Journal of Magnetism and Magnetic Materials 272–276 (2004) e1525–e1526 Spin-dependent tunneling in magnetic tunnel junctions with Al 2 O 3 , MgO, NiO and hybrid structures Laurianne Gabillet a, *, Boucar Diouf a , Jean-Fran@ois Bobo a , David Serrate b , Jose-Maria De Teresa b a Laboratoire de Physique de la Mati " ere Condens ! ee, FRE 2686, CNRS, 135 avenue de Rangueil, INSA, 31077 Toulouse Cedex 4, France b Instituto de Ciencia de Materiales de Aragon, Universidad de Zaragoza, CSIC, Pza. S. Francisco, s/n, Zargoza 50009, Spain Abstract The aim of this study is to investigate spin polarization in magnetic tunnel junctions and to separate the barrier and interfaces nature contributions. NiO/Co/tunnel barrier/Co junctions were sputtered with two kinds of tunnel barriers: (i) ‘‘classical’’ barriers, formed by a 10–20 ( A of elemental oxide Al 2 O 3 or MgO, (ii) ‘‘hybrid’’ barriers, formed by three subnanometric layers : Al 2 O 3 /MgO/Al 2 O 3 , MgO/Al 2 O 3 /MgO or Al 2 O 3 /NiO/Al 2 O 3 . r 2004 Elsevier B.V. All rights reserved. PACS: 73.40.Gk; 73.40.Rw Keywords: Magnetic tunnel junctions; Hybrid barrier; Spin polarization 1. Introduction Despite the large research efforts on magnetic tunnel junctions (MTJ) and the potential applications they promise like MRAM, the exact mechanism of spin- dependent tunneling (SDT) is still not well understood. Julli " ere [1], in his initial model, assumed tunnel magnetoresistance (TMR) based on the bulk spin polarizations of the two ferromagnetic (F) electrodes. Further studies [2] have indicated that the tunnel barrier nature and its interfaces with the F electrodes also play an important role on the spin polarization of the tunnel current and the TMR value. 2. Experimental details MTJ were sputtered on float glass (FG) substrates, with the following layer sequence FG/NiO (300 ( A)/Co (50 ( A)/ tunnel barrier/Co (100 ( A). Contact masks were used to pattern junctions areas ranging from 100 Â 100 to200 Â 500 mm 2 .TheNiOantiferromagnetic(AF)layer acts as a pinning layer of the bottom Co electrode. Moreover a magnetic field of 1kOe was applied during deposition, leading to an unidirectional anisotropy in the F layer. Junctions with two kinds of tunnel barriers wereprepared:(i)‘‘classical’’tunnelbarrier,formedbya 10 to 20 ( A-thick Al 2 O 3 or MgO elemental oxide, (ii) ‘‘hybrid’’ barriers, formed by three subnanometric layers: Al 2 O 3 /MgO/Al 2 O 3 , MgO/Al 2 O 3 /MgO or Al 2 O 3 / NiO/Al 2 O 3 . Oxides layers were obtained by natural oxidation of Al and Mg metal layers. I–V curves and TMR loops of the junctions were measured at room temperature with a standard four- probe set-up and an electromagnet operating up to 1500Oe. 3. Results In previous studies [3], we have investigated the optimal barrier thickness for classical tunnel junctions. With our fabrication process, classical junctions with Al 2 O 3 barrier give a reproducible maximum TMR of ARTICLE IN PRESS *Corresponding author. Tel.: +33 (0)5-61-55-96-67; fax: +33 (0)5-61-55-96-97. E-mail address: gabillet@insa-tlse.fr (L. Gabillet). 0304-8853/$-see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.1056