Persistent magnetic coherence in magnets

巻内 崇彦*; 日置 友智*; 清水 祐樹*; 星 幸治郎*; Elyasi, M.*; 山本 慧; 横井 直人*; Serga, A. A.*; Hillebrands, B.*; Bauer, G. E. W.*; et al.

Nature Materials, 23(5), p.627 - 632, 2024/05

https://doi.org/10.1038/s41563-024-01798-z

When excited, a magnetization in a magnet precesses around the field in an anticlockwise manner on a time scale governed by viscous magnetization damping, after which any information carried by the initial actuation seems to be lost. The damping appears to be a fundamental bottleneck for the use of magnets in information processing. Nevertheless, here we demonstrate recall of the magnetization-precession phase after times that far exceed the damping time scale by two orders of magnitude using dedicated two-colour microwave pump-and-probe experiments for a YFeO microstructured film. Time- resolved magnetization state tomography confirms the magnetic persistent coherence (MPC) by revealing a double-exponential decay of the magnetization correlation. We attribute MPC to a feedback effect, viz. coherent coupling of the uniform precession with long-lived excitations at the minima of the spin-wave dispersion relation. Our finding liberates magnetic systems from the strong damping in nanostructures tyranny that limits their use in coherent information storage and processing.

Fast Magnons modulated by slow mechanical rotation

山本 慧

JPSJ News and Comments (Internet), 21, p.07_1 - 07_2, 2024/04

https://doi.org/10.7566/JPSJNC.21.07

A novel mechanism for generating net magnetization from phonons in chiral crystals builds upon the modern notion of geometric phases and extends the scope of century-old research efforts to understand the transfer of angular momentum between lattice and spins.

Valley-selective phonon-magnon scattering in magnetoelastic superlattices

Liao, L.*; Puebla, J.*; 山本 慧; Kim, J.*; 前川 禎通*; Hwang, Y.*; Ba, Y.*; 大谷 義近*

Physical Review Letters, 131(17), p.176701_1 - 176701_6, 2023/10

https://doi.org/10.1103/PhysRevLett.131.176701

Phonons and magnons are engineered by periodic potential landscapes in phononic and magnonic crystals, and their combined studies may enable valley phonon transport tunable by the magnetic field. Through nonreciprocal surface acoustic wave transmission, we demonstrate valley-selective phonon-magnon scattering in magnetoelastic superlattices. The lattice symmetry and the out-of-plane magnetization component control the sign of nonreciprocity. The phonons in the valleys play a crucial role in generating nonreciprocal transmission by inducing helical strains that couple with the magnons. The transmission spectra show a nonreciprocity peak near a transmission gap, matching the phononic band structure. Our results open the way for manipulating valley phonon transport through periodically varying magnon-phonon coupling.

Nonlinear magnon polaritons

Lee, O.*; 山本 慧; 埋田 真樹; Zollitsch, C. W.*; Elyasi, M.*; 吉川 貴史*; 齊藤 英治; Bauer, G. E. W.*; 紅林 秀和*

Physical Review Letters, 130(4), p.046703_1 - 046703_6, 2023/01

https://doi.org/10.1103/PhysRevLett.130.046703

We experimentally and theoretically demonstrate that nonlinear spin-wave interactions suppress the hybrid magnon-photon quasiparticle or "magnon polariton" in microwave spectra of an yttrium iron garnet film detected by an on-chip split-ring resonator. We observe a strong coupling between the Kittel and microwave cavity modes in terms of an avoided crossing as a function of magnetic fields at low microwave input powers, but a complete closing of the gap at high powers. The experimental results are well explained by a theoretical model including the three-magnon decay of the Kittel magnon into spin waves. The gap closure originates from the saturation of the ferromagnetic resonance above the Suhl instability threshold by a coherent back reaction from the spin waves.

二酸化炭素地中貯留のキーマテリアルである非晶質炭酸マグネシウムの構造とその特性

興野 純*; 山本 弦一郎*; 米田 安宏; 岡田 慧*

Isotope News, (783), p.23 - 27, 2022/10

二酸化炭素の地中貯蔵法として鉱物トラップが注目されている。二酸化炭素を含んだ地下水と玄武岩を反応させてマグネサイトを形成させる。形成される炭酸マグネシウム相は多様に変化するが、そのすべてがアモルファス炭酸マグネシウムを経るためその構造を明らかにすることを試みた。高エネルギーX線回折を用いた2体相関分布関数からアモルファス炭酸マグネシウムはハイドロマグネサイトと同様の構造を持つことが明らかとなり、長期的にも安定な構造であることからマグネシウムを含んだ炭酸塩化は二酸化炭素貯留技術として安全な隔離法と言える。

Long decay length of magnon-polarons in BiFeO/LaSrMnO heterostructures

Zhang, J.*; Chen, M.*; Chen, J.*; 山本 慧; Wang, H.*; Hamdi, M.*; Sun, Y.*; Wagner, K.*; He, W.*; Zhang, Y.*; et al.

Nature Communications (Internet), 12, p.7258_1 - 7258_8, 2021/12

https://doi.org/10.1038/s41467-021-27405-2

Magnons can transfer information in metals and insulators without Joule heating, and therefore are promising for low-power computation. The on-chip magnonics however suffers from high losses due to limited magnon decay length. In metallic thin films, it is typically on the tens of micrometre length scale. Here, we demonstrate an ultra-long magnon decay length of up to one millimetre in multiferroic/ferromagnetic BiFeO(BFO)/LaSrMnO (LSMO) heterostructures at room temperature. This decay length is attributed to a magnon-phonon hybridization and is more than two orders of magnitude longer than that of bare metallic LSMO. The long-distance modes have high group velocities of 2.5 km as detected by time-resolved Brillouin light scattering. Numerical simulations suggest that magnetoelastic coupling via the BFO/LSMO interface hybridizes phonons in BFO with magnons in LSMO. Our results provide a solution to the long-standing issue on magnon decay lengths in metallic magnets and advance the bourgeoning field of hybrid magnonics.

