arXiv subjects

Hiroshi Imamura

Publications and source records attributed to Hiroshi Imamura.

At least 55 records · Page 3Linked to original sources

arXiv2012

Dependence of spin torque switching probability on electric current

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 $b$.

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arXiv2012

Theory of Spin Torque Assisted Thermal Switching of Single Free Layer

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.

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arXiv2011

Electron density distribution of bilayer nanographene and band structures of boron-carbon-nitride systems

Bilayer graphene nanoribbon with zigzag edge is investigated with the tight binding model. Two stacking structures, alpha and beta, are considered. The band splitting is seen in the alpha structure, while the splitting in the wave number direction is found in the beta structure. The local density of states in the beta structure tend to avoid sites where interlayer hopping interactions are present. The calculation is extended to the boron-carbon-nitride systems. The qualitative properties persist when zigzag edge atoms are replaced with borons and nitrogens.

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arXiv2011

Minimization of the Switching Time of a Synthetic Free Layer in Thermally Assisted Spin Torque Switching

We theoretically studied the thermally assisted spin torque switching of a synthetic free layer and showed that the switching time is minimized if the condition H_J=|H_s|/(2 alpha) is satisfied, where H_J, H_s and alpha are the coupling field of two ferromagnetic layers, the amplitude of the spin torque, and the Gilbert damping constant. We also showed that the coupling field of the synthetic free layer can be determined from the resonance frequencies of the spin-torque diode effect.

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arXiv2011

Edge States and Stacking Effects in Nanographene Systems

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arXiv2011

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.

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arXiv2010

Proposal of a full Bell state analyzer for spin qubits in a double quantum dot

We propose a scheme for full Bell state measurement of spin qubits in a double quantum dot. Our scheme consists of Pauli spin-blockade measurements and biaxial electron-spin resonance. In order to eliminate the average of the Zeeman fields, the double quantum dot is designed so that the Landé g-factors of first and second dots satisfy g1=g2 with the use of g-factor engineering. Thus, we can swap one of the three spin-triplet states for the spin-singlet state without disturbing the other states. Our study shows that the sequential spin-to-charge conversions enable us to implement the full Bell state measurement of electron-spin qubits.

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arXiv2009

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 non-perturbative wave function of the conduction electrons and the non-equilibrium 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 (>0.8) and a small period (about 1-2 nm).

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arXiv2009

Dipolar field effect on microwave oscillation in a domain wall spin-valve

We examined dipolar field effects on the microwave generation in the domain wall spin-valve by solving simultaneously the Landau-Lifshitz-Gilbert and Zhang-Levy-Fert diffusion equations. By numerically analyzing dipolar field dependence, we showed that the microwave generation needs the dipole-dipole interaction for a 180$^\circ$ domain wall and the amplitude of the microwave voltage signal depends strongly on the exchange length. In order to design a microwave generator using the domain wall spin-valve with high efficiency, we propose that the materials with short exchange length are preferred.

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arXiv2009

Electrical measurement of a two-electron spin state in a double quantum dot

We propose a scheme for electrical measurement of two-electron spin states in a semiconductor double quantum dot. We calculated the adiabatic charge transfer when surface gates are modulated in time. Because of spin-orbit coupling in the semiconductor, spatial displacement of the electrons causes a total spin rotation. It follows that the expectation value of the transferred charge reflects the relative phase as well as the total spin population of a prepared singlet-triplet superposition state. The precise detection of the charge transfer serves to identify the quantum superposition.

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arXiv2009

Conductance oscillation due to the geometrical resonance in FNS double junctions

We theoretically analyzed the Andreev reflection in ferromagnetic metal/nonmagnetic metal/superconductor double junctions with special attention to the electron interference effect in the nonmagnetic metal layer. We showed that the conductance oscillates as a function of the bias voltage due to the geometrical resonance. We found that the exchange field, and therefore the spin polarization of the ferromagnetic metal can be determined from the period of the conductance oscillation, which is proportional to the square root of the exchange field.

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arXiv2009

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.

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arXiv2009

Current-induced microwave excitation of a domain wall confined in a magnetic wire with bi-axial anisotropy

We studied the current-induced magnetization dynamics of a domain wall confined in a magnetic wire with bi-axial anisotropy. We showed that above the threshold current density, breathing-mode excitation, where the thickness of the domain wall oscillates, is induced by spin-transfer torque. We found that the breathing-mode can be applied as a source of microwave oscillation because the resistance of the domain wall is a function of the domain wall thickness. In a current sweep simulation, the frequency of the breathing-mode exhibits hysteresis because of the confinement.

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arXiv2009

Simulation of current-induced microwave oscillation in geometrically confined domain wall

We studied magnetization dynamics of a geometrically confined domain wall under dc current by solving simultaneously the Landau-Lifshitz-Gilbert equation and diffusion equation for spin accumulation. We showed that the oscillation motion of the domain wall is driven by the spin-transfer torque and the dc current is converted to the ac voltage signal. The results means that the geometrically confined domain wall is applicable as a source of microwave oscillator.

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arXiv2009

Current-perpendicular-to-plane magnetoresistance of a domain wall confined in a nano-oxide-layer

We theoretically study the current-perpendicular-to-plane magnetoresistance of a domain wall confined in a current-confined-path (CCP) structure made of a nano-oxide-layer (NOL). In order to calculate the MR ratio of the system, the continuity equations for charge and spin currents are numerically solved with the three-dimensional CCP geometry by use of finite element method. It is confirmed that the MR ratio is enhanced by the CCP structure, which is consistent with the experimental results.

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arXiv2009

Dependence of critical current of spin transfer torque-driven magnetization dynamics on free layer thickness

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.

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arXiv2009

Magnetic structure of domain walls confined in a nano-oxide layer

In the recent years, a spin-valve was developed with a current-confined-path structure consisting of a non-oxide-layer (NOL). We analyze magnetic structures of the current-confinedpath in the nano-oxide layer sandwiched between ferromagnetic electrodes and clarify the dependency of the magnetic structure on the shape and size of the current-confined-path. Our results of stiffness energy density and thermal stability shows that we should fabricate a CPP spin-valve with narrow current-confinedpaths with large aspect ratio and strong constriction in order to enhance the MR ratio.

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arXiv2008

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 {\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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