Low B2 crystallization temperature and high tunnel magnetoresistance in Co2FeAl/MgO/CoFe magnetic tunnel junctions

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Abstract

We present tunnel magnetoresistance values of up to 147% at room temperature and 273% at 13 K for MgO-based magnetic tunnel junctions with Co2FeAl and Co–Fe electrodes. The magnetic moment and coercive field were examined as a function of the annealing temperature by alternating gradient magnetometer investigations. This is compared with X-ray diffraction studies of the same samples and all results are contrasted to similar layer stacks based on the Heusler compound Co2MnSi.

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 EF. These material classes are oxide compounds such as Fe3O4 and CrO2 [1], perovskites (e.g. LaSrMnO3 [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 TC [5]. A Heusler compound is given by the composition X2YZ in the L21 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 L21- type-structured Heusler compounds as electrodes: 217% for Co2MnSi [9] and 220% for Co2Fe0.5Al0.5Si [10]. A maximum TMR ratio of about 50% was found for B2-type-structured Co2FeAl 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: L21 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 Co2FeAl0.5Si0.5 [18], because Co2FeSi is easy to fabricate in the L21 structure, whereas Co2FeAl has only B2-type structure. Here, we present high room temperature TMR ratios for the Heusler compound Co2FeAl. Additionally, the low B2 crystallization temperature allows us to propose Co2FeAl 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 1.0×107mbar of the sputtering system can be achieved; the Argon process pressure is about 1.5×103mbar. 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 Co2FeAl (or Co2MnSi) 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 Co2FeAl 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 Co2FeAl. We have shown that the full magnetization value of 4.9μB and high room temperature TMR ratios of about 150% can be reached. The Co2FeAl 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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