Katsuyoshi Matsushita

Microwave Generation on Geometrically Constrained Magnetic Wall: Effect of Twist Angle

We studied the current-induced magnetization dynamics of geometrically constrained magnetic walls (GCMWs) with twist angles of 90–180°, in order to examine their possibility as microwave generators using the effects of the twist angles. We numerically solved the Landau–Lifshitz–Gilbert equation with a spin-transfer torque term and found that the magnetic structures of GCMWs with certain twist angles oscillate between the Bloch and Neel walls (OBN) with microwave frequency, even though the twist angle is not 180°. We also found the critical twist angle below which the oscillation of the magnetic structure is inhibited. We showed that the oscillation frequency shows a hysteresis when we vary the twist angle of a GCMW or the corresponding applied magnetic field. Then, we also discussed the microwave generation induced by the periodic-deformation dynamics of GCMWs in the OBN.

Frustration-induced protein intrinsic disorder

Spontaneous folding into a specific native structure is the most important property of protein to perform their biological functions within organisms. Spontaneous folding is understood on the basis of an energy landscape picture based on the minimum frustration principle. Therefore, frustration seemingly only leads to protein functional disorder. However, frustration has recently been suggested to have a function in allosteric regulation. Functional frustration has the possibility to be a key to our deeper understanding of protein function. To explore another functional frustration, we theoretically examined structural frustration, which is designed to induce intrinsic disorder of a protein and its function through the coupled folding and binding. We extended the Wako-Saitô-Muñoz-Eaton model to take into account a frustration effect. With the model, we analyzed the binding part of neuron-restrictive silencer factor and showed that designed structural frustration in it induces intrinsic disorder. Furthermore, we showed that the folding and the binding are cooperative in interacting with a target protein. The cooperativity enables an intrinsically disordered protein to exhibit a sharp switch-like folding response to binding chemical potential change. Through this switch-like response, the structural frustration may contribute to the regulation function of interprotein interaction of the intrinsically disordered protein.

AC-Driven Breathing Mode of Confined Magnetic Domain Wall

We theoretically clarified the properties of ac-driven breathing modes of a confined magnetic domain wall. By numerically solving equations of motion for the domain wall, we found three characteristic breathing modes: forced harmonic, regular nonharmonic, and irregular nonharmonic modes. The latter two modes are characterized by the commensurability of the oscillation amplitude of the azimuthal angle of magnetization in the domain wall with a period of the magnetic anisotropy potential. They periodically and alternatively appear as ac amplitude increases above a threshold. We also showed that the voltage signal due to the domain wall depends on the mode.

Thermal stability of the geometrically constrained magnetic wall and its effect on a domain-wall spin valve

We studied thermal fluctuations in magnetoresistance (MR) signals originating from geometrically confined magnetic walls (GCMWs) of nanometer size. To this end, we developed a novel numerical simulation method which quantitatively evaluates the magnitude of thermal fluctuations in MR signals of magnetic nanostructures. Using the method, we first investigated the case when the twist angle Θ between the magnetization in a fixed layer and that in a free layer is 180°. We found that the thermal fluctuations of the magnetic structure of the 180° GCMW do not induce any crucial fluctuations in the MR signal because there is no significant difference among the MR values of the magnetic structures caused by the thermal fluctuations. We next investigated the dependence on the twist angle Θ of thermal fluctuations in MR signals. Since the GCMW is stabilized by decreasing Θ from 180°, the standard deviation (SD) of the MR signal is reduced with decreasing Θ. On the contrary, the SD/M ratio (M is the mean of the MR si...

Current-Induced Exchange Length and Geometrically Constrained Magnetic Wall

We analyzed stable magnetic structures of a geometrically constrained magnetic wall under direct current (dc) and found that there exists a characteristic length induced by the competition between the spin transfer torque and the torque due to exchange interaction. This current-induced exchange length is inversely proportional to the applied dc, and the magnetic ripple structure appears when the current-induced exchange length is smaller than the nanoscale constriction. We also found that the amplitude of the magnetic ripple structure oscillates as a function of the applied dc, which can be observed as an oscillation of the resistance against the applied dc.

Cell-alignment patterns in the collective migration of cells with polarized adhesion

Dictyostelium discoideum (Dd) utilizes inhomogeneities in the distribution of cell-cell adhesion molecules on cell membranes for collective cell migration. A simple example of an inhomogeneity is a front-side (leading-edge) polarization in the distribution at the early streaming stage. Experiments have shown that the polarized cell-cell adhesion induces side-by-side contact between cells [Beug et al., Nature (London) 274, 445 (1978)NATUAS0028-083610.1038/274445a0]. This result is counterintuitive, as one would expect cells to align front to front in contact with each other on the basis of front-side polarization. In this work, we theoretically examine whether front-side polarization induces side-by-side contact in collective cell migration. We construct a model for expressing cells with this polarization based on the two-dimensional cellular Potts model. By a numerical simulation with this model, we find cell-cell alignment wherein cells form lateral arrays with side-by-side contacts as observed in the experiments.

Design of Domain Wall Spin Torquemeter

We investigate an AC-driven oscillation of a domain wall in a one-dimensional clean ferromagnetic wire in order to design an effective domain wall spin torquemeter. The amplitude of the oscillation is evaluated by the collective coordinate method, and its dependence on the nonadiabatic torque constant β is clarified. We showed that the amplitude has an oscillatory component as a function of the AC amplitude. The oscillation period is inversely proportional to |β − α|, where α is the Gilbert damping constant. This result indicates that the measurement of the amplitude enables us to precisely evaluate β.

Magnon Turbulence in Ferromagnetic Nanocontact

We theoretically examine the minimal factors for the emergence of magnon turbulence under current. We show that the turbulence emerges even in the nanocontact, where only the exchange term and the spin-transfer torque contribute in the determination of the magnetic structure and dynamics therein. Namely, the sufficient factors for the emergence of the turbulence are only the exchange and the spin-transfer torque. We also show that the critical spin current density of the magnon turbulence is related to a current-induced exchange length. The emergence of the magnon turbulence is characterized by a crossover of the length and the system size.

Chaos in AC-Driven Motion of Confined Magnetic Domain Wall

We examined ac-driven motion of a confined magnetic domain wall for its time periodicity in order to explore current-induced chaos of domain wall motion. It is known that the motion is a time periodic or nonperiodic, depending on ac amplitude. We showed that the transition from the time periodic to nonperiodic motion occurs with a period doubling cascade with increasing ac amplitude. In the transition, the attractor changes from a periodic to chaotic one through the cascade and further changes into another chaotic one in a crisis at a higher ac amplitude. We also showed that the transition from nonperiodic to periodic motion with increasing ac amplitude originates from the emergence of the periodic attractor with the disappearance of the chaotic attractor.

Swing Casting Boost for Confined Domain Wall Breathing

A steady breathing mode excitation of the confined domain wall under dc is a candidate signal source in future telecommunication devices. The breathing mode exhibits a non-linear response to dc with a threshold. Before driving the breathing mode, if we resonantly swing the domain wall by ac, the domain wall is expected to be cast to a breathing mode even below the threshold dc. To examine the effect of “swing casting”, we investigate the resonant mode under ac. We show that overdamped, underdamped, chaotic, and periodic running modes emerge depending on the applied ac amplitude. Furthermore, we demonstrate that swing casting achieves the steady breathing mode excitation down to one quarter of the threshold dc.