Showing papers by "Jonathan Z. Sun published in 2009"

A three-terminal spin-torque-driven magnetic switch

Abstract: A three-terminal spin-torque-driven magnetic switch is experimentally demonstrated. The device uses nonlocal spin current and spin accumulation as the main mechanism for current-driven magnetic switching. It separates the current-induced write operation from that of a magnetic tunnel junction based read. The write current only passes through metallic structures, improving device reliability. The device structure makes efficient use of lithography capabilities, important for robust process integration.

Bias and angular dependence of spin-transfer torque in magnetic tunnel junctions

Abstract: We use spin-transfer-driven ferromagnetic resonance (ST-FMR) to measure the spin-transfer torque vector $\mathbit{\ensuremath{\tau}}$ in MgO-based magnetic tunnel junctions as a function of the offset angle between the magnetic moments of the electrodes and as a function of bias, $V$. We explain the conflicting conclusions of two previous experiments by accounting for additional terms that contribute to the ST-FMR signal at large $|V|$. Including the additional terms gives us improved precision in the determination of $\mathbit{\ensuremath{\tau}}(V)$, allowing us to distinguish among competing predictions. We determine that the in-plane component of $d\mathbit{\ensuremath{\tau}}/dV$ has a weak but nonzero dependence on bias, varying by 30%--35% over the bias range where the measurements are accurate, and that the perpendicular component can be large enough to be technologically significant. We also make comparisons to other experimental techniques that have been used to try to measure $\mathbit{\ensuremath{\tau}}(V)$.

Bias and Angular Dependence of Spin-Transfer Torque in Magnetic Tunnel Junctions

Abstract: We use spin-transfer-driven ferromagnetic resonance (ST-FMR) to measure the spin-transfer torque vector $\\mathbit{\\ensuremath{\\tau}}$ in MgO-based magnetic tunnel junctions as a function of the offset angle between the magnetic moments of the electrodes and as a function of bias, $V$. We explain the conflicting conclusions of two previous experiments by accounting for additional terms that contribute to the ST-FMR signal at large $|V|$. Including the additional terms gives us improved precision in the determination of $\\mathbit{\\ensuremath{\\tau}}(V)$, allowing us to distinguish among competing predictions. We determine that the in-plane component of $d\\mathbit{\\ensuremath{\\tau}}/dV$ has a weak but nonzero dependence on bias, varying by 30%--35% over the bias range where the measurements are accurate, and that the perpendicular component can be large enough to be technologically significant. We also make comparisons to other experimental techniques that have been used to try to measure $\\mathbit{\\ensuremath{\\tau}}(V)$.

High-bias backhopping in nanosecond time-domain spin-torque switches of MgO-based magnetic tunnel junctions

Abstract: For CoFeB∕MgO-based magnetic tunnel junctions, the switching probability has an unusual dependence on bias voltage V and bias magnetic field H for bias voltage pulse durations t long enough to allow thermally activated reversal. At high junction bias close to 1V, the probability of magnetic switching in spin-torque-driven switches sometimes appears to decrease. This is shown to be due to a backhopping behavior occurring at high bias, and it is asymmetric in bias voltage, being more pronounced in the bias direction for antiparallel-to-parallel spin-torque switch, i.e., in the direction of electrons tunneling into the free layer. This asymmetry hints at processes involving hot electrons within the free-layer nanomagnet.

Back-hopping after spin torque transfer induced magnetization switching in magnetic tunneling junction cells

Abstract: In some cases such as junctions with low magnetic thermal activation energy, the magnetization of the free layer in MgO-based magnetic tunnel junctions (MTJs) can back hop to its original direction after successful spin torque induced switching. The back-hopping is observed in both current directions corresponding to parallel-to-antiparallel and antiparallel-to-parallel switchings. For bias voltage pulses with increasing pulse width, the threshold voltage for back-hopping appears to decrease together with spin-torque switching and junction breakdown thresholds, but its rate of decrease is less. Increasing the anisotropy field Hk by increasing the MTJ aspect ratio can raise the threshold voltage of back-hopping significantly.

