We developed a fabrication process of an epitaxial MgAl2O4 barrier for magnetic tunnel junctions (MTJs) using a direct sputtering method from an MgAl2O4 spinel sintered target. Annealing the sputter-deposited MgAl2O4 layer sandwiched between Fe electrodes led to the formation of a (001)-oriented cation-disorder spinel with atomically sharp interfaces and lattice-matching with the Fe electrodes. A large tunnel magnetoresistance ratio up to 245% at 297 K (436% at 3 K) was achieved in the Fe/MgAl2O4/Fe(001) MTJ as well as an excellent bias voltage dependence. These results indicate that the direct sputtering is an alternative method for the realization of high performance MTJs with a spinel-based tunnel barrier.
Intersubband transitions in ZnO∕MgZnO multiple quantum wells (MQWs) are investigated by a photocurrent spectroscopy. Photocurrent peaks are observed in the energy range from 300to400meV and shifted to higher energy by reducing the ZnO well thickness. Polarization-resolved photocurrent spectra show that these peaks are observed when the polarization of incident lights is TM mode, following the intersubband selection rule. Calculation indicates that the photocurrent peaks are the intersubband transition from the first to the third subband in ZnO∕MgZnO MQWs.
Epitaxial Fe/magnesium gallium spinel oxide (MgGa2O4)/Fe(001) magnetic tunnel junctions (MTJs) were fabricated by magnetron sputtering. Tunnel magnetoresistance (TMR) ratio up to 121% at room temperature (196% at 4 K) was observed, suggesting a TMR enhancement by the coherent tunneling effect in the MgGa2O4 barrier. The MgGa2O4 layer had a spinel structure and it showed good lattice matching with the Fe layers owing to slight tetragonal lattice distortion of MgGa2O4. Barrier thickness dependence of the tunneling resistance and current-voltage characteristics revealed that the barrier height of the MgGa2O4 barrier is much lower than that in an MgAl2O4 barrier. This study demonstrates the potential of Ga-based spinel oxides for MTJ barriers having a large TMR ratio at a low resistance area product. * Electronic mail: sukegawa.hiroaki@nims.go.jp 2 Magnetic tunnel junctions (MTJs) have played a central role in spintronic devices such as read heads of hard disk drives and non-volatile magnetoresistive random access memories (MRAMs) for the last two decades, and many efforts have been made to improve their performance. 1 One of the most prominent achievements which accelerated the practical applications was the realization of giant tunnel magnetoresistance (TMR) ratios by using rock-salt type MgO crystalline barrier. [2][3][4][5] The giant TMR effect is attributed to the spin-dependent coherent tunneling through the 1 Bloch state in MgO(001). Toward ultra-high density spin transfer torque (STT)-MRAM applications, MTJs with low resistance area (RA) product of around 1 m 2 are needed as well as high TMR ratios. In MgO-based MTJs, the thickness of the MgO barrier must be reduced to a few monoatomic layers for achieving such a low RA, which causes substantial reduction of TMR ratios. This means the necessity of alternative barriers having a low barrier height. Doping other elements into MgO has been known to reduce the MgO barrier height; e.g., Zn 6,7 and Ti 8 dopings were reported to provide lower RA values although such dopings are likely to cause reduction of TMR ratios.Spinel oxide MgAl2O4(001) barrier also exhibits the coherent tunneling effect, 9,10 and over 300% TMR ratios at room temperature (RT) were reported in MgAl2O4-based MTJs. 11,12 Very recently, a TMR ratio at RT of 92% in an MgO/spinel-type -Ga2O3(001) bilayer barrier MTJ 13 and that of 120% in a Li-Mg-Al-O quaternary spinel-based barrier MTJ were reported, 14 showing the capability of the spinel based barriers for MTJs. The lattice spacing of MgAl2O4 is 4% smaller than that of MgO. In 3 addition, the lattice spacing can further be tuned by the Mg/Al composition, leading to the highly lattice-matched interfaces with various ferromagnetic materials. 9,12 On the other hand, the experimental band gap of MgAl2O4 is reported to be 7.8 eV, 15 which is similar to that of MgO (7.587.8 eV); 15 therefore, no reduction in RA values using an MgAl2O4 barrier was observed. 16 Some of spinel oxides other than MgAl2O4, such as ZnAl2O4, SiMg2O4 and SiZ...
Although single-crystalline spinel (MgAl2O4)-based magnetic tunnel junctions (MTJs) are known to show a good bias voltage dependence of a tunnel magnetoresistance (TMR) ratio over MgO-based MTJs, no polycrystalline MgAl2O4-based MTJs exhibiting large TMR ratios have been grown previously due to the lack of crystallinity of the MgAl2O4 barrier. In this work, we demonstrate the growth of polycrystalline-based MTJs with large TMR ratios exceeding 240% and an improved bias voltage dependence compared to that of MgO-based MTJs. An ultra-thin CoFe/MgO seed layer on the amorphous CoFeB layer induced the growth of a highly (001)-textured MgAl2O4 barrier, which worked as a template layer for the solid epitaxy of CoFe grains during the crystallization of the CoFeB layers. High resolution scanning transmission electron microscopy shows lattice-matched epitaxy between the (001)-textured MgAl2O4 barrier and CoFe grains. This study demonstrates the industrial viability of MgAl2O4-based polycrystalline MTJs with an improved bias voltage dependence.
We investigated the tunneling electroresistance (TER) in metal/wurtzite-MgZnO/metal junctions for applications in nonvolatile random-access memories. A resistive switching was detected utilizing an electric-field cooling at ±1 V and exhibited a TER ratio of 360%–490% at 2 K. The extracted change in the average barrier height between the two resistance states gave an estimation of the MgZnO electric polarization at 2.5 μC/cm2 for the low-temperature limit. In addition, the temperature-dependent TER ratio and the shift of the localized states energies at the barrier interface supported the ferroelectric behavior of the MgZnO tunnel-barrier. From the first-principles calculations, we found a similar effect of the barrier height change coming from the reversal of ZnO electric polarization. The possibility of using metal electrodes and lower growth temperatures, in addition to the ferroelectric property, make the ZnO-based memory devices suitable for CMOS integration.
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