Abstract
We investigated the effect of an Mg-Al layer insertion at the bottom interface of epitaxial Fe/MgAl2O4/Fe(001) magnetic tunnel junctions (MTJs) on their spin-dependent transport properties. The tunnel magnetoresistance (TMR) ratio and differential conductance spectra for the parallel magnetic configuration exhibited clear dependence on the inserted Mg-Al thickness. A slight Mg-Al insertion (thickness < 0.1 nm) was effective for obtaining a large TMR ratio above 200% at room temperature and observing a distinct local minimum structure in conductance spectra. In contrast, thicker Mg-Al (> 0.2 nm) induced a reduction of TMR ratios and featureless conductance spectra, indicating a degradation of the bottom-Fe/MgAl2O4 interface. Therefore, a minimal Mg-Al insertion was found to be effective to maximize the TMR ratio for a sputtered MgAl2O4-based MTJ.
Highlights
Obtaining a large tunnel magnetoresistance (TMR) ratio exceeding 100% at room temperature (RT) in a magnetic tunnel junction (MTJ) is very essential to the development of various spintronic devices such as magnetoresistive random access memories, magnetic logic circuits and high sensitive magnetic sensors.[1]
We found that the Mg-Al insertion modified the MTJs properties such as, the zero bias TMR ratio, resistance-area (RA) product and its bias voltage dependence
I.e. tMgAl < 0.05 nm, the largest TMR ratio of 245% was observed closely to that reported in our previous report.[16]
Summary
Obtaining a large tunnel magnetoresistance (TMR) ratio exceeding 100% at room temperature (RT) in a magnetic tunnel junction (MTJ) is very essential to the development of various spintronic devices such as magnetoresistive random access memories, magnetic logic circuits and high sensitive magnetic sensors.[1]. Spinel MgAl2O4(001) barrier-based MTJs have shown large TMR ratios over 100% at RT due to the occurrence of spin-dependent coherent tunneling similar to MgO(001) barriers.[13,14,15] Importantly, the lattice constant of an MgAl2O4 barrier prepared by a post-oxidation method can be tuned by the Mg-Al composition in MgAl2O4, leading to nearly a perfect lattice-matching with bcc Co-Fe alloys[13,14] and Co2FeAl Heusler alloy.[15] Recently, using Fe as electrodes we developed a latticematched and very flat MgAl2O4(001) barrier with a cation-disordered spinel structure by a direct rf sputtering of a MgAl2O4 sintered target, instead of the conventional post-oxidation of Mg-Al alloys.[16] The TMR ratio of a direct-sputtered epitaxial Fe/MgAl2O4/Fe(001) MTJ reached 245% at RT, which is larger than typical TMR ratios in epitaxial Fe/MgO/Fe MTJs.[3,17] In general, spin-dependent transport properties of an MTJ is significantly affected by the barrier/ferromagnetic interface conditions. The interface modification by Mg-Al insertion under the sputtered MgAl2O4 barrier formed an insufficient oxidation interface, which weakened the spin-dependent coherent tunneling effect. A minimal insertion (less than 0.1 nm) was effective to prevent the interface oxidation at the bottom-Fe and enhance the TMR ratio to maximum values (240–245%)
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