Interface Stability in Hybrid Transition Metal-Oxide Magnetic Junctions
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Abstract Recent experiments revealed an apparently bias-dependent tunneling magnetoresistance between a transition metal (such as Fe, Co) and an oxide barrier such as SrTiO 3. We examine the materials issues involved in this type of hybrid transition metal-oxide junctions. The junction interface is shown to be unstable against thermal treatment or high-bias current stress. We conclude that the junction magnetoresistance is largely determined by the formation of an interface oxide layer different from the barrier or the transition metal electrode themselves.
Introduction Doped manganites such as La0.6 7Sr 0. 33MnO 3 (LSMO) have attracted much attention because of their strong conduction band spin-polarization. Recently, for study of spin-polarized tunneling, junctions were fabricated between LSMO and a ferromagnetic transition metal such as Co[1-4]. In some of such structures, a thin layer of eptaxially grown SrTiO 3 (STO) was used as tunneling barrier[2-4]. In these junctions it was observed that the sign of the junction's tunneling magnetoresistance is bias-dependent. Similar junction MR sign-reversal upon changing bias condition has also been observed in magnetic tunneling junctions between two Permalloy (Fe2oNiso) electrodes with a double-layer barrier material A120 3/Ta 2 0 5 sandwiched in between[5]. Since this type of tunneling spectroscopy has the potential of revealing the electronic structure of the electrodes involved[2-5], it is important to understand and control the role materials chemistry play at the junction interface. This is the objective of this study. Two model systems were chosen. One is a trilayer structure of LSMO-STO-Fe, the other of LSMO-STO-CosoFe20 (LSMO-STO-CoFe). Junctions made from these trilayer films reveal an unstable interi'ace between the transition metal and the STO barrier. The interface property appears to be the controlling factor for the junction's magnetoresistance behavior - both in terms of its field-dependence and its bias dependence, including the sign of the junction magnetoresistance (MR).
Materials synthesis and device fabrication The trialyers were deposited on (110) cut NdGaO 3 substrates, 1 cm x 1 cm in size. The bottom layer was a 600 A thick LSMO film, epitaxially grown using laser ablation. Then a thin layer of SrTiO 3 film, nominally 30 A thick, was epitaxially deposited under the same condition using laser ablation. For koth layers, the deposition was carried out in 300 mTorr of oxygen background pressure at a substrate block temperature of 780 C. The substrates were thermally anchored to the holder with silver paste. A Nd-YAG laser was used for the ablation process, operating in frequency tripled mode (355nm) with a repetition rate of 10 Hz. The energy density on target surface was around 2 J/cm2 per pulse. This deposition procedure is similar to what has been used before for all-oxide deposition, the films were cooled to ambient temperature in 300 Torr of oxygen. They were then transported to another vacuum system, where the transition metals, 100 A of eit
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