Glassy ferromagnetism in La 0.75 Sr 0.25-x K x CoO 3 polycrystalline perovskite cobaltites N. Mahfoudh & M. Koubaa & W. Cheikhrouhou-Koubaa & A. Cheikhrouhou Received: 25 January 2013 /Accepted: 11 November 2013 # Springer Science+Business Media New York 2013 Abstract In this paper, the structural and magnetic properties of La 0.75 Sr 0.25-x K x CoO 3 (0≤x≤0.15) powder samples are re- ported. X-ray diffraction analysis using Rietveld refinement show that all our samples crystallize in the rhombohedral structure with R 3c space group. At low temperatures, with increasing K amount, the samples change from ferromagnetic like behavior (x =0, 0.05 and 0.1) to spin glass one (x =0.15). The ferromagnetic-paramagnetic transition temperature de- creases with increasing K amount from 235K (x =0) to 200K (x =0.1). In the paramagnetic phase, all our synthesized samples obey the Curie-Weiss law. In the vicinity of T C , the magnetic entropy change |ΔS M | for La 0.75 Sr 0.2 K 0.05 CoO 3 sample reached maximum values of 0.47Jkg/K and 2.27Jkg/ K under magnetic field variation in the range 1T and 7T, respectively. The magnetization hysteresis loops demonstrate a weakening the ferromagnetism with increasing K content. Keywords Cobaltites . Magnetic properties . Spin state . Hysteresis 1 Introduction Cobalt oxides with perovskite structure having the general for- mula Ln 1-x M x CoO 3 (Ln is a rare earth element and M is a divalent alkali earth element) have been the subject of avid interest in the recent years for their unusual magnetic behavior and intriguing transport properties as well as practical applications [1–5]. The strong competition between the crystal-field splitting energy and the intra-atomic Hund ex- change interaction leads to several possible spin states for Co 3+ and Co 4+ ions [6, 7]. Co 3+ ions (3d 6 ) can be in low-spin (LS, S =0, t 2g 6 ), intermediate-spin (IS, S =1, t 2g 5 e g 1 ) or high-spin (HS, S =2, t 2g 4 e g 2 ) states. Co 4+ ions have a 3d 5 configuration with LS (S =½, t 2g 5 ), IS (S =3/2, t 2g 4 e g 1 ) or HS (S =5/2, t 2g 3 e g 2 ). Partic- ularly, it is the temperature, the applied magnetic field, the pressure and the chemical tuning that determine the spin states for both Co 3+ and Co 4+ ions. The magnetic properties in these compounds are highly sensitive to several perturbations such as (i) the doping level x (the Co 4+ amount), (ii) the average size of the A cation site <r A > and (iii) the disorder degree called mismatch effect in the A site and given by σ 2 ¼ X y i r 2 i − r A h i 2 ð1Þ (y i : fractional occupancy) [8–11]. Thus, cobaltites are char- acterized by a complex interplay between structural, magnetic and electrical properties leading to rich magnetoelectronic phase diagram. Monovalent element doping is usually used to adjust the ratio of Co 3+ /Co 4+ ; however, the variations of <r A > and σ 2 are introduced at the same time. The present work is devoted to investigate the influence of K doping in La 0.75 Sr 0.25-x K x CoO 3 system to obtain further information how the properties are affected as a result of increasing <r A >. 2 Experimental techniques Polycrystalline La 0.75 Sr 0.25-x K x CoO 3 (0≤x≤0.2) samples were synthesized using the standard solid state reaction method at high temperature, by mixing La 2 O 3 , SrCO 3 ,K 2 CO 3 and Co 3 O 4 up to 99.9% purity in the desired proportions. The starting materials were intimately mixed in an agate mortar N. Mahfoudh : M. Koubaa (*) : W. Cheikhrouhou-Koubaa : A. Cheikhrouhou Laboratoire de Physique des Matériaux, Faculté des Sciences de Sfax, B.P. 1171, 3000 Sfax, Tunisia e-mail: mohamedkoubaa@yahoo.fr A. Cheikhrouhou Institut NEEL, CNRS, B.P.166, 38042 Grenoble cedex9, France J Electroceram DOI 10.1007/s10832-013-9877-x