Nonreciprocal surface acoustic wave propagation via magneto-rotation coupling

Xu, M.*; 山本 慧; Puebla, J.*; Baumgaertl, K.*; Rana, B.*; 三浦 勝哉*; 高橋 宏昌*; Grundler, D.*; 前川 禎通*; 大谷 義近*

Science Advances (Internet), 6(32), p.eabb1724_1 - eabb1724_4, 2020/08

https://doi.org/10.1126/sciadv.abb1724

One of the most fundamental forms of magnon-phonon-interaction is an intrinsic property of magnetic materials, the "magnetoelastic coupling." This particular form of interaction has been the basis for describing magnetic materials and their strain related applications, where strain induces changes of internal magnetic fields. Different from the magnetoelastic coupling, more than 40 years ago, it was proposed that surface acoustic waves may induce surface magnons via rotational motion of the lattice in anisotropic magnets. However, a signature of this magnon-phonon coupling mechanism, termed magneto-rotation coupling, has been elusive. Here, we report the first observation and theoretical framework of the magneto-rotation coupling in a perpendicular anisotropic ultra-thin lim Ta/CoFeB/MgO, which consequently induces nonreciprocal acoustic wave attenuation with an unprecedented ratio up to 100% rectification at a theoretically predicted optimized condition. Our work not only experimentally demonstrates a fundamentally new path for investigating magnon-phonon coupling, but also justifies the feasibility of the magneto-rotation coupling based application.

Experimental realization of a passive Gigahertz frequency-division demultiplexer for magnonic logic networks

Heussner, F.*; Talmelli, G.*; Geilen, M.*; Heinz, B.*; Brcher, T.*; Meyer, T.*; Ciubotaru, F.*; Adelmann, C.*; 山本 慧; Serga, A. A.*; et al.

Physica Status Solidi; Rapid Research Letters, 14(4), p.1900695_1 - 1900695_6, 2020/04

https://doi.org/10.1002/pssr.201900695

The emerging field of magnonics employs spin waves and their quanta, magnons, to implement wave-based computing on the micro- and nanoscales. Multi-frequency magnon networks would allow for parallel data processing within single logic elements whereas this is not the case with conventional transistor-based electric logic. However, a lack of experimental proven solutions to efficiently combine and separate magnons of different frequencies has impeded the intensive use of this concept. In this Letter, the experimental realization of a spin-wave demultiplexer enabling frequency-dependent separation of magnetic signals in the GHz range is demonstrated. The device is based on two-dimensional magnon transport in the form of spin-wave beams in unpatterned magnetic films. The intrinsic frequency-dependence of the beam direction is exploited to realize a passive functioning obviating an external control and additional power consumption. This approach paves the way to magnonic multiplexing circuits enabling simultaneous information transport and processing.

Topological characterization of classical waves; The Topological origin of magnetostatic surface spin waves

山本 慧; Thiang, G. C.*; Pirro, P.*; Kim, K.-W.*; Everschor-Sitte, K.*; 齊藤 英治*

Physical Review Letters, 122(21), p.217201_1 - 217201_5, 2019/05

AA2018-0627.pdf:0.98MB

https://doi.org/10.1103/PhysRevLett.122.217201

We propose a topological characterization of Hamiltonians describing classical waves. Applying it to the magnetostatic surface spin waves that are important in spintronics applications, we settle the speculation over their topological origin. For a class of classical systems that includes spin waves driven by dipole-dipole interactions, we show that the topology is characterized by vortex lines in the Brillouin zone in such a way that the symplectic structure of Hamiltonian mechanics plays an essential role. We define winding numbers around these vortex lines and identify them to be the bulk topological invariants for a class of semimetals. Exploiting the bulk-edge correspondence appropriately reformulated for these classical waves, we predict that surface modes appear but not in a gap of the bulk frequency spectrum. This feature, consistent with the magnetostatic surface spin waves, indicates a broader realm of topological phases of matter beyond spectrally gapped ones.

Spin transfer torques and spin-dependent transport in a metallic F/AF/N tunneling junction

山本 慧; Gomonay, O.*; Sinova, J.*; Schwiete, G.*

Physical Review B, 98(1), p.014406_1 - 014406_24, 2018/07

https://doi.org/10.1103/PhysRevB.98.014406

We study spin-dependent electron transport through a ferromagnetic-antiferromagnetic-normal metal tunneling junction, in the absence of spin-orbit coupling. We derive microscopic formulas for various types of spin torque acting on the antiferromagnet as well as for charge and spin currents flowing through the junction. The obtained results are applicable in the limit of slow magnetization dynamics. We identify a parameter regime in which an unconventional damping-like torque can become comparable in magnitude to the conventional Slonczewski's torque familiar from ferromagnets. Moreover, we show that the antiferromagnetic sublattice structure opens up a channel of electron transport which does not have a ferromagnetic analog and that this mechanism leads to a pronounced field-like torque. Both charge conductance and spin current transmission through the junction depend on the relative orientation of the ferromagnetic and the antiferromagnetic order parameters.