• Journal of Magnetics
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• v.9 no.1
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• pp.13-16
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• 2004

Magnetic tunnel junctions (MTJs) comprising amorphous CoNbZr layers have been investigated. $Co_{85.5}Nb_8Zr_{6.5}$(in at. %) layers were employed to substitute the traditionally used Ta layers with an emphasis given on under-standing underlayer effect. The typical junction structure was $SiO_2/CoNbZr$ or Ta 2/CoFe 8/IrMn 7.5/CoFe 3/Al 1.6 + oxidation/CoFe 3/CoNbZr or Ta 2 (nm). For both as-deposited state and after annealing, the CoNbZr-underlayered structure showed superior surface smoothness up to the tunnel barrier than Ta-underlayerd one (rms roughness of 0.16 vs. 0.34 nm). CoNbZr-based MTJs was proven beneficial for increasing thermal stability and increasing $V_h$ (the bias voltage where MR ratio becomes half) characteristics than Ta-based MTJs. This is because the CoNbZr-based junctions offer smoother interface structure than the Ta-based one.

• Proceedings of the Korean Magnestics Society Conference
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• 2014.05a
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• pp.57-57
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• 2014

• Journal of Magnetics
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• v.7 no.2
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• pp.59-62
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• 2002

Magnetic tunnel junctions (MTJs) were fabricated with high bias for plasma oxidation and the effects of annealing on the temperature dependence of tunneling magnetoresistance (TMR) were investigated experimentally. As-grown, TMR increases, peaks around 160 K, and decreases with increasing temperature from 80 K to 300 K. When MTJs are annealed, $T_{max}$, the temperature at which maximum TMR is obtained, decreases as annealing temperature increases to the optimal point. In order to explain this abnormal temperature dependence of TMR, the difference of conductance between parallel and antiparallel alignments of magnetizations as a function of temperature is also analyzed. The shifts of $T_{max}$ due to annealing process are described phenomenologically with spin-dependent transfer rates of electrons tunnel through the barrier.

• Journal of the Korean Magnetics Society
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• v.25 no.2
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• pp.47-51
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• 2015

To achieve a high tunneling magneto resistance (TMR) of sputtered magnetic tunnel junctions (MTJs) with an MgO barrier, the annealing process is indispensable. The structural and compositional changes as consequences of the annealing greatly affect the spin-dependent transport properties of MTJs. Higher TMR could be obtained for MTJs annealed at higher annealing temperature. The diffusion of Ru, Mn and/or Ta in the MTJs may occur during annealing process, which is known to be detrimental to spin-dependent tunneling effect. The rapid thermal annealing (RTA) process was used for annealing the MTJs with synthetic antiferromagnets. To suppress the diffusion of Mn, Ru and/or Ta in the MTJs, the process time and temperature of RTA were minutely controlled.

• Journal of the Korean Magnetics Society
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• v.16 no.2
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• pp.136-139
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• 2006

The tunneling magnetoresistance (TMR) ratios of magnetic tunnel junctions (MTJs), in general, decrease abruptly above 250$^{\circ}C$ due to Mn interdiffusion from an antiferromagnet IrMn layer to a ferromagnetic CoFe and/or a tunnel barrier. To improve thermal stability, we prepared MTJs with nano-oxide layers. Using a MTJ structure consisting of underlayer CoNbZr 4/bufferlayer CoFe 10/antiferromaget IrMn 7.5/pinned layer CoFe 3/tunnel barrier AlO/freelayer CoFe 3/capping CoNbZr 2 (nm), we placed a nano-oxide layer (NOL) into the underlayer or bufferlayer. Then, the thermal, structural and magneto-electric properties were measured. The TMR ratio, surface flatness, and thermal stability of the MTJs with NOLs were promoted.

• Journal of Magnetics
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• v.16 no.2
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• pp.140-144
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• 2011

We report on a numerical calculation study of two new functional properties in magnetic tunnel junctions (MTJs), negative dynamic resistance and RF amplification. The magnetic dynamics in a conventional CoFeB/MgO/CoFeB MTJ with in-plane magnetization was investigated using a macro-spin model simulation. To examine the influence of thermal fluctuations, random external magnetic fields were also included. Using a voltage controlled bias circuit, the negative dynamic resistance was obtained from time averaged I-V characteristics at both 0 K and 300 K under appropriate external magnetic fields and bias voltages. Using this negative dynamic resistance property, we demonstrated RF amplification with a 100 MHz high frequency signal. Sizable RF amplification gain was observed without thermal fluctuation. However, at 300 K, the RF signal was not amplified because low frequency magnetization dynamics were dominant.

