Susceptibility of CoFeB/AlO x/Co Magnetic Tunnel Junctions to Low-Frequency Alternating Current

Yuan-Tsung Chen; Zu-Gao Chang

doi:10.3390/nano3040574

Susceptibility of CoFeB/AlO _x/Co Magnetic Tunnel Junctions to Low-Frequency Alternating Current

Nanomaterials (Basel). 2013 Oct 15;3(4):574-582. doi: 10.3390/nano3040574.

Authors

Yuan-Tsung Chen¹, Zu-Gao Chang²

Affiliations

¹ Department of Materials Science and Engineering, I-Shou University, Kaohsiung 840, Taiwan. ytchen@isu.edu.tw.
² Department of Materials Science and Engineering, I-Shou University, Kaohsiung 840, Taiwan. isu10107014m@cloud.isu.edu.tw.

Abstract

This investigation studies CoFeB/AlO_x/Co magnetic tunneling junction (MTJ) in the magnetic field of a low-frequency alternating current, for various thicknesses of the barrier layer AlO_x. The low-frequency alternate-current magnetic susceptibility (χ_ac) and phase angle (θ) of the CoFeB/AlO_x/Co MTJ are determined using an c_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 and CoFeB as amorphous phases. The full width at half maximum (FWHM) of the Co(0002) peak, decreases as the AlO_x thickness increases; revealing that the Co layer becomes more crystalline with increasing thickness. χ_ac result demonstrates that the optimal resonance frequency (f_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 barrier layer thickness of 30 Å because the Co(0002) texture induces magneto-anisotropy, which improves the indirect CoFeB and Co spin exchange-coupling strength and the χ_ac value. The results concerning magnetism indicate that the magnetic characteristics are related to the crystallinity of Co.

Keywords: indirect exchange coupling; low-frequency alternate-current magnetic susceptibility (χac); magnetic tunnel junctions (MTJs); resonance frequency (fres).