T. Daibou

Low-frequency noise in MgO magnetic tunnel junctions

Noise measurements have been performed in MgO-based tunnel junctions with normalized resistance in the range of 105–107Ωμm2 and various magnetoresistance ratios were investigated. Noise measurements in the frequency range of 1–1000Hz shows magnetically dependent pure 1∕f power spectra at low frequency. The 1∕f noise scales with bias voltage, indicating that the 1∕f noise can be attributed to magnetic tunnel junction resistance fluctuations. Bias voltage dependence of random telegraph noise (RTN) was observed, indicating electronic origin due to the charge-trapping mechanism. In the presence of the easy-axis bias field, our data exhibit a magnetic-field dependence of RTN that originates from magnetization fluctuations. A phenomenological noise parameter, defined for the comparison of noise levels in different junctions, was shown to be independent of the junction resistance-area product in the range investigated. This observation suggests that the decrease in tunnel resistance does not play an important ro...

Tunneling Spin Polarization and Magnetic Properties of Co–Fe–B Alloys and Their Dependence on Boron Content

Tunneling spin polarization (TSP) of (Co50Fe50)100-xBx alloys with various boron contents is measured by superconducting tunneling spectroscopy (STS). Because magnetic tunnel junctions (MTJs) using Co–Fe–B alloys exhibit a large tunnel magnetoresistance (TMR) effect, they are highly valued materials; in addition, the TSP of Co–Fe–B alloys is expected to be high. The maximum observed TSP value is 49% for (Co50Fe50)80B20, which is higher than that of pure Co50Fe50 (37%). An X-ray diffraction pattern showed that the structure of (Co50Fe50)80B20 is amorphous. We infer that the structure is changed to the amorphous state by increasing the boron content, which enhances the TSP of Co–Fe–B alloys.

High-Magnetoresistance Tunnel Junctions Using Co75Fe25 Ferromagnetic Electrodes

Tunnel magnetoresistance (TMR) effect and the applied voltage dependence of the TMR ratio in the tunnel junctions Ta(5 nm)/Ni79Fe21(3 nm)/Cu(20 nm)/Ni79Fe21(3 nm)/ Ir22Mn78(10 nm)/Co75Fe25(4 nm)/Al(0.8 nm)-oxide/Co75Fe25(4 nm)/ Ni79Fe21(20 nm)/Ta(5 nm) were investigated. MR ratio, effective barrier height and width, and breakdown voltage of the junctions can be remarkably enhanced after annealed at 300°C for an hour using Co75Fe25 as ferromagnetic electrodes and Cu as bottom conduction electrode. High MR ratio of 49.7% at room temperature and 69.1% at 4.2 K for the TMR junctions were observed.

Tunneling spectroscopy in CoFeB∕MgO∕CoFeB magnetic tunnel junctions

The d2V∕dI2-V measurements were used to investigate the tunneling mechanism in CoFeB∕MgO∕CoFeB magnetic tunnel junctions (MTJs), which showed a giant tunnel magnetoresistance ratio up to 200% at room temperature. The d2V∕dI2-V spectra of CoFeB∕MgO∕CoFeB junctions resemble those of single-crystal Fe(001)∕MgO(001)∕Fe(001) MTJs. Broad peaks appeared around ±600mV in spectra for antiparallel magnetic configurations. A complex structure was apparent in the spectra for parallel configurations. We inferred that giant tunnel magnetoresistance observed in CoFeB∕MgO∕CoFeB junctions originates in coherent tunneling between the Δ1 bands of crystallized CoFeB electrodes.

Tunnel Magnetoresistance Effect in CoFeB/MgO/Co

We have fabricated MgO-based magnetic tunnel junctions (MTJs) with the CoFeB bottom electrode and top electrodes of poly crystalline Co2FeSi and Co2MnSi Heusler alloys. We have measured temperature dependence of the TMR ratio and TMR-V characteristics at 6 K. We have achieved a high TMR ratio of 90% at RT for the MTJ with Co2FeSi electrode after annealing at 325degC. The MTJ with Co2MnSi electrode showed a significant annealing temperature dependence of the TMR ratio. The increase of TMR ratio by annealing is due to the crystallization of the Co2MnSi Heusler layer. Furthermore, the strong temperature dependence of TMR ratio and the anomalous TMR-V characteristics have been observed in the MTJ with Co2MnSi

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Magnetic tunnel junctions (MTJs) were fabricated using an Al–O insulating layer prepared on an epitaxially grown Ni80Fe20 bottom electrode and on a polycrystalline Ni80Fe20 bottom electrode. Crystallographic orientations and surface morphology of the films were examined using x-ray diffraction and atomic force microscopy, respectively. The MTJ with an epitaxial bottom electrode showed a tunnel magnetoresistance (TMR) ratio of 51% after annealing at 250 °C. This value was about two times larger than that of the MTJ with a polycrystalline bottom electrode (27%). The applied bias voltage dependences of the TMR ratios were also much different. The Vhalf values of epitaxial and polycrystalline samples were about 750 and 400 mV, respectively.

FeSi and Co

In this study, we investigated the TMR effects in CoFeB/MgO/Heusler alloys (Co2FeSi and Co2MnSi) magnetic tunnel junctions. Both Co2FeSi and Co2MnSi heusler alloys have half-metallic band structure and high Curie temperature of 1200K and 985K, respectively. Moreover, the lattice mismatch between the Heusler alloys and MgO barrier is relatively small, which indicates that the highly textured Heusler alloys can be grown on the MgO barrier. In addition, it could be noted that spin-fluctuation at finite temperature can reduce spin-polarization of Heusler alloys even though the MgO/Heusler alloys is ideal HMFs at low temperature.