Event Details

Dr. Daigo Oue (Lisbon) Space-time metasurfaces: Cherenkov radiation and noncontact friction and Dr. Junta Igarashi (Lorraine) Sub-picosecond counterintuitive magnetization reversal in ferromagnetic spin valves

Space-time metasurfaces: Cherenkov radiation and noncontact friction

Daigo Oue - 1, 2

1. Instituto de Telecomunicações, Instituto Superior Técnico, University of Lisbon

2. The Blackett Laboratory, Department of Physics, Imperial College London

In this presentation, I will introduce electromagnetic phenomena generated by temporally and spatially modulating the surface of dielectric materials. In the first part of the presentation, I will discuss how patterns of heterogeneous electric dipoles are generated on the material's surface following the spatiotemporal modulation. These patterns are responsible for light emission [1]. The rate of change of electric dipole patterns is characterised by the "peristaltic velocity Ω/g," which is determined by the parameter Ω (g) describing the temporal (spatial) modulation. When this peristaltic velocity exceeds the speed of light, radiation occurs from the medium's surface through the Cherenkov mechanism. If the peristaltic velocity is below the speed of light, evanescent waves cling to the material's surface.

In the latter part of the presentation, I will discuss the energy consumption associated with the "oscillatory motion" of the material's surface and the generation of "frictional forces" on the material's surface [2]. Peristaltic motion generates electromagnetic fields around the material's surface, resulting in a resistance force as a reaction. When a conductive material is brought close to the modulated surface, it receives energy and momentum from it, leading to energy consumption and resistance even below the Cherenkov threshold due to oscillatory motion.

References

[1] D. Oue, K. Ding & J. B. Pendry, Phys. Rev. Research 4 , 013064 (2022).

[2] D. Oue, K. Ding & J. B. Pendry, Phys. Rev. A 107 , 063501 (2023).

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Sub-picosecond counterintuitive magnetization reversal in ferromagnetic spin valves

Junta Igarashi - Institut Jean Lamour, Université de Lorraine

The manipulation of magnetic materials without the use of magnetic fields is of great fundamental and technical interest. The discovery of spin-transfer torque (STT) [1,2] allowed us to control the magnetization direction electrically in magnetic devices within nanoseconds, which paved the way for non-volatile applications such as spin-transfer-torque magnetoresistive random access memory (STT-MRAM). However, current-induced STT switching below a few hundred picoseconds with low power consumption while maintaining high thermal stability is still challenging. Ultra-short optical pulses can also be used to manipulate the magnetization direction without a magnetic field, which is called all-optical switching (AOS). So far, only specific materials containing ferrimagnet such as GdFeCo have shown ultrafast AOS [3,4]. Thus, these methods to manipulate magnetization have been mostly developed independently within the fields of spintronics and ultrafast magnetism. In this seminar, I will show our recent work, where we demonstrated sub-picosecond magnetization reversal of a Co/Pt ferromagnetic layer induced by a single femtosecond laser pulse within a common [Co/Pt]/Cu/[Co/Pt] perpendicular spin-valve structure [5,6].

References

[1] J. C. Slonczewski, Journal of Magnetism and Magnetic Materials 159, L1–L7 (1996).

[2] L. Berger, Phys. Rev. B 54, 9353–9358 (1996).

[3] I. Radu et al., Nature 472, 205–208 (2011).

[4] T. A. Ostler et al., Nat. Commun. 3, 1–6 (2012).

[5] S. Mangin et al., Nature Mater. 5, 210–215 (2006)

[6] J. Igarashi et al., Nature Mater. 22, 725-730 (2023).