Revival of metastable behavior in phase separated La 0.5 Ca 0.5 MnO 3þd Affia Aslam a, * , Wiqar Hussain Shah a , Bakhtyar Ali a , S.K. Hasanain a , M.J. Akhtar b , M. Nadeem b a Department of Physics, Quaid-i-Azam University, Islamabad 45320, Pakistan b Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan Received 14 March 2003; received in revised form 23 July 2003; accepted 7 October 2003 by T.T.M. Palstra Abstract The La x Ca 12x MnO 3þd compositions close to charge ordering ðx , 0:5Þ show a gradual relaxation from a metallic/ferro- magnetic state to an insulating/antiferromagnetic state with thermal cycling. Here, we report on the magnetic relaxation in the metastable state and also the revival of the metastable state (in a relaxed sample) due to high temperature thermal treatment. We also show the changes in the magnetization and the thermoelectric power as the revived metastable state is cycled. We find that the changes in the thermoelectric power extend well into the region above the charge ordering temperatures. This suggests that the micro-structural changes accompanying the thermal cycling leave their imprint in the paramagnetic insulating state as well. q 2003 Published by Elsevier Ltd. PACS: 71.30. þ h; 75.30.Vn; 75.30.Kz; 75.60.Lr; 72.60. þ g Keywords: A. Magnetically ordered materials 1. Introduction The mixed valence manganates of the general formula R 12x A x MnO 3 (R ¼ rare earth metals and A ¼ alkaline earth metals, e.g. Ca þ2 , Sr þ2 , or Ba þ2 ) have attracted consider- able attention due to the wide variety of spin, charge and orbital degrees of freedom they manifest [1]. The main focus of attention in these materials is currently on the question of phase coexistence and separation, particularly close to the x ¼ 0:50 compositions, and the relationship of the same to the phenomenon of colossal magnetoresistance (CMR) [2–4]. The CMR composition La 0.5 Ca 0.5 MnO 3 represents the boundary between competing ferromagnetic (FM) and charge ordered anti-ferromagnetic (CO-AFM) ground states [2,5]. At this boundary the FM-metallic state becomes unstable to an insulating CO-AFM state [6,7]. The physical properties close to this phase boundary arise from a strong competition among ferromagnetic double exchange inter- action, an antiferromagnetic superexchange interaction, and the spin-phonon coupling [8–12]. A variety of measure- ments [13,14] have shown the co-existence of the two phases in the form of FM micro-domains embedded in a CO-AFM matrix [15,16]. Recent measurements have also shown the presence of meta-stable states in the two-phase region of these compositions and the conversion of one phase into another [2,6]. In our previous work [6], we presented data on the resistivity, AC susceptibility and the irreversible changes produced by thermal cycling in the x ¼ 0:50 and x ¼ 0:52 compositions. The unique aspect of our work was the observation of these effects in a nominally ‘true’ or unperturbed composition unlike many other cases where dopants lead to instabilities. We showed that the metastable state at low temperatures gradually relaxes into the stable antiferromagnetic and more resistive state with thermal cycling. Two main questions are posed by the metastable 0038-1098/$ - see front matter q 2003 Published by Elsevier Ltd. doi:10.1016/j.ssc.2003.10.004 Solid State Communications 129 (2004) 267–271 www.elsevier.com/locate/ssc * Corresponding author. Tel.: þ92-512-829-472; fax: þ 92-519- 210-256. E-mail address: afiaaslam@yahoo.com (A. Aslam).