Journal of Magnetism and Magnetic Materials 226}230 (2001) 1359}1361 In#uence of annealing time on the magnetic properties of La Sr CoO compound
N.V Khiem *, D.N.H Nam , N.X Phuc , N.N Toan Department of Science and Technology, Hongduc University, Thanhhoa, Viet Nam Laboratory of Magnetism and Superconductivity, Institute of Materials Science, NCST, Hoang Quoc Viet Str., Caugiay, Hanoi, Viet Nam Abstract For the La Sr CoO compounds with x)0.3, the formation of homogenous samples appear to depend strictly on \V V the preparation conditions that cause the electronic and magnetic data reported scatter strongly In the present contribution, the results of systematic investigation of the in#uence of annealing time (t ) on various magnetic parameters are reported for the x"0.2 substance, i.e near the percolation threshold between spin-glass and cluster-glass phases High Sr-concentration phases are initially formed at short t but gradually transformed to lower Sr-concentration ones with longer t This transformation process apparently "nishes at t +200 h and homogenous samples with the purely expected stoichiometric phase can be obtained with longer t The results from transport measurements are also mentioned in comparison to the magnetic data 2001 Elsevier Science B.V All rights reserved Keywords: Magnetic oxides; Impurity e!ects; Annealing e!ect; Phase transitions*inhomogeneity}homogeneity Partial substitution of La> with Sr> in La Sr CoO introduces very important changes in \V V the system: while the parent compound shows high resistivity and antiferromagnetic exchange interactions, the one with 0(x)0.5 evolves to a ferromagnetic and metallic behavior Magnetic diagram of this system reported by Itoh et al [1] shows a spin-glass (SG) phase for 0(x)0.18 and a cluster-glass (CG) one for 0.18(x)0.5 Nevertheless, there are still controversies regarding the exact origins of the magnetic anomalies and threshold for cluster formation The results reported are rather scattered and depend much on the conditions used for the preparation [1}6] From a crystallographic point of view the samples are of single phase, but from a magneto-electronic point of view they are inhomogeneous: upon Sr> doping, the materials segregate into hole-rich region, which are ferromagnetic and metal- * Correspondence address Laboratory of Magnetism and Superconductivity, Institute of Materials Science, NCST, Hoang Quoc Viet Str., Caugiay, Hanoi, Viet Nam Tel./fax: #84-48364403 E-mail address: dnhnam@ims.ncst.ac.vn (N.V Khiem) lic, and a hole-poor matrix similar to the undoped compound In order to get a better understanding in this problem, we have carried out the preparation of La Sr CoO samples using the solid state reaction
method with the same annealing temperature of 10503C and the annealing time t varying in a broad range, from to 550 h X-ray di!ractograms (XRD) of di!erent samples reported in Ref [6] con"rm the single-phase nature of all the samples Fig shows the in-phase susceptibility (¹) curves and the inset illustrates the Curie transition temperature, ¹ , which is determined as the position where (¹) decreases to zero, as proposed in Ref [5], for samples annealed for various t A remark from this "gure is that ¹ is strongly a!ected by t , i.e it decreases from 240 to 185 K when t increases from to 550 h As shown in Ref [1], for the La Sr CoO system the \V V ferromagnetic ordering temperature increases as x increases, i.e ¹ +180 K for x"0.2 to ¹ +250 K for x"0.5 So even a small impurity amount of compositions with x'0.2 in the nominal La Sr CoO
sample can give rise to a prominent ferromagnetic ordering at ¹'180 K This implies that the x"0.2 sample with t "3 h is highly compositional and inhomogeneous This sample may contain various phases 0304-8853/01/$ - see front matter 2001 Elsevier Science B.V All rights reserved PII: S - 8 ( 0 ) - 1360 N.V Khiem et al / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1359}1361 Fig (¹) measured at H "10 G and f"170 Hz The inset ! shows the t dependence of ¹ for the La Sr CoO samples
