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10:00-11:00
講師 : Hua Chen　氏　(テキサス大学オースティン校)
標題 : Orbital moments and current-induced magnetization dynamics in noncollinear antiferromagnets
要旨 :
Now that the anomalous Hall effect is established as a convenient order parameter probe in a class of noncollinear antiferromagnets, it is timely to examine effects that can be used for order parameter manipulation. In this talk I will discuss some of our recent theoretical results on the coupling between the noncollinear magnetic order parameter and external electric and magnetic fields. First I will briefly explain the physics of orbital magnetic moments which arise from the coupling between magnetic fields and the orbital motion of electrons. A nonzero orbital moment is present in all ferromagnets, but is usually much smaller than the total spin moments. In noncollinear antiferromagnets like Mn3Ir or Mn3Sn, which have an anomalous Hall effect, sizable orbital moments exist while the total spin moment nearly vanishes. This orbital moment can be used to manipulate the order parameter using magnetic fields. Separately, in certain noncollinear antiferromagnets with broken atomic-site-inversion symmetry, a nonzero site-dependent spin polarization can be induced by external electric fields or currents, which can exert a torque on the local spin moments and can potentially change the direction of the noncollinear magnetic order parameter or induce its nontrivial dynamics.

11:00-12:00
講師：野村健太郎　氏　(東北大学金属材料研究所)
標題 : Spin-electromagnetic responses in topological matters
要旨 :
The electrical control of spin magnetization aims to be used in next-generation magnetic devices, allowing information to be written electronically. Recently, spintronics phenomena in topological materials have been drawn interests for achieving novel electrical manipulation of the magnetization, and generation of spin currents. In this presentation we discuss theoretical proposals of spintronics phenomena in topological insulators and magnetic Weyl semimetals.
In the first part of the talk, spin-electricity conversion at the interface between a ferromag-netic material and a topological insulator[1] is discussed. Injected spins by spin pumping are converted into a charge current due to spin-momentum locking on the surface state. We for-mulate a theoretical model for spin dynamics and the spin-electricity conversion effect on the topological surface. The dumping constant is expressed in terms of the conductivity of the surface Dirac fermions. The electrically induced spin current is calculated using the perturba-tion theory.
In the second part, we discuss spintronics phenomena in magnetic Weyl semimetals. A Weyl semimetal is a new type of topologically protected gapless quantum state, with either time-reversal or spatial inversion symmetries broken in three dimensions. Weyl semimetals with broken time-reversal symmetry are more interesting and rewarding for spintronics applications. We derive an effective free energy functional of magnetization which describes low energy excitations and magnetic textures[2]. We also demonstrate that Weyl electrons in a magnetically doped Weyl semimetal exert a spin torque on the local magnetization, without a flowing current, when the chemical potential is modulated in a magnetic field. The spin torque is proportional to the anomalous Hall conductivity, and its effective field strength may overcome the Zeeman field. Using this effect, the direc-tion of the local magnetization is switched by gate control in a thin film. We also discuss dynamics of local magnetization by solving the Landau-Lifshitz-Gilbert equation.

[1] Y. Shiomi, K. Nomura, Y. Kajiwara, K. Eto, M. Novak, K. Segawa, Y. Ando, E. Saitoh, Phys. Rev. Lett. 113, 196601 (2014).
[2] K. Nomura and D. Kurebayashi, Phys. Rev. Lett. 115, 127201 (2015).
[3] D. Kurebayashi and K. Nomura, arXiv:1604.03326.

13:30-14:30
講師：Lucile Savary 氏　(マサチューセッツ工科大学）
標題 : Disorder-induced entanglement in spin ice pyrochlores
要旨 :
I will discuss a proposal according to which, in a certain class of magnetic materials, known as non-Kramers ‘spin ice,’ disorder induces quantum entanglement. Instead of driving glassy behavior, disorder provokes quantum superpositions of spins throughout the system, and engenders an associated emergent gauge structure and set of fractional excitations. More precisely, disorder transforms a classical phase governed by a large entropy, classical spin ice, into a quantum spin liquid governed by entanglement. As the degree of disorder is increased, the system transitions between (i) a “regular” Coulombic spin liquid, (ii) a phase known as “Mott glass,” which contains rare gapless regions in real space, but whose behavior on long length scales is only modified quantitatively, and (iii) a true glassy phase for random distributions with large width or large mean amplitude. These results may be applicable to Pr2Zr2O7, in which random crystal field splittings have already been observed, and to classical spin ices, such as Ho2Ti2O7, upon chemical doping.

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