Research Group for Spin-energy Science
Group Leader: SAITOH Eiji
Members: IEDA Jun’ichi, MORI Michiyasu, ONISHI Hiroaki, CHUDO Hiroyuki, NAKATAKoki, YAMAMOTO Kei, ARAKI Yasufumi, IMAI Masaki, SATO Nana, UMEDA Maki

We investigate solid-state physics stemming from "spin", aquantum of angular momentum, i.e., spintronics, magnonics,gyromagnetics, strongly correlated systems, and topologicalphysics. Exploiting the rectification nature of spin, wetheoretically and experimentally pursue the potential of spin
toward quantum science innovation that will be the basis of nextgeneration energy-saving and energy-creating technologies.
New development in power control research with thequantum relativistic effect of electron spins
Inductors are basic electric-circuit components for transformers,
noise filtering, switching, etc. The mechanism relies on classicalelectromagnetism: a conducting coil stores the magnetic fieldenergy when an ac electric current flows through it, leading to theinduction of an electromotive force that opposes the current
change. An inductance, L, of a solenoid coil is proportional to thecross-sectional area, A, of the solenoid [Fig. 1(a)]. This size
dependence limits miniaturizing inductors, and thus, to overcome
the fundamental difficulty, a new principle beyond the classicalelectromagnetic induction is requisite.
Recently, the spin-extension of inductor operation in magnetic
nanostructures, the so-called “emergent inductor,” was invented
by using a spiral magnet [Fig. 1(b)]. When an electric current
flows in a spiral magnet, it stores the energy in the spiralmagnetization texture via its exchange coupling with conduction
electron spins. Contrary to the coil inductance, the emergent
inductance, L, is inversely proportional to the area, A, that thecurrent passes through (L ∝ A-1). The concept of the emergent
inductor is not limited to spiral magnetization dynamics. In fact,
we have unveiled that a novel inductance of a spin-orbit coupling(SOC) origin is possible [1], where a SOC mediates the energy
conversion with the prominent size dependence [Fig. 1(c)]. The
spin-orbit inductance with spatially uniform magnetization is ofparticular interest; it provides nearly frequency-independent
inductance, except in the vicinity of the ferromagnetic resonancefrequency (typically a few GHz). This finding clarifies that theinductor function can be realized by standard magnetic materials
without "twists" like conventional coils and magnetic spiralstructures.

Modification of magnetic properties in a magnetic insulator
Insulating materials have garnered attention toward future
information and communication technologies because of the low
dissipation of energy, diversity of material choices, and a varietyof couplings among elementary excitations in solids. By using theangular momentum transport of spin waves (magnons) inmagnetic insulators, spin information can be transferred to solids
despite the electrons being immobile. For this purpose,
ferrimagnetic rare-earth garnets are particularly suitable because
they have extremely stable crystals and long-lived wellcharacterized magnetic excitations. To further enrich thepotential of this material series, we have demonstrated a magnetic
property modification in a magnetic insulator caused by theaccumulation of nanocolumnar defects based on numericalsimulations and heavy-ion beam irradiation [2].
With an increase in the ion irradiation dose, Φ, modifications inthe saturation magnetization, Ms, and magnetic coercivity, Hc,
were observed [Fig. 2(a)]; the decrease in saturation
magnetization caused by ion-beam damage was monotonic withincreasing beam fluence [Fig. 2(b)], whereas the enhancement ofmagnetic coercivity exhibited a fluence threshold [Fig. 2(c)].
These behaviors qualitatively agree with the numericalsimulations and models based on the continuum percolation
theory. The present method can be used for magnetic property
modification, opening a new pathway to a microstructuring
technique for magnetic insulators.

Fig. 2 Results of the ion-beam irradiation experiments. (a) Magnetization
hysteresis of the irradiated samples with ion dose of Φ = 0-2×1012
ions/cm2 (data offset for clarity). Beam fluence dependence of (b)the saturation magnetization, Ms, and of (c) the magnetic coercivity,
Hc, where the shaded area indicates over the percolation threshold.
Fig. 1 Schematics of the inductance, L, vs cross-sectional area, A, (upperpanels) for the (a) conventional coil inductor, (b) emergent
inductor based on a spiral magnet, and (c) spin-orbit emergent
inductor based on a collinear magnet showing SOC (lower panels)
where I represents the applied ac current. Arrows in the lowerpanels of (b) and (c) illustrate magnetization direction.

References[1] Y. Yamane, S. Fukami and J. Ieda, Phys. Rev. Lett. 128,147201 (2022).
[2] K. Harii, M. Umeda, H. Arisawa, T. Hioki, N. Sato, S.
Okayasu, and J. Ieda, J. Phys. Soc. Jpn. 92, 073701 (2023).