2020提案書 （Web公開用）

作成者： Bauer Gerrit （東北大 AIMR & 金研）

研究プロジェクト名： Towards Rare Earth Spintronics
概要： A

promising research direction in spintronics initiated at Tohoku University
addresses voltage‐induced magnetization dynamics. Rare earths are heavy atoms
characterized by a large spin‐orbit interaction that can strongly lock the charge andspin degrees of freedom. This phenomenon might allow manipulation of rare‐earthmagnetic moments by electric field gradients, occurring naturally at the interfaces ofgated magnetic insulator | non‐magnetic metal heterostructures, thereby leading topower saving new devices. We continue researching the physics of rare earthmagnetic moments in spintronic materials and devices.
コアメンバー： Alejandro Leon, Adam Cahaya, and Gerrit Bauer
期待される研究成果： Dilute alloys of rare earths
have played a vital role in understanding magnetic
phenomena. Here, we model the ground state ofdilute 4f rare‐earth impurities in light metals.
Conventional magnetic impurities are known toinduced spin density (RKKY) oscillations. When the4f subshells are open (but not half‐filled), the spin‐
orbit coupling might imprint a rotational charge
current of conduction electrons around rare‐earthatoms. The sign and amplitude of the current
oscillate remarkably similar to the RKKY spinpolarization. We will compute these current
vortices and related observable effects, viz. theassociated Oersted fields.

2018年度 報告書 （Web公開用）

作成者： Bauer, Gerrit （東北大金研）

研究プロジェクト名： Towards Rare Earth Spintronics
概要： Spintronics studies the flow and conversion of the charge and spin of electrons.
Rare earths are heavy atoms characterized by a large spin–orbit interaction that canstrongly lock the charge and spin degrees of freedom. This phenomenon allows themanipulation of rare-earth magnetic moments by electric field gradients, which occur
naturally at the interfaces of gated magnetic-insulator | nonmagnetic-metal
heterostructures. A time-dependent voltage can induce, for example, magnetization
reversal and ferromagnetic resonance.
研究成果（実施状況）: We theoretically analyzed heterostructures containing rareearth atoms, such as dysprosium and holmium, with emphasis on theinterface of magnetic insulators and nonmagnetic metals, such as copper. An
applied electric field does not penetrate the metal, which implies that theinterface atoms are subject to a non-uniform field that can reorient a charge
quadrupole. This, via the spin-orbit-induced LS spin-charge locking, generates
a torque on the magnetization. We formulate the coupling between themagnetic moment and the applied voltage in terms of the electrostatic energyof charged ellipsoids (the 4f-sub shells), oriented along the magnetization [1].
Our estimates of the coupling strength imply a superior efficiency comparedto the known values for transition-metal-based devices. Interface momentsare embedded into the magnetization by strong exchange interactions, suchthat the interface torque acts on the total magnetic order. When the voltage
is time-dependent, a plethora of dynamical responses mediated by interface
rare earths, such as ferromagnetic and parametric resonances, and fast
precessional magnetization switching, can be generated.

LS coupling

主要発表論文等： [1] A. O. Leon, A. B. Cahaya, and G.E.W. Bauer, Voltage control of interface

rare-earth magnetic moments, Phys. Rev. Lett. 120, 027201 (2018).

2019年度 報告書 （Web公開用）

作成者：Bauer Gerrit （東北大 AIMR&金研）

研究プロジェクト名： Towards Rare Earth Spintronics
概要: The control of magnetic materials and devices by voltages without electric currents holds the promiseof power-saving technologies. Rare earths are heavy atoms characterized by a large spin-orbit interactionthat can strongly lock the charge and spin degrees of freedom. This phenomenon allows manipulation of
rare-earth magnetic moments by electric field gradients that occur naturally at the interfaces of gatedmagnetic insulator|non-magnetic metal heterostructures, and may lead to power saving new devices.

コアメンバー： Alejandro Leon, Adam Cahaya, and Gerrit Bauer
研 究 成 果 （ 実 施 状 況 ） : We studied thetemperature-dependent voltage control ofthe magnetic anisotropy caused by rareearth (RE) local moments at an interface
between a magnetic metal and a nonmagnetic insulator, such as Co|(RE)|MgO.
Based on a Stevens operator representationof crystal and applied field effects, wepredict large, dominantly quadrupolar
intrinsic andfield-induced interface
anisotropies, even at room temperature. We
suggestimprovedfunctionalities
oftransition metal tunnel junctions afterdusting their interfaces with rare earths.

Voltage-controlled magnetic anisotropy constants(MACs) DK1 (qudrupolar, solid line), DK2 (square-dashedline) and DK3 (dashed line) of rare earth moments at thesurface of Co at low temperature.

主要発表論文等 ： [1] A.O. Leon and G.E.W. Bauer, Voltage- and temperature-dependence of the magnetic
anisotropy of interfacial rare-earth local moments, J. Phys.: Condens. Matter (invited) arXiv:2004.00320.

2020年度 報告書 （Web公開用）

作成者： Bauer Gerrit （東北大 AIMR & 金研）

研究プロジェクト名： Towards Rare Earth Spintronics
概要： A promising research direction in spintronics initiated at Tohoku University addresses voltage-

induced magnetization dynamics. Rare earths are heavy atoms characterized by a large spin-orbit
interaction that can strongly lock the charge and spin degrees of freedom. This phenomenon might allow
manipulation of rare-earth magnetic moments by electric field gradients, occurring naturally at theinterfaces of gated magnetic insulator | non-magnetic metal heterostructures, thereby leading to powersaving new devices. We continued researching the physics of rare earth magnetic moments in spintronicmaterials and devices.

研 究 成 果 （ 実 施 状 況 ） ： Alloys with rare earths
played a vital role in understanding magnetic
phenomena. Here, we model the ground state ofdilute 4f rare-earth impurities in light metals. Whenthe 4f subshells are open (but not half-filled), thespin-orbit coupling imprints a rotational charge
current of conduction electrons around rare-earth
atoms. The sign and amplitude of the current
oscillate similar to the Ruderman-Kittel-Kasuya-Yosida(RKKY) spin polarization. We compute the observableeffects, such as the Ørsted field and Knight generated
by the current vortices.
主要発表論文等： A.B. Cahaya, A.O. Leon, M.R. Aliabad, and G.E.W. Bauer, Phys. Rev. B 103, 064433 (2021) .