Evidence for strontium segregation in La 0:7 Sr 0:3 MnO 3 thin films grown by pulsed laser deposition: consequences for tunnelling junctions R. Bertacco a, * , J.P. Contour b , A. Barth elemy b , J. Olivier c a INFM, Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy b Unit e Mixte de Physique CNRS/Thales, 91404 Orsay Cedex, France c TRT, 91404 Orsay Cedex, France Received 18 January 2002; accepted for publication 8 March 2002 Abstract This work addresses the issue of the quality of La 0:7 Sr 0:3 MnO 3 (LSMO) surfaces integrated in tunnelling junctions. From a detailed X-ray photoemission spectroscopy analysis it results that Sr segregates at the surface of LSMO films grown by pulsed laser deposition at high O 2 pressure (350 mTorr). This fact is attributed to the presence of an out- ermost SrO layer, which is critical for the subsequent formation of a good LSMO/insulator interface. In particular we found that the unique properties of the junctions employing SrTiO 3 as insulator, are strictly correlated to the inte- gration of the SrO segregated layer in the SrTiO 3 structure. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Surface segregation; Magnetic interfaces; Heterojunctions; X-ray photoelectron spectroscopy; Photoelectron diffraction 1. Introduction Since the discovery of colossal magneto resis- tance, perovskite manganese oxides have attracted more and more attention [1]. The very high spin polarization at the Fermi level (E F ) of some com- pounds, reflecting their half metallic behaviour [2,3], combined with the relatively high value of the Curie temperature, makes them quasi ideal materials to be employed in magnetoelectronic devices like tunnelling junctions. This is the case of La 0:7 Sr 0:3 MnO 3 (LSMO) which has been success- fully employed as spin injector and spin analyser in various structures as LSMO/insulator/ferromagnet [4] and LSMO/insulator/LSMO [5,6]. In the group of Paris, much work has been done on the trans- port and magnetic properties of these junctions and also on the bulk properties of the films inte- grated in the junctions. However only a few monolayers near the interface are really important for the spin dependent transport phenomena, so that a systematic study of the surfaces and inter- faces involved constitutes a central issue for un- derstanding the physics of real devices and improving the deposition technique. In particular the growth of well defined super- lattices presents some technical problems to be Surface Science 511 (2002) 366–372 www.elsevier.com/locate/susc * Corresponding author. Tel.: +39-02-23996132; fax: +39-02- 23996126. E-mail address: riccardo.bertacco@polimi.it (R. Bertacco). 0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0039-6028(02)01546-7