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This investigation studies CoFeB/AlO_{x}_{x}_{ac}) and phase angle (θ) of the CoFeB/AlO_{x}_{ac} analyzer. The driving frequency ranges from 10 to 25,000 Hz. These multilayered MTJs are deposited on a silicon substrate using a DC and RF magnetron sputtering system. Barrier layer thicknesses are 22, 26, and 30 Å. The X-ray diffraction patterns (XRD) include a main peak at 2θ = 44.7° from hexagonal close-packed (HCP) Co with a highly (0002) textured structure, with AlO_{x}_{x}_{ac} result demonstrates that the optimal resonance frequency (_{res}) that maximizes the χ_{ac} value is 500 Hz. As the frequency increases to 1000 Hz, the susceptibility decreases rapidly. However, when the frequency increases over 1000 Hz, the susceptibility sharply declines, and almost closes to zero. The experimental results reveal that the mean optimal susceptibility is 1.87 at an AlO_{x}_{ac} value. The results concerning magnetism indicate that the magnetic characteristics are related to the crystallinity of Co.

_{ac})

_{res})

Recently, ferromagnetic exchange-coupling in magnetic fields has been extensively examined. Exchange-coupling has been discussed following the discovery of spintronics [_{s}) and coercivity (_{c}) [_{ac}) and the optimal resonance frequency (_{res}). Susceptibility to low-frequency alternate-current magnetism and the effect of the thickness of the tunneling barrier on indirect exchange coupling between FM1 and FM2 in the CoFeB/AlO_{x}_{x}_{x}_{ac} reveal that the susceptibility distribution is optimal at low frequency. The three highest values of susceptibility for each sample are averaged. When the thickness of the AlO_{x}_{x}_{x}

_{x}_{x}

X-ray diffraction patterns of CoFeB(75 Å)/AlO_{x}

The full width at half maximum (FWHM, _{x}_{α1} line; _{x}_{x}_{x}

Full width at half maximum (FWHM) (_{x}

_{ac} measurement. According to the result, the in-plane χ_{ac}signal of MTJ is larger than the perpendicular χ_{ac}signal. It indicates that in-plane direction is easy-axis magnetization of MTJ.

(_{ac}signal of MTJ sample when the external field is 30 mOe; (_{ac}signal of MTJ sample when the external field is 50 mOe; (_{ac}signal of MTJ sample when the external field is 30 mOe; (_{ac}signal of MTJ sample when the external field is 50 mOe.

_{ac}) of the multilayered CoFeB/AlO_{x}_{x}_{ac} has the following physical meaning. The magnetic material under the external AC magnetic field shows a magnetic property called multiple-frequency AC magnetic susceptibility χ_{ac}. The origin of χ_{ac} is due to the association between magnetic spin interactions. The frequency of the applied AC magnetic field equals the frequency of oscillation of the magnetic dipole. Hence, the frequency of the peak of the low-frequency magnetic susceptibility corresponds to the resonant frequency of the oscillation of the magnetic dipole moment inside domains. The χ_{ac} peak indicates the spin exchange-coupling interaction and dipole moment of domain under frequency [_{ac} peaks are corresponding to high exchange coupling. From _{res}) of 500 Hz for independently of AlO_{x}_{ac} corresponds to the maximum spin sensitivity at _{res}. Additionally, the susceptibility of the films sharply fell to zero as the frequency was increased over 1000 Hz. The MTJ with a AlO_{x}_{ac} of approximately 1.87 at a _{res} of 500 Hz because Co(0002) texture induces a magneto nanocrystalline anisotropy with the maximum χ_{ac} and _{res} effect [

Susceptibility (χ_{ac}) of CoFeB/AlO_{x}

_{x}_{ac} and phase angle increase with the AlO_{x}_{ac}. The results in _{ac} increases with the phase angle. The χ_{ac} and the phase angle follow vary similarly. Moreover, an increasing phase angle is associated with increasingly sensitive to spin exchange-coupling strength, which is observed as a high susceptibility.

Mean optimal susceptibility and average phase angle as functions of AlO_{x}

High optimal susceptibility, average optimal susceptibility, and resonance frequency of maximum mean susceptibility.

Optimal susceptibility No. 1 | Optimal susceptibility No. 2 | Optimal susceptibility No. 3 | Average optimal susceptibility | Optimal resonance frequency | |
---|---|---|---|---|---|

22 | 1.9861 | 0.9452 | 0.6536 | 1.1950 | 500 Hz |

26 | 2.2841 | 1.1405 | 0.7594 | 1.3947 | 500 Hz |

30 | 2.4905 | 2.2711 | 0.8487 | 1.8700 | 500 Hz |

A multilayered MTJ was deposited on a Si(100) substrate by DC and RF magnetron sputtering. The typical base chamber pressure was less than 2 × 10^{−7} Torr, and the Ar working chamber pressure was 5 × 10^{−3} Torr. The MTJs had the structure Si(100)/CoFeB(75 Å)/AlO_{x}_{x}_{x}_{2} in the ratio 9:16. The plasma oxidation time varied from 50 to 70 s as the initial thickness of the Al layer increased from 22 to 30 Å. To examine the microstructure, the degree of Co(0002) layer texturing was characterized by X-ray diffraction (XRD) using CuK_{α1} radiation. The in-plane low-frequency alternate-current magnetic susceptibility (χ_{ac}) of MTJ was investigated using an χ_{ac} analyzer (XacQuan, MagQu Co. Ltd., Sindian City, Taiwan). About the χ_{ac} measurement, the referenced standard sample is calibrated by χ_{ac} analyzer with an external field. The size of standard sample is 5 mm × 5 mm. After the calibration is finish, the same size MTJ is placed in χ_{ac} measurement. The driving frequency ranged from 10 to 25,000 Hz. The χ_{ac} is determined through the magnetization measurement. All measured samples had the same shape and size to eliminate the demagnetization factor. The χ_{ac} valve is unitless, because the χ_{ac} result is corresponding to referenced standard sample.

The CoFeB/AlO_{x}_{ac}, in a low-frequency alternating magnetic field, for various thicknesses of the barrier layer AlO_{x}_{x}_{x}_{ac} value is obtained at the optimal _{res} of 500 Hz. As the frequency increased further to 1000 Hz, the susceptibility rapidly declined. Beyond 1000 Hz, it decreased sharply, almost to zero. As the barrier layer thickness increased, the χ_{ac} and phase angle increased because the magneto-anisotropy of the Co(0002) texture induced strong indirect spin exchange coupling between CoFeB and Co, increasing χ_{ac}. Finally, the average optimal susceptibility of CoFeB(75 Å)/AlO_{x}_{x}

This work was supported by the National Science Council, under Grant No. NSC100-2112-M-214-001-MY3.

The authors declare no conflict of interest.

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