Tomohiro Taniguchi

Thermally assisted spin transfer torque switching in synthetic free layers

We studied the magnetization reversal rates of thermally assisted spin transfer torque switching in a ferromagnetically coupled synthetic free layer theoretically. By solving the Fokker-Planck equation, we obtained the analytical expression of the switching probability for both the weak and the strong coupling limit. We found that the thermal stability is proportional to Delta_{0}(1-I/I_{c})^{2}, not Delta_{0}(1-I/I_{c}) argued by Koch et al. [Phys. Rev. Lett. 92, 088302 (2004)], where I and I_{c} are the electric current and the critical current of spin transfer torque switching at absolute zero temperature. The difference in the exponent of (1-I/I_{c}) leads to a significant underestimation of the thermal stability Delta_{0}. We also found that fast switching is achieved by choosing the appropriate direction of the applied field.

Effect of the number of layers on determination of spin asymmetries in current-perpendicular-to-plane giant magnetoresistance

Current-perpendicular-to-plane giant-magnetoresistance effect in a ferromagnetic/nonmagnetic multilayer is studied theoretically by solving the diffusion equation of the spin accumulation without assuming the periodic boundary condition. We show that the Valet and Fert theory underestimates spin asymmetries with decreasing the number of layers.

Critical current of spin-transfer-torque-driven magnetization dynamics in magnetic multilayers

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 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. We also showed that we could control the critical current by varying the spin-diffusion length of the nonmagnetic electrode adjacent to the free layer.

Theory of spin accumulation and spin-transfer torque in a magnetic domain wall

We study spin accumulation and spin transfer torque in a domain wall by solving the Boltzmann equation with a diffusion approximation. We obtained the analytical expressions of spin accumulation and spin transfer torque. Both the adiabatic and the non-adiabatic components of the spin transfer torque oscillate with the thickness of the domain wall. We show that the oscillation plays a dominant role in the non-adiabatic torque when the domain wall thickness is less than the spin-flip length, which is defined by the product of the Fermi velocity and the spin-flip scattering time.

Penetration Depth of Transverse Spin Current in Ferromagnetic Metals

The line width of the ferromagnetic resonance (FMR) spectrum of Cu/CoFeB/Cu/Co/Cu is studied. Analyzing the FMR spectrum by the theory of spin pumping, we determined the penetration depth of the transverse spin current in the Co layer. The obtained penetration depth of Co is 1.7 nm.

Angle dependence of the magnetoresistance of CCP-CPP-GMR system

Angle dependence of the magnetoresistance (MR) effect due to a magnetic domain wall confined in a current-confined-path current-perpendicular-to-plane giant magnetoresistance system was studied by solving the Boltzmann equation. We calculate the scattering rates by using the nonperturbative wave function of the conduction electrons, which enables us to invesigate the MR ratio of a sufficiently thin domain wall. We find that the dependence of the MR ratio on the rotation angle of the domain wall is approximately described by cos , which has a good agreement with the recent experimental results.

Boltzmann theory of magnetoresistance due to a spin spiral

We studied the magnetoresistance due to a spin spiral by solving the Boltzmann equation. The scattering rates of conduction electrons are calculated by using the nonperturbative wave function of the conduction electrons and the nonequilibrium distribution function is obtained by numerically solving the Boltzmann equation. These enable us to calculate the resistivity of a sufficiently thin spin spiral. A magnetoresistance ratio of more than 50% is predicted for a spin spiral with high spin polarization

SPIN PUMPING IN FERROMAGNETIC MULTILAYERS

(ensuremath{ge}0.8)

Critical current density of domain wall oscillation due to spin-transfer torque

and a small period (about 1char21{}2 nm).

Time evolution of spin accumulation and spin current in a magnetic domain wall

We present a brief review of our recent study on spin pumping in ferromagnetic multilayers. First, we present theoretical models describing spin pumping induced by ferromagnetic resonance (FMR). Then we apply the spin-pumping theory to FMR in ferromagnetic multilayers and show that the line width of the FMR spectrum depends on the thickness of the ferromagnetic metal layer which is not in resonance. We also show that the penetration depths of transverse spin current in ferromagnetic metals can be determined by analyzing the line width of the FMR spectrum. The obtained penetration depths of the transverse spin current were 3.7 nm for Py, 2.5 nm for CoFe, 12.0 nm for CoFeB, and 1.7 nm for Co, respectively.