Kyoto University · Physics and Astronomy
Professor Yoichi Shiota's research lab specializes in spintronics and nanomagnetic devices, focusing on voltage-controlled magnetic phenomena, spin wave dynamics, and ultra-low power magnetization switching. The lab investigates fundamental mechanisms of magnon-magnon coupling, nonreciprocal spin wave propagation, and voltage-induced perpendicular magnetic anisotropy in synthetic antiferromagnets and magnetic tunnel junctions. Their work aims to enable energy-efficient, high-speed spintronic devices through precise control of magnetic states using electric fields and spin-wave engineering. Key achievements include demonstrating sub-nanosecond switching with extremely low error rates and giant nonreciprocal frequency shifts for logic applications.
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We report an experimental observation of magnon-magnon coupling in interlayer exchange coupled synthetic antiferromagnets of FeCoB/Ru/FeCoB layers. An anticrossing gap of spin-wave resonance between acoustic and optic modes appears when the external magnetic field points to the direction tilted from the spin-wave propagation. The magnitude of the gap (i.e., coupling strength) can be controlled by changing the direction of the in-plane magnetic field and also enhanced by increasing the wave numbe
Abstract We investigated the write error rate (WER) for voltage-driven dynamic switching in magnetic tunnel junctions with perpendicular magnetization. We observed a clear oscillatory behavior of the switching probability with respect to the duration of pulse voltage, which reveals the precessional motion of magnetization during voltage application. We experimentally demonstrated WER as low as 4 × 10 −3 at the pulse duration corresponding to a half precession period (∼1 ns). The comparison betwe
We report a voltage-induced perpendicular magnetic anisotropy (PMA) change in sputter-deposited Ta|CoFeB|MgO and Ru|CoFeB|MgO junctions. The PMA change is quantitatively evaluated by the field dependence of the tunneling magnetoresistance for various bias voltages. We find that both the sign and amplitude of the voltage effect depend on the underlayer, Ta or Ru, below the CoFeB layer. The rf voltage-induced ferromagnetic resonance spectra also support the underlayer-material-dependent direction
The nonreciprocity of propagating spin waves, i.e., the difference in amplitude and/or frequency depending on the propagation direction, is essential for the realization of spin wave-based logic circuits. However, the nonreciprocal frequency shifts demonstrated so far are not large enough for applications because they originate from interfacial effects. In addition, switching of the spin wave nonreciprocity in the electrical way remains a challenging issue. Here, we show a switchable giant nonre
We investigated pulse voltage-induced dynamic magnetization switchings in magnetic tunneling junctions with a high resistance-area product of 2 kΩ μm2. We found that bistable switching and the oscillatory behavior of switching probability as a function of voltage pulse duration are realized at a lower current density (−1.1 × 105 A/cm2) than in conventional spin-transfer-torque-induced magnetization switching. In addition, the switching probability at different voltage pulse strengths confirmed t
In this study, we demonstrate voltage-driven dynamic magnetization switching for the write error rate (WER) of the order of 10−5. The largest voltage effect on the perpendicular magnetic anisotropy in Ta/(CoxFe100–x)80B20/MgO structure (x = 0, 10, 31, 51) is obtained for x = 31 after annealing at 250 °C. Based on investigations using perpendicularly magnetized magnetic tunnel junctions that have different (Co31Fe69)80B20 free layer thicknesses, we demonstrate that the improvement in the thermal
We investigated the voltage-induced ferromagnetic resonance (FMR) with various DC bias voltage and input RF power in magnetic tunnel junctions. We found that the DC bias monotonically increases the homodyne detection voltage due to the nonlinear FMR originating in an asymmetric magnetization-potential in the free layer. In addition, the linear increase of an output voltage to the input RF power in the voltage-induced FMR is more robust than that in spin-torque FMR. These characteristics enable u
Unlike ferromagnets, collinear antiferromagnets have two precession modes exhibiting opposite chirality, which enables the use of the polarization degree of freedom of magnons. In this study, we experimentally measure the polarization-selective spectra of magnetic resonance in perpendicularly magnetized synthetic antiferromagnets using a wideband crossed microstrip circuit. The magnetic resonances with right-handed and left-handed precession modes are directly detected, and theoretical analysis
In this study, we investigated the electric field effect on interfacial perpendicular magnetic anisotropy (i-PMA) and interfacial Dzyaloshinskii–Moriya interaction (i-DMI) by propagating spin wave spectroscopy with a high degree of accuracy in Pt/Co/MgO structures. We estimated i-PMA and i-DMI energy densities of 0.71 and 0.45 mJ/m2 from the nonreciprocity of spin wave propagation, and the electric field effect on them of 2.5 and 2.0 fJ/(V·m) from symmetrical and asymmetrical electrical modulati
We investigated caustic-like spin wave beams radiated from a narrow waveguide to a continuous film using a Permalloy. For this purpose, we developed a spatially resolved optical heterodyne detection of propagating spin waves by using the polar Kerr effect, which allows the measurement of the intensity and the phase information of the spin waves. We characterized magnetostatic surface spin wave modes in one-dimensional (1D) propagation and caustic-like spin wave beams in two-dimensional (2D) prop
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