Tunnel magnetoresistance in alumina, magnesia and composite tunnel barrier magnetic tunnel junctions

https://doi.org/10.1016/j.jmmm.2011.01.011Get rights and content

Abstract

Using magnetron sputtering, we have prepared Co–Fe–B/tunnel barrier/Co–Fe–B magnetic tunnel junctions with tunnel barriers consisting of alumina, magnesia, and magnesia–alumina bilayer systems. The highest tunnel magnetoresistance ratios we found were 73% for alumina and 323% for magnesia-based tunnel junctions. Additionally, tunnel junctions with a unified layer stack were prepared for the three different barriers. In these systems, the tunnel magnetoresistance ratios at optimum annealing temperatures were found to be 65% for alumina, 173% for magnesia, and 78% for the composite tunnel barriers. The similar tunnel magnetoresistance ratios of the tunnel junctions containing alumina provide evidence that coherent tunneling is suppressed by the alumina layer in the composite tunnel barrier.

Research highlights

► Transport properties of Co–Fe–B/tunnel barrier/Co–Fe–B magnetic tunnel junctions. ► Tunnel barrier consists of MgO, Al-Ox, or MgO/Al-Ox bilayer systems. ► Limitation of TMR-ratio in composite barrier tunnel junctions to Al-Ox values. ► Limitation indicates that Al-Ox layer is causing incoherent tunneling.

Introduction

In recent years, magnetic tunnel junctions (MTJs) have garnered much interest due to the large number of possible applications, such as magnetic random access memory (MRAM) and magnetic logic [1], [2], [3]. In most cases, a large tunnel magnetoresistance (TMR) ratio is desired.

The TMR effect was discovered at room temperature in alumina-based magnetic tunnel junctions [4], [5], as this material was well studied from tunneling experiments with superconductors [6], [7]. The highest measured TMR ratio has gradually increased over time and has been measured to be as high as 80% at room temperature [8], [9].

In addition to alumina, other materials, such as strontium-titanate [10], [11] and titanium-oxide [12], were used as tunnel barriers in MTJs. In 2001, higher TMR ratios were predicted for Fe/MgO/Fe systems with crystalline tunnel barriers and electrodes [13], [14] and were subsequently experimentally verified [15], [16]. Now, TMR ratios of up to 604% are observed in MgO-based MTJs at room temperature [17].

In this manuscript, we investigate magnetic tunnel junctions with alumina and magnesia barriers and compare them to MTJs with alumina–magnesia bilayers as the tunnel barrier. For all the junctions studied, we examine the transport properties as a function of the annealing temperature.

The goal of our investigation is to find evidence for non-coherent tunneling processes in the bilayer magnetic tunnel junctions. We expect to find TMR ratios of the bilayer that are comparable to the pure alumina system, since the coherence is destroyed by the alumina layer. This is in contrast to simple spin-polarization models by e.g. Jullière that would predict values in-between the values for MgO and alumina junctions [18].

Section snippets

Preparation

We studied MTJs with tunnel barriers that consist either of a single layer of Al2O3 or MgO. Additionally, we investigated MgO–Al2O3 bilayer structures as tunnel barrier materials. The thickness of each layer forming a tunnel barrier was always larger than 1.2 nm, to avoid pinholes. The layer stack and the annealing process varied for the respective samples and are provided in the results and discussion sections. The samples were structured using UV optical lithography and Ar-ion beam etching,

Magnesia and alumina reference samples

We prepared two reference samples to optimize the alumina and MgO preparation processes. The layer stacks, sputter conditions, and annealing temperatures were adjusted to yield the highest TMR values. The TMR vs. magnetic field (H) curves of these samples are shown in Fig. 1. First, we discuss the details of the MgO preparation, after which the alumina sample preparation will be outlined.

The layer stack of the MgO sample was Ta 20/Co–Fe–B 5.3/MgO 2.4/Co–Fe–B 3.2/Ta 20 (all values in nm) with a

Unified layer stack

The unified layer stacks consisted of Ta/Ru/Ta/Ru/Mn17Ir83 under-layers, followed by a Co40Fe40B20 (2.5 nm)/tunnel barrier/Co40Fe40B20 (3 nm) tri-layer. A Ta/Ru/Au cover stack provided protection for the upper electrode and a reliable electrical contact.

To form a hard magnetic electrode, the lower Co–Fe–B layer was exchange coupled to the underlying anti-ferromagnetic Mn–Ir layer. The exchange bias was activated in a post-annealing and field-cooling step similar to the alumina samples. The

Discussion

The highest TMR ratio with bilayer tunnel barriers in the present work is 78%. This ratio is in the range of values reported for Al2O3 based MTJs. This is true for the low temperature values as well. A TMR ratio of 118% was measured at 20 K, compared to 114% for the pure alumina MTJs [22]. The small increase in the TMR ratio might be attributed to the higher interface quality of the Co–Fe–B/MgO layer. The TMR ratio is still smaller than the highest value observed in alumina junctions [8], [9].

Summary

In summary, we have investigated the transport properties of MTJs with tunnel barriers consisting of single layers and bilayers of Al2O3 and MgO. The highest observed TMR ratio (78%) of the bilayer at room temperature is on the order of the highest reported values for MTJs with Al2O3 tunnel barriers in other works. This indicates that the observed limitation of the TMR ratio in Al2O3-based MTJs is caused by incoherent tunneling through the amorphous Al2O3 layer.

Acknowledgments

The authors would like to thank C.A. Jenkins from Mainz University for helpful discussions. We also would like to acknowledge the MIWF of the NRW state government and the German Research Foundation DFG for financial support.

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