Spin-torque driven ferromagnetic resonance in a nonlinear regime

Abstract: Spin-valve based nanojunctions incorporating Co|Ni multilayers with perpendicular anisotropy were used to study spin-torque driven ferromagnetic resonance (ST-FMR) in a nonlinear regime. Perpendicular field swept resonance lines were measured under a large amplitude microwave current excitation, which produces a large angle precession of the Co|Ni layer magnetization. With increasing rf power the resonance lines broaden and become asymmetric, with their peak shifting to lower applied field. A nonhysteretic step jump in ST-FMR voltage signal was also observed at high powers. The results are analyzed in in terms of the foldover effect of a forced nonlinear oscillator and compared to macrospin simulations. The ST-FMR nonhysteretic step response may have applications in frequency and amplitude tunable nanoscale field sensors.

Spin-torque driven ferromagnetic resonance in a nonlinear regime

Abstract: Spin-valve based nanojunctions incorporating Co∣Ni multilayers with perpendicular anisotropy were used to study spin-torque driven ferromagnetic resonance (ST-FMR) in a nonlinear regime. Perpendicular field swept resonance lines were measured under a large amplitude microwave current excitation, which produces a large angle precession of the Co∣Ni layer magnetization. With increasing rf power the resonance lines broaden and become asymmetric, with their peak shifting to lower applied field. A nonhysteretic step jump in ST-FMR voltage signal was also observed at high powers. The results are analyzed in terms of the foldover effect of a forced nonlinear oscillator and compared to macrospin simulations. The ST-FMR nonhysteretic step response may have applications in frequency and amplitude tunable nanoscale field sensors.

Thermal-magnetic noise measurement of spin-torque effects on ferromagnetic resonance in MgO-based magnetic tunnel junctions

Abstract: Thermal-magnetic noise at ferromagnetic resonance (T-FMR) can be used to measure magnetic perpendicular anisotropy of nanoscale magnetic tunnel junctions (MTJs). For this purpose, T-FMR measurements were conducted with an external magnetic field up to 14 kOe applied perpendicular to the film surface of MgO-based MTJs under a dc bias. The observed frequency-field relationship suggests that a 20 A CoFeB free layer has an effective demagnetization field much smaller than the intrinsic bulk value of CoFeB, with 4PiMeff = (6.1 +/- 0.3) kOe. This value is consistent with the saturation field obtained from magnetometry measurements on extended films of the same CoFeB thickness. In-plane T-FMR on the other hand shows less consistent results for the effective demagnetization field, presumably due to excitations of more complex modes. These experiments suggest that the perpendicular T-FMR is preferred for quantitative magnetic characterization of nanoscale MTJs.

Thermal-magnetic noise measurement of spin-torque effects on ferromagnetic resonance in MgO-based magnetic tunnel junctions

Abstract: Thermal-magnetic noise at ferromagnetic resonance (T-FMR) can be used to measure magnetic perpendicular anisotropy of nanoscale magnetic tunnel junctions (MTJs). For this purpose, T-FMR measurements were conducted with an external magnetic field up to 14 kOe applied perpendicular to the film surface of MgO-based MTJs under a dc bias. The observed frequency-field relationship suggests that a 20 A CoFeB free layer has an effective demagnetization field much smaller than the intrinsic bulk value of CoFeB, with 4πMeff=6.1±0.3 kOe. This value is consistent with the saturation field obtained from magnetometry measurements on extended films of the same CoFeB thickness. In-plane T-FMR on the other hand shows less consistent results for the effective demagnetization field, presumably due to excitations of more complex modes. These experiments suggest that the perpendicular T-FMR is preferred for quantitative magnetic characterization of nanoscale MTJs.

Field and bias dependence of high-frequency magnetic noise in MgO-based magnetic tunnel junctions

Abstract: We present room-temperature measurements of high-frequency magnetization fluctuation (mag noise) in MgO-based nanopillar magnetic tunnel junctions (MTJs) biased with a direct current (dc). In the frequency range of 1–13 GHz, double mag-noise peaks are observed for some MTJs while others only show a single mag-noise peak. The in-plane field dependence of the mag-noise peak frequency is consistent with the Kittel formula. For all MTJs measured, the bias-dependent shift in the mag-noise peak frequency has a pronounced asymmetry. In addition, we find nonmonotonic variations in peak linewidth as a function of the external in-plane magnetic field and of the dc bias current. These suggest the possible involvement of nonmacrospin modes in spin-torque-dependent thermal mag-noise generation.