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Search Results: 1 - 10 of 1920 matches for " Tomohiro Taniguchi "
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Nonlinear analysis of magnetization dynamics excited by spin Hall effect
Tomohiro Taniguchi
Physics , 2015, DOI: 10.1103/PhysRevB.91.104406
Abstract: We investigate the possibility of exciting self-oscillation in a perpendicular ferromagnet by the spin Hall effect on the basis of a nonlinear analysis of the Landau-Lifshitz-Gilbert (LLG) equation. In the self-oscillation state, the energy supplied by the spin torque during a precession on a constant energy curve should equal the dissipation due to damping. Also, the current to balance the spin torque and the damping torque in the self-oscillation state should be larger than the critical current to destabilize the initial state. We find that the second condition in the spin Hall system is not satisfied by deriving analytical solutions of the energy supplied by the spin transfer effect and the dissipation due to the damping from the nonlinear LLG equation. This indicates that the self-oscillation of a perpendicular ferromagnet cannot be excited solely by the spin Hall torque.
Magnetization reversal condition for a nanomagnet within a rotating magnetic field
Tomohiro Taniguchi
Physics , 2014, DOI: 10.1103/PhysRevB.90.024424
Abstract: The reversal condition of magnetization in a nanomagnet under the effect of rotating magnetic field generated by a microwave is theoretically studied based on the Landau-Lifshitz-Gilbert equation. In a rotating frame, the microwave produces a dc magnetic field pointing in the reversed direction, which energetically stabilizes the reversed state. We find that the microwave simultaneously produces a torque preventing the reversal. It is pointed out that this torque leads to a jump in the reversal field with respect to the frequency. We derive the equations determining the reversal fields in both the low- and high-frequency regions from the energy balance equation. The validities of the formulas are confirmed by a comparison with the numerical simulation of the Landau-Lifshitz-Gilbert equation.
Magnetization switching by microwaves initially rotating in opposite direction to precession
Tomohiro Taniguchi
Physics , 2015, DOI: 10.7567/APEX.8.123002
Abstract: A common understanding of magnetization switching in microwave-assisted magnetization reversal is that the rotation direction of the microwaves should be the same with the precession direction of the magnetization. In this letter however, we show that microwaves initially rotating opposite to the magnetization precession destabilize the magnetization at an equilibrium and induces the switching more efficiently, when the microwave frequency depends on time. This argument is analytically deduced from energy balance equation. We also establish a model satisfying this condition, and confirm magnetization switching solely by microwaves by using numerical simulation.
Linewidth of Power Spectrum Originated from Thermal Noise in Spin Torque Oscillator
Tomohiro Taniguchi
Physics , 2014, DOI: 10.7567/APEX.7.053004
Abstract: A theoretical formula of the linewidth caused by the thermal activation in a spin torque oscillator with a perpendicularly magnetized free layer and an in-plane magnetized pinned layer was developed by solving the stochastic Landau-Lifshitz-Gilbert equation in the energy-phase representation. It is shown that the linewidth can be suppressed down to 0.1 MHz by applying a large current (10 mA for typical material parameters). A quality factor larger than 10^{4} is predicted in the large current limit, which is two orders of magnitude larger than the recently observed experimental value.
Magnetization switching by microwaves synchronized in the vicinity of precession frequency
Tomohiro Taniguchi
Physics , 2015, DOI: 10.7567/APEX.8.083004
Abstract: We propose a theoretical framework of a magnetization switching induced solely by a microwave. The microwave frequency is always close to but slightly different from the oscillation frequency of the magnetization. By efficiently absorbing energy from the microwave, the magnetization climbs up the energy landscape to synchronize the precession with the microwave. We introduced a dimensionless parameter $\epsilon$ determining the difference between the microwave frequency and the instant oscillation frequency of the magnetization. We analytically derived the condition of $\epsilon$ to switch the magnetization, and confirmed its validity by the comparison with numerical simulations.
Dependence of critical current of spin transfer torque-driven magnetization dynamics on free layer thickness
Tomohiro Taniguchi,Hiroshi Imamura
Physics , 2009, DOI: 10.1063/1.3070622
Abstract: The dependence of the critical current of spin transfer torque-driven magnetization dynamics on the free-layer thickness was studied by taking into account both the finite penetration depth of the transverse spin current and spin pumping. We showed that the critical current remains finite in the zero-thickness limit of the free layer for both parallel and anti-parallel alignments. We also showed that the remaining value of the critical current of parallel to anti-parallel switching is larger than that of anti-parallel to parallel switching.
Enhancement of the Gilbert damping constant due to spin pumping in noncollinear ferromagnet/nonmagnet/ferromagnet trilayer systems
Tomohiro Taniguchi,Hiroshi Imamura
Physics , 2007, DOI: 10.1103/PhysRevB.76.092402
Abstract: We analyzed the enhancement of the Gilbert damping constant due to spin pumping in non-collinear ferromagnet / non-magnet / ferromagnet trilayer systems. We show that the Gilbert damping constant depends both on the precession angle of the magnetization of the free layer and on the direction of the magntization of the fixed layer. We find the condition to be satisfied to realize strong enhancement of the Gilbert damping constant.
Theory of Spin Torque Assisted Thermal Switching of Single Free Layer
Tomohiro Taniguchi,Hiroshi Imamura
Physics , 2012, DOI: 10.1109/TMAG.2012.2196979
Abstract: The spin torque assisted thermal switching of the single free layer was studied theoretically. Based on the rate equation, we derived the theoretical formulas of the most likely and mean switching currents of the sweep current assisted magnetization switching, and found that the value of the exponent $b$ in the switching rate formula significantly affects the estimation of the retention time of magnetic random access memory. Based on the Fokker-Planck approach, we also showed that the value of $b$ should be two, not unity as argued in the previous works.
Dependence of spin torque switching probability on electric current
Tomohiro Taniguchi,Hiroshi Imamura
Physics , 2012, DOI: 10.1166/jnn.2012.6557
Abstract: Dependence of the thermally assisted spin torque switching probability on the sweep electric current was investigated theoretically. The analytical expressions of the switching times for $b=1$ and $b=2$ are derived based on the rate equation, where $b$ is the exponent of the current term in the switching rate. The switching current is approximately proportional to the temperature $T$ and the logarithm of the sweep rate $v$ for both $b=1$ and $b=2$ in the experimentally performed ranges of $T$ and $v$. Experiments in very low temperature range are required to determine the exponent $b$.
Critical current of spin transfer torque-driven magnetization dynamics in magnetic multilayers
Tomohiro Taniguchi,Hiroshi Imamura
Physics , 2008, DOI: 10.1103/PhysRevB.78.224421
Abstract: The critical current of the spin transfer torque-driven magnetization dynamics was studied by taking into account both spin pumping and the finite penetration depth of the transverse spin current. We successfully reproduced the recent experimental results obtained by Chen et al. [Phys. Rev. B {\bf 74}, 144408 (2006)] and found that the critical current remains finite even in the zero-thickness limit of the free layer. We showed that the remaining value of the critical current is determined mainly by spin pumping.
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