• Journal of the Korean Magnetics Society
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• v.14 no.1
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• pp.13-17
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• 2004

We have investigated the effect of Hf insertion in the Al oxide tunnel barrier on the properties of magnetic tunnel junctions (MTJs). MTJs with Hf inserted barrier show the higher tunnel magnetoresistance (TMR) ratio and less temperature and bias voltage dependence of TMR than MTJs with a conventional Al$_2$O$_3$ barrier. The enhancement of TMR ratio and the reduction of the temperature and bias voltage dependence might be due to the reduction of defects in the barrier. Al-Hf oxide was formed by depositing Al and Hf simultaneously, and oxidizing the compound films. The TMR ratio of 36% was almost the same value as that with Hf inserted barrier. This implies that the inserted Hf layers mixed with Al layers during deposition or oxidation, and they might form Al Hf oxide barriers. This compound Al Hf oxide formation may be responsible to reduction of defect concentration which enhanced the TMR ratio and reduced temperature and bias-voltage dependence.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.14 no.8
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• pp.688-693
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• 2001

A series of Fe/CeO$_2$/Fe$_{75}$Co$_{25}$ tunnel junctions (Magnetic Tunnel Junction, MTJ) having CeO$_2$ barrier layers from 30 to 90$\AA$ in thickness were prepared by ion beam sputtering (IBS) method. In order to compare the properties of MTJs, Fe/Al oxide/Fe-Co tunnel junctions were also prepared. Some junctions with a CeO$_2$ barrier layer showed the ferromagnetic tunneling effect and the highest MR ratio at room temperature was 5%. The electric resistance of junctions with a CeO$_2$ barrier layer was higher that that of junctions with an Al oxide barrier. On the other hand, The interface analysis of the Fe/CeO$_2$ bilayer was conducted by means of X-ray photoelectron spectroscopy (XPS). It was found that CeO$_2$ was decomposed to Ce and $O_2$ during sputtering, and Fe was oxidized with these decomposed $O_2$ molecules. The reduction of both electric resistance and MR ratio may be associated with the decomposed Ce in the barrier layer.r.r.

• Journal of Magnetics
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• v.9 no.1
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• pp.17-22
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• 2004

We studied the magnetotransport properties of tunnel junctions with AlO and AlN barriers fabricated using microwave-excited plasma. The plasma nitridation process provided wider controllability than the plasma oxidization for the formation of MTJs with ultra-thin insulating layer, because of the slow nitriding rate of metal Al layers, comparing with the oxidizing rate of them. High tunnel magnetoresistance (TMR) ratios of 49 and 44% with respective resistance-area product $(R{\times}A) of 3 {\times} 10^4 and 6 {\times} 10^3 {\Omega}{\mu}m^2$ were obtained in the Co-Fe/Al-N/Co-Fe MTJs. We conclude that AlN is a hopeful barrier material to realize MTJs with high TMR ratio and low $R{\times}A$ for high performance MRAM cells. In addition, in order to clarify the annealing temperature dependence of TMR, the local transport properties were measured for Ta $50{\AA} /Cu 200 {\AA}/Ta 50 {\AA}/Ni_{76}Fe_{24} 20 {\AA}/Cu 50 {\AA}/Mn_{75}Ir_{25} 100 {\AA}/Co_{71}Fe_{29} 40 {\AA}/Al-O$ junction with $d_{Al}= 8 {\AA} and P_{O2}{\times}t_{0X}/ = 8.4 {\times} 10^4$ at various temperatures. The current histogram statistically calculated from the electrical current image was well in accord with the fitting result considering the Gaussian distribution and Fowler-Nordheim equation. After annealing at $340^{\circ}C$, where the TMR ratio of the corresponding MTJ had the maximum value of 44%, the average barrier height increased to 1.12 eV and its standard deviation decreased to 0.1 eV. The increase of TMR ratio after annealing could be well explained by the enhancement of the average barrier height and the reduction of its fluctuation.

• Journal of the Korean Magnetics Society
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• v.16 no.3
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• pp.186-191
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• 2006

Ferromagnetic amorphous $Ni_{16}Fe_{62}Si_8B_{14}$ and $Co_{70.5}Fe_{4.5}Si_{15}B_{10}$ layers have been devised and incorporated as free layers of magnetic tunnel junctions (MTJs) to improve MRAM reading and writing performance. The NiFeSiB and CoFeSiB single-layer film exhibited a lower saturation magnetization ($Ms=800emu/cm^3,\;and\;560emu/cm^3$, respectively) compared to that of a $Co_{90}Fe_{10}(Ms=1400emu/cm^3)$. Because amorphous ferromagnetic materials have lower Ms than crystalline ones, the MTJs incorporating amorphous ferromagnetic materials offer lower switching field ($H_{sw}$) values than that of the traditional CoFe-based MTJ. The double-barrier MTJ with an amorphous NiFeSiB free layer offered smooth surface resulting in low bias voltage dependence, and high $V_h\;and\;V_{bd}$ compared with the values of the traditional CoFe-based MTJ.