Polarized phonons carry angular momentum in ultrafast demagnetization

Abstract

Magnetic phenomena are ubiquitous in our surroundings and indispensable for modern science and technology, but it is notoriously difficult to change the magnetic order of a material in a rapid way. However, if a thin nickel film is subjected to ultrashort laser pulses, it can lose its magnetic order almost completely within merely femtosecond times [1]. This phenomenon, in the meantime also observed in many other materials [2-7], has connected magnetism with femtosecond optics in an efficient, ultrafast and complex way, offering opportunities for rapid information processing [8-12] or ultrafast spintronics at frequencies approaching those of light [8,9,13]. Consequently, the physics of ultrafast demagnetization is central to modern material research [1-7,14-28], but a crucial question has remained elusive: If a material loses its magnetization within only femtoseconds, where is the missing angular momentum in such short time? Here we use ultrafast electron diffraction to reveal in nickel an almost instantaneous, long-lasting, non-equilibrium population of anisotropic highfrequency phonons that appear as quickly as the magnetic order is lost. The anisotropy plane is perpendicular to the direction of the initial magnetization and the atomic oscillation

Angular momentum transfer via relativistic spin-lattice coupling from first principles

“…Analyzing the properties of these parameters, in particular their dependence on spin-orbit coupling, we find that even in bcc Fe the leading term which is responsibla for the exchange of angular momentum between the spin system and the lattice is a Dzyaloshiskii-Moriya-type interaction, which emerges due to the symmetry breaking distortion of the lattice. Our findings, hence, stress the importance of relativistic effects for the transfer of angular momentum from magnonic excitations to circularily polarized phonons [5,[7][8][9][10]…”

“…In certain ferrimagnets, a single laser pulse can switch the magnetization orientation on a sub-picosecond time scale [2], due to the exchange of spin angular momentum between the two magnetic sublattices [3,4]. However, recent work on ultrafast demagnetization in ferromagnets has demonstrated that angular momentum can also be transferred from the spin system to the lattice on similar time scales [5]. In the lattice, the spin angular momentum is absorbed in terms of phonons carrying the angular