Effect of Low-Frequency AC Magnetic Susceptibility and Magnetic Properties of CoFeB/MgO/CoFeB Magnetic Tunnel Junctions.

In this investigation, the low-frequency alternate-current (AC) magnetic susceptibility (χ) and hysteresis loop of various MgO thickness in CoFeB/MgO/CoFeB magnetic tunneling junction (MTJ) determined coercivity () and magnetization () and correlated that with χ maxima. The multilayer films were sputtered onto glass substrates and the thickness of intermediate barrier MgO layer was varied from 6 to 15 Å. An experiment was also performed to examine the variation of the highest χ and maximum phase angle (θ) at the optimal resonance frequency (), at which the spin sensitivity is maximal. The results reveal that χ falls as the frequency increases due to the relationship between magnetization and thickness of the barrier layer. The maximum χ is at 10 Hz that is related to the maximal spin sensitivity and that this corresponds to a MgO layer of 11 Å. This result also suggests that the spin sensitivity is related to both highest χ and maximum phase angle. The corresponding maximum of χ is related to high exchange coupling. High coercivity and saturation magnetization contribute to high exchange-coupling χ strength.