of La Sr CoO having di!erent x values greater than \V V 0.2, although there is no strange phase resolvable in the XRD patterns With further annealing time, ¹ shifts to lower temperatures and reaches 185 K as obtained for the sample with t "550 h The zero-"eld-cooled (ZFC) and "eld-cooled (FC) magnetization show some typical spinglass-like behavior, i.e an irreversibility between M (¹) $! and M (¹) below a temperature ¹ and a cusp shape 8$! of M (¹) at a temperature ¹ Not only ¹ but ¹ and 8$! ¹ also depend either on t or on the Sr content [6] In an attempt to check the validity of the de Almeida}Thouless line (originally proposed for ordinary SG) [7], we plotted the reduced temperature vs the external "eld H in Fig and found that could scale with H as "¹ /¹ J1!(C /¹ )HL One can clearly note 2 systematic dependence of the "tting parameters that, with increasing the annealing time from to 550 h, the parameter n decreases from 0.51 to 0.229 and C in 2 creases from 1.6 to 7.8, correspondingly It is worth to remark that with the shortest t the n value is the largest and almost close to that of the x"0.5 sample (n+0.58) [5] This once more supports our above conclusion deduced from the ¹ behavior that high Sr-content phases are predominantly easier to form in the whole preparation procedure of the La Sr CoO system The inset \V V of Fig shows that two regions of t , with border at about 200 h, can be clearly distinguished High Sr-content phases are initially formed at short t and they are transformed to lower Sr-concentration phases with longer t This transformation process apparently "nishes at t +200 h and homogenous samples with the purely expected stoichiometric phase can be obtained with longer t when the magnetic parameters are almost saturated Fig shows the temperature dependence of the resistivity (¹) for several studied samples Although the metallic behavior decreases in accordance with the Fig Reduced temperature versus H for the x"0.2 samples The inset presents the t dependence of the n and C para 2 meters Fig The (¹) curves for the di!erent x"0.2 samples transformation from high to low Sr-content, the absolute value of resistivity decreases with longer t We suppose that, in the samples annealed for short t , the high Srcontent impurity phases are in the form of clusters embedded in a much lower Sr-content matrix and high resistivity of these samples can be attributed to the contributions of the media between the clusters Sr-inhomogeneity and therefore a distribution of the ferromagnetic cluster sizes dominates the characteristic features of magnetism of the studied samples Since for the La Sr CoO system, in the literature &Sr-in\V V homogeneity' and &impurity' it has been used with the same meaning, various extrinsic magnetic and transport behavior of this system have been reported While the XRD method is not su$ciently sensitive to detect the compositional inhomogeneities, the magnetic measurements with temperature and "eld variation seem to be very useful to qualify the samples This work was supported by the National Program on Basic Research and SAREC/Sida Project on Rare Earth Material The authors thank Dr P.V Phuc for the X-ray di!raction measurements and phase analyses N.V Khiem et al / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1359}1361 References [1] M Itoh, I Natori, S Kubota, K Motoya, J Phys Soc Jpn 63 (1994) 1468 [2] M.A Sen armH s-RodrmH guez et al., J Magn Magn Mater 177}181 (1998) 935 [3] P.S Anil Kumar et al., J Appl Phys 83 (1998) 7375 1361 [4] J Mira et al., Phys Rev B 59 (1999) 123 [5] D.N.H Nam, K Jonason, P Nordblad, N.V Khiem, N.X Phuc, Phys Rev B 59 (1999) 4189 [6] N.V Khiem, D.N.H Nam, N.X Phuc, N.N Toan, N.M Proceedings of the third International Workshop on Materials Science, IWOMS'99, Hanoi, November 1999, p 349 [7] D De Almeida, Thouless, J Phys A 11 (1978) 983 ... Sr-content matrix and high resistivity of these samples can be attributed to the contributions of the media between the clusters Sr-inhomogeneity and therefore a distribution of the ferromagnetic... 136 0 N.V Khiem et al / Journal of Magnetism and Magnetic Materials 226} 230 (2001) 135 9} 136 1 Fig (¹) measured at H "10 G and f"170 Hz The inset ! shows the t dependence of ¹ for the La Sr... form in the whole preparation procedure of the La Sr CoO system The inset V V of Fig shows that two regions of t , with border at about 200 h, can be clearly distinguished High Sr-content