Low B2 crystallization temperature and high tunnel magnetoresistance in magnetic tunnel junctions
Introduction
To date, four kinds of materials have been theoretically predicted to show half metallic behavior, i.e. they are 100% spin polarized at the Fermi level . These material classes are oxide compounds such as and [1], perovskites (e.g. [2]), zinc-blende-type CrAs [3] and Heusler compounds [4]. In particular, Co-based Heusler compounds are the promising materials for spintronics applications due to the required high Curie temperatures [5]. A Heusler compound is given by the composition in the structure, where X and Y are transition metal elements and Z is a group III, IV or V element.
In 2004, room temperature tunnel magneto resistance (TMR) ratios of more than 100% were reported for MgO-based magnetic tunnel junctions (MTJs) [6], [7]. Recently, Ikeda et al. presented TMR ratios of over 600% at room temperature and over 1100% at low temperatures [8]. High room temperature TMR ratios have also been reported for magnetic tunnel junctions containing - type-structured Heusler compounds as electrodes: 217% for [9] and 220% for [10]. A maximum TMR ratio of about 50% was found for B2-type-structured so far [10], [11].
The predicted half-metallicity for Heusler compounds should lead to much higher TMR ratios. Nevertheless, one has to meet two challenges to achieve half-metallicity: structure of the Heusler electrode(s) and coherent interfaces of the Heusler compound and the MgO tunnel barrier. It was reported by Tezuka et al. that Si is important for a good ordering of the Heusler compound [18], because is easy to fabricate in the structure, whereas has only B2-type structure. Here, we present high room temperature TMR ratios for the Heusler compound . Additionally, the low B2 crystallization temperature allows us to propose as a buffer layer for other Heusler compounds.
Section snippets
Preparation
DC/RF magnetron sputtering was used for the preparation of our magnetic tunnel junctions. All films were deposited at room temperature. A base pressure of of the sputtering system can be achieved; the Argon process pressure is about . The layers were deposited on an MgO (0 0 1) substrate covered by a 5 nm thick MgO buffer layer to coat surface contaminations. Thereafter, the lower electrode containing of 20 nm (or ) was deposited from a stoichiometric target
Results and discussion
The magnetic major and minor loop of the junction with the highest TMR ratio is shown in Fig. 1. A TMR ratio of about 147% at room temperature and more than 270% at 13 K was achieved. This is about 3 times the value previously reported by Inomata/Tezuka et al. for this compound [10], [11].
Fig. 2 shows the magnetization for different annealing temperatures of the layers investigated by an alternating gradient field magnetometer (AGM) at room temperature. The calculated bulk magnetic
Summary
In summary, we investigated the magnetic, structural and transport properties of the Heusler compound . We have shown that the full magnetization value of and high room temperature TMR ratios of about 150% can be reached. The layers show (0 0 1) texture in the B2 structure already in the as prepared state.
The authors gratefully acknowledge the German Bundesministerium für Bildung und Forschung (BMBF) for financial support within the project HeuSpin. We are indebted to C.
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