xNaxMnO3 (x = 0 to x =0.2) [13]. The agglomeration degree defined by the ratio SEM G to crystallite size CS [23] is about 16. This ratio confirms that the grains observed by SEM consist of multiple crystallites [24]. The average angle was used to determine the tilt angle in the plane of the bond, which is an argument in cosine function in eq. 1. The average bond length listed in the Tab. 1 is in turn used to determine the bandwidth W according to the following approximate formula [25, 26]: