IEEE Magnetics Society Distinguished Lecturers for 2020 Antiferromagnetic Insulatronics—Spintronics Without Magnetic Fields and Moving Electrons

Novel schemes for optically controlling ferromagnetic order at a femtosecond time scale [1] receive great scientiﬁc interest. In the strong non-equilibrium regime, it has become possible not only to quench magnetic order but also to deterministically switch the magnetic state by a single femtosecond laser pulse. Moreover, it has been shown that pulsed laser excitation can induce spin currents over several tens of nanometers. This development triggered a merging of the ﬁelds of “femto-magnetism” and spintronics—opening a fascinating playground for novel physical phenomena. In this lecture, I will not only discuss the underlying principles but also envision their exploitation in terahertz magnonics and integrated spintronic–photonic memory. After a brief review of the ﬁeld, mechanisms for ultrafast loss of magnetic order upon femtosecond laser heating [2] as well as all-optical switching will be explained. Next, different processes that give rise to laser-induced spin currents will be distinguished. In particular, I will address experiments that have demonstrated laser-induced spin transfer torque on a free magnetic layer [3]. These femtosecond spin currents are absorbed within a few nanometers, providing ideal conditions for exciting and exploring terahertz spin waves [4]. Finally, I will argue that synthetic layered ferrimagnets provide an ideal platform for combining femtosecond optical control with advanced spintronic functionality. I will show how magnetic bits can be written “on-the-ﬂy” by femtosecond laser pulses in a so-called magnetic racetrack, where they are immediately transported by a dc current [5]. Such schemes may lead to a novel class of integrated photonics, in which information is transferred back and forth between the photonic and magnetic domains without any intermediate electronic steps. The dynamics and the damping of magnetization are of paramount importance to understand and predict the properties of magnetic materials used in a variety of applications. For example, spin-transfer torque magnetic random-access memory (STT-MRAM) cells are expected to switch fast, reliably, and with low power consumption, which requires low damping materials. The Landau–Lifshitz–Gilbert equation of motion [1] and extensions thereof have been extremely successful in enabling predictions of the magnetization dynamics of materials both analytically and using micromagnetic simulations. Experimentally, magnetization dynamics can be elegantly probed in the frequency domain using ferromagnetic resonance (FMR). Recent progress in broadband FMR techniques has provided valuable new insights in the magnetization dynamics and the damping mechanisms of magnetic materials.Inthislecture, I will introduce the fundamentals of magnetization dynamics and damping. I will discuss their importance for many applications, including hard disk drive read heads, STT-MRAMs, and new skyrmion-based devices. I will talk about various mechanisms that can contribute to the damping of the magnetization in thin ﬁlms, including spin–orbit relaxation, spin pumping, and two-magnon scattering. The presentation will show how recent developments in broadband FMR enable precise measurements of the dynamics and damping in thin magnetic ﬁlms and multilayers especially when combined with angle-and temperature-dependent measurements [2]. This will include a discussion of the recently discovered anisotropic damping in exchange-biased ﬁlms [3], [4].