arXiv subjects

Tomohiro Taniguchi

Publications and source records attributed to Tomohiro Taniguchi.

At least 73 records · Page 4Linked to original sources

arXiv2015

Critical current destabilizing perpendicular magnetization by the spin Hall effect

The critical current needed to destabilize the magnetization of a perpendicular ferromagnet via the spin Hall effect is studied. Both the dampinglike and fieldlike torques associated with the spin current generated by the spin Hall effect is included in the Landau-Lifshitz-Gilbert equation to model the system. In the absence of the fieldlike torque, the critical current is independent of the damping constant and is much larger than that of conventional spin torque switching of collinear magnetic systems, as in magnetic tunnel junctions. With the fieldlike torque included, we find that the critical current scales with the damping constant as $α^{0}$ (i.e., damping independent),$α$, and $α^{1/2}$ depending on the sign of the fieldlike torque and other parameters such as the external field. Numerical and analytical results show that the critical current can be significantly reduced when the fieldlike torque possesses the appropriate sign, i.e. when the effective field associated with the fieldlike torque is pointing opposite to the spin direction of the incoming electrons. These results provide a pathway to reducing the current needed to switch magnetization using the spin Hall effect.

cond-mat.mes-hall↗

arXiv2015

Magnetization switching by microwaves synchronized in the vicinity of precession frequency

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 $ε$ determining the difference between the microwave frequency and the instant oscillation frequency of the magnetization. We analytically derived the condition of $ε$ to switch the magnetization, and confirmed its validity by the comparison with numerical simulations.

cond-mat.mes-hall↗

arXiv2015

Current driven asymmetric magnetization switching in perpendicularly magnetized CoFeB/MgO heterostructures

The flow of in-plane current through ultrathin magnetic heterostructures can cause magnetization switching or domain wall nucleation owing to bulk and interfacial effects. Within the magnetic layer, the current can create magnetic instabilities via spin transfer torques (STT). At interface(s), spin current generated from the spin Hall effect in a neighboring layer can exert torques, referred to as the spin Hall torques, on the magnetic moments. Here, we study current induced magnetization switching in perpendicularly magnetized CoFeB/MgO heterostructures with a heavy metal (HM) underlayer. Depending on the thickness of the HM underlayer, we find distinct differences in the inplane field dependence of the threshold switching current. The STT is likely responsible for the magnetization reversal for the thinner underlayer films whereas the spin Hall torques cause the switching for thicker underlayer films. For the latter, we find differences in the switching current for positive and negative currents and initial magnetization directions. We find that the growth process during the film deposition introduces an anisotropy that breaks the symmetry of the system and causes the asymmetric switching. The presence of such symmetry breaking anisotropy enables deterministic magnetization switching at zero external fields.

cond-mat.mtrl-sci↗

arXiv2015

Nonlinear analysis of magnetization dynamics excited by spin Hall effect

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.

cond-mat.mes-hall↗

arXiv2015

Large amplitude oscillation of magnetization in spin-torque oscillator stabilized by field-like torque

Oscillation frequency of spin torque oscillator with a perpendicularly magnetized free layer and an in-plane magnetized pinned layer is theoretically investigated by taking into account the field-like torque. It is shown that the field-like torque plays an important role in finding the balance between the energy supplied by the spin torque and the dissipation due to the damping, which results in a steady precession. The validity of the developed theory is confirmed by performing numerical simulations based on the Landau-Lifshitz-Gilbert equation.

cond-mat.mes-hall↗

arXiv2014

Role of Magnetic Field in Self-Oscillation of Nanomagnet Excited by Spin Torque

The critical current of the self-oscillation of spin torque oscillator (STO) consisting of a perpendicularly magnetized free layer and an in-plane magnetized pinned layer was studied by solving the Landau-Lifshitz-Gilbert (LLG) equation. We found that the critical current diverged at certain field directions, indicating that the self-oscillation does not occur at these directions. It was also found that the sign of the critical current changed depending on the applied field direction.

cond-mat.mes-hall↗

arXiv2014

Dissipation due to pure spin-current generated by spin pumping

Based on spin-dependent transport theory and thermodynamics, we develop a generalized theory of the Joule heating in the presence of a spin current. Along with the conventional Joule heating consisting of an electric current and electrochemical potential, it is found that the spin current and spin accumulation give an additional dissipation because the spin-dependent scatterings inside bulk and ferromagnetic/nonmagnetic interface lead to a change of entropy. The theory is applied to investigate the dissipation due to pure spin-current generated by spin pumping across a ferromagnetic/nonmagnetic/ferromagnetic multilayer. The dissipation arises from an interface because the spin pumping is a transfer of both the spin angular momentum and the energy from the ferromagnet to conduction electrons near the interface. It is found that the dissipation is proportional to the enhancement of the Gilbert damping constant by spin pumping.

cond-mat.mes-hall↗

arXiv2014

Spin transfer torques generated by the anomalous Hall effect and anisotropic magnetoresistance

Spin-orbit coupling in ferromagnets gives rise to the anomalous Hall effect and the anisotropic magnetoresistance, both of which can be used to create spin-transfer torques in a similar manner as the spin Hall effect. In this paper we show how these effects can be used to reliably switch perpendicularly magnetized layers and to move domain walls. A drift-diffusion treatment of the anomalous Hall effect and the anisotropic magnetoresistance describes the spin currents that flow in directions perpendicular to the electric field. In systems with two ferromagnetic layers separated by a spacer layer, an in-plane electric field cause spin currents to be injected from one layer into the other, creating spin transfer torques. Unlike the related spin Hall effect in non-magnetic materials, the anomalous Hall effect and the anisotropic magnetoresistance allow control of the orientation of the injected spins, and hence torques, by changing the direction of the magnetization in the injecting layer. The torques on one layer show a rich angular dependence as a function of the orientation of the magnetization in the other layer. The control of the torques afforded by changing the orientation of the magnetization in a fixed layer makes it possible to reliably switch a perpendicularly magnetized free layer. Our calculated critical current densities for a representative CoFe/Cu/FePt structure show that the switching can be efficient for appropriate material choices. Similarly, control of the magnetization direction can drive domain wall motion, as shown for NiFe/Cu/NiFe structures.

cond-mat.mtrl-sci↗

arXiv2014

Fluctuation Theorem for a Small Engine and Magnetization Switching by Spin Torque

We consider a reversal of the magnetic moment of a nano-magnet by the fluctuating spin-torque induced by a non-equilibrium current of electron spins. This is an example of the problem of the escape of a particle from a metastable state subjected to a fluctuating non-conservative force. The spin-torque is the non-conservative force and its fluctuations are beyond the description of the fluctuation-dissipation theorem. We estimate the joint probability distribution of work done by the spin torque and the Joule heat generated by the current, which satisfies the fluctuation theorem for a small engine. We predict a threshold voltage above which the spin-torque shot noise induces probabilistic switching events and below which such events are blocked. We adopt the theory of the full-counting statistics under the adiabatic pumping of spin angular momentum. This enables us to account for the backaction effect, which is crucial to maintain consistency with the fluctuation theorem.

cond-mat.mes-hall↗

arXiv2014

Magnetization reversal condition for a nanomagnet within a rotating magnetic field

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.

cond-mat.mes-hall↗

arXiv2014

Linewidth of Power Spectrum Originated from Thermal Noise in Spin Torque Oscillator

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.

cond-mat.mes-hall↗

arXiv2014

Self-Oscillation in Spin Torque Oscillator Stabilized by Field-like Torque

The self-oscillation of the magnetization in a spin torque oscillator (STO) with a perpendicularly magnetized free layer and an in-plane magnetized pinned layer in the absence of an applied magnetic field was studied by numerically solving the Landau-Lifshitz-Gilbert equation. It was pointed out that field-like torque was necessary to realize stable self-oscillation in this type of STO at zero field. The numerical simulation at finite temperature showed that the presence of the field-like torque led to a high power with a relatively high oscillation frequency.

cond-mat.mes-hall↗

arXiv2014

Current dependence of spin torque switching rate based on Fokker-Planck approach

The spin torque switching rate of an in-plane magnetized system in the presence of an applied field is derived by solving the Fokker-Planck equation. It is found that three scaling currents are necessary to describe the current dependence of the switching rate in the low-current limit. The dependences of these scaling currents on the applied field strength are also studied.

cond-mat.mes-hall↗

arXiv2013

Thermally activated switching rate of a nanomagnet in the presence of spin torque

The current dependence of the spin torque switching rate in a thermally activated region of an in-plane magnetized system was studied. The logarithm of the switching rate depended nonlinearly on current in the high-current region, I_{c} < I < I_{c}^{*}, where I_{c} and I_{c}^{*} are critical currents distinguishing the stability of the magnetization. We also found that the attempt frequency had a minimum around I_{c}, and that the attempt frequency at I_{c} was three orders of magnitude smaller than that at zero current, contrary to the assumption in previous analyses of experiments that it remains constant.

cond-mat.mes-hall↗

arXiv2013

Critical Field of Spin Torque Oscillator with Perpendicularly Magnetized Free Layer

The oscillation properties of a spin torque oscillator consisting of a perpendicularly magnetized free layer and an in-plane magnetized pinned layer are studied based on an analysis of the energy balance between spin torque and damping. The critical value of an external magnetic field applied normal to the film plane is found, below which the controllable range of the oscillation frequency by the current is suppressed. The value of the critical field depends on the magnetic anisotropy, the saturation magnetization, and the spin torque parameter.

cond-mat.mes-hall↗

arXiv2013

Current Dependence of Spin Torque Switching Barrier

The current dependence of the switching barrier for spin torque switching of an in-plane magnetized ferromagnet was studied. Two scaling currents, I_{c}$ and I_{c}^{*}(>I_{c}), were introduced to distinguish the magnetization stability. In the low-current region I<I_{c}, the switching barrier is linear to the current with another scaling current \tilde{I}_{c}, while such linear scaling does not hold in the high-current region I_{c} < I < I_{c}^{*}. The linear scaling is valid for the high temperature and the long current pulse duration time.

cond-mat.mes-hall↗

arXiv2013

Dependence of spin torque diode voltage on applied field direction

The optimum condition of an applied field direction to maximize spin torque diode voltage was theoretically derived for a magnetic tunnel junction with a perpendicularly magnetized free layer and an in-plane magnetized pinned layer. We found that the diode voltage for a relatively small applied field is maximized when the projection of the applied field to the film-plane is parallel or anti-parallel to the magnetization of the pinned layer. However, by increasing the applied field magnitude, the optimum applied field direction shifts from the parallel or anti-parallel direction. These analytical predictions were confirmed by numerical simulations.

cond-mat.mes-hall↗

arXiv2013

Theoretical Study of Spin-Torque Oscillator with Perpendicularly Magnetized Free Layer

The magnetization dynamics of spin torque oscillator (STO) consisting of a perpendicularly magnetized free layer and an in-plane magnetized pinned layer was studied by solving the Landau-Lifshitz-Gilbert equation. We derived the analytical formula of the relation between the current and the oscillation frequency of the STO by analyzing the energy balance between the work done by the spin torque and the energy dissipation due to the damping. We also found that the field-like torque breaks the energy balance, and change the oscillation frequency.

cond-mat.mes-hall↗