Speaker
Prof.
Peter Oppeneer
(Uppsala University)
Description
Magnetism in condensed matter systems continuous to be a burgeoning research field with new areas emerging regularly. In this talk I shall address several recent theoretical developments in magnetic phenomena of materials, which emphasize the need for experimental facilities that can probe magnetic properties on ultrashort time and length scales, in addition to be able to detect emergence of long-range order.
Magnetic order is a common phenomenon in three dimensional materials; however, in two dimensions long-range magnetic order caused by Heisenberg exchange interactions is forbidden by the famous Mermin-Wagner theorem. Topological Kosterlitz-Thouless vortex phases have been proposed as a possible route to overcome this limitation. Here we consider another route to realize long-range magnetic order in a two-dimensional system. Specifically, we consider a two-dimensional supramolecular lattice, consisting of Mn-phthalocyanine and hexadeca-fluorinated Fe-phthalocyanine molecules selfassembled into a molecular checkerboard on a gold (111) surface. The two spin-bearing molecules physisorb on the Au surface, i.e. without chemical reaction. The Au surface electrons feel nonetheless the presence of the magnetic moment on the central transition-metal ion, giving rise to an incomplete Kondo screening of the molecular moment. Using the element-specific x-ray magnetic circular dichroism (XMCD) and first-principles electronic structure calculations we show that ferrimagnetic long-range order is realized at low temperatures in this two-dimensional checkerboard. Our ab initio calculations reveal that the Ruderman-Kittel-Kasuya-Yosida interaction, which is transmitted through the gold electrons, is responsible for establishing the long-range order [1]. This discovery can open the way for applications of magnetic molecules in molecular quantum spintronics.
Ultrafast magnetism is a recent branch of magnetism, in which femtosecond laser pulses are employed to ultrafast demagnetize a magnetic material or to switch its magnetization. However, currently not much is known about the fundamental microscopic mechanisms that can dissipate spin angular momentum on ultrashort time scales. Here we investigate, both theoretically and experimentally, several proposed mechanisms, in particular, the Elliott-Yafet electron-phonon spin-flip scattering, ultrafast magnon generation, and the non-local transport of spin through superdiffusive spin currents. For thin ferromagnetic films on metallic substrates superdiffusive spin currents give a sizeable and fast contribution to demagnetization, whereas for films on insulating substrates, we surprisingly find that ultrafast magnon generation provides a major channel for fast dissipation of spin angular momentum [2]. Among the possible applications of these ultrafast magnetic processes are spintronic devices operating at THz frequencies and ultrafast laser-assisted magnetic recording.
[1] J. Girovsky et al., Nature Commun. 8, 15388 (2017).
[2] E. Turgut et al., Phys. Rev. B 94, 220408(R) (2016).
Primary author
Prof.
Peter Oppeneer
(Uppsala University)
