Structural and tunneling properties of magnetic tunnel junctions with Al–O and MgO barrier
Introduction
Magnetic tunnel junctions (MTJs) [1] are a key component for a new generation of spintronic device applications such as magnetic random access memory (MRAM) [2], ultrahigh-density magnetic recording heads [3] and spin-logic devices [4] due to their high-tunnel magnetoresistance (TMR), and large and tunable ranges of resistance-area (RA) product from several Ω μm2 to MΩ μm2. A basic MTJ stack consists of two ferromagnetic electrodes (pinned and free layers) separated by a thin tunnel barrier such as amorphous Al–O or polycrystalline MgO. Recently, it was shown that replacing the commonly used Al–O tunnel barrier by highly oriented MgO(1 0 0) film leads to very large TMR ratio up to 472% at room temperature [5]. In this paper, we report on the influence of film texture and interface roughness on the TMR, RA and interlayer coupling of MTJs with amorphous Al–O and crystalline MgO(1 0 0) barrier.
Section snippets
Experimental
The spin valve MTJs were deposited by magnetron sputtering. Samples were grown on thermally oxidized silicon wafers in vacuum chamber under a base pressure of ∼10–7 mbar. Two types of spin valves were prepared. One with Al–O barrier and layers configuration Si(1 0 0)/SiO2 47 nm/buffer/IrMn 12 nm/CoFe 2.5 nm/Al–O 1.4 nm/NiFe 3 nm/Ta 5 nm. The stack of the second type of junctions was as follows: substrate Si(1 0 0)/SiO2 47 nm/buffer/IrMn 10 nm/CoFeB 3 nm/MgO 2 nm/CoFeB 4 nm/Ta 5 nm. Al–O barrier was obtained by
Results and discussion
Fig. 1 shows θ–2θ profiles of the samples with buffers Cu (A) and Ta/Cu (B). Intensities of diffraction peaks of cubic Cu(1 1 1) and IrMn(1 1 1) reveal strong differences in crystalline structure depending on a buffer type. Peaks from the junction deposited on the B buffer are two orders higher than that of the A buffer. It indicates a very strong [1 1 1] texture of the junctions with the B buffer in comparison to the A buffer. For the A buffer small intensity peak of Cu(2 0 0) crystallites was
Conclusions
The highest texture has been obtained for the samples with B buffers (Ta 5 nm/Cu 25 nm). The MTJ stack is textured in columnar-like fashion which produces roughness. The texture and roughness increase Néel coupling field. Junctions with the MgO barrier have the higher TMR value and lower RA product than that with the Al–O barrier. The proper design of seed-buffer layers allows to optimize magnetic and tunneling parameters of MTJs.
Acknowledgments
The authors like to thank Volker Drewello for assisting the preparation of MgO based MTJs. This work was supported by the bilateral agreement of Polish Ministry of Science and Higher Education, and DAAD in the frame of Grant 40/DAA/2006/01. One of the authors (G. R.) gratefully acknowledges financial support by the Deutsche Forschungsgemeinschaft DFG.
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