Influence of MgO tunnel barrier thickness on spin-transfer ferromagnetic resonance and torque in magnetic tunnel junctions

Spin-transfer ferromagnetic resonance (ST-FMR) in symmetric magnetic tunnel junctions (MTJs) with a varied thickness of the MgO tunnel barrier ($0.75\text{nm}<t_{\mathrm{MgO}}<1.05\text{nm}$) is studied using the spin-torque diode effect. The application of an rf current into nanosized MTJs generates a dc mixing voltage across the device when the frequency is in resonance with the resistance oscillations arising from the spin-transfer torque. Magnetization precession in the free and reference layers of the MTJs is analyzed by comparing ST-FMR signals with macrospin and micromagnetic simulations. From ST-FMR spectra at different dc bias voltage, the in-plane and perpendicular torkances are derived. The experiments and free electron model calculations show that the absolute torque values are independent of tunnel barrier thickness. The influence of coupling between the free and reference layer of the MTJs on the ST-FMR signals and the derived torkances are discussed.

Figure 1

TMR ratio vs magnetic field loops of samples S1–S3.

Figure 2

ST-FMR spectra of samples S1 (a), S2 (b), and S3 (c) measured with various magnetic field applied at an angle of $\beta ={70}^{\circ }$ with respect to the easy magnetization axis. Only the rf signal (without dc bias voltage) was supplied to the MTJ. For sample S3 (c) two closely spaced peaks are visible.

Figure 3

Dispersion relation of sample S3 measured with the magnetic field applied at an angle of $\beta ={70}^{\circ }$ (a) and $\beta ={30}^{\circ }$ (b) with respect to the easy magnetization axis. The solid and dashed lines represent macrospin simulations of the FL and RL, respectively. (a) At an angle of $\beta ={70}^{\circ }$, the resonance frequency of two slightly separated FL modes increases with increasing magnetic field. (b) At an angle of $\beta ={30}^{\circ }$, magnetization precessions in both the FL and RL are measured.

Figure 4

Simulated ST-FMR curves for weak (a) and strong (b) ferromagnetic interlayer exchange coupling. In oommf simulations, a voltage step was used to excite magnetization precession in the FL of a MTJ structure. The dimensions of the simulated and experimental junctions are identical. The existence of a closely spaced double-peak ST-FMR signal for strong coupling is independent of the anisotropy constant.

Figure 5

Bias dependence of the in-plane torkance (a), in-plane torque (b), perpendicular torkance (c), and perpendicular torque (d) for MTJs with different MgO barrier thickness. The solid lines in (a) represent calculations based on Eq. (3). The torque values are numerically integrated from experimentally determined torkances. ${\tau }_{\perp }$ for sample S3 was compensated for an error originating from asymmetric ST-FMR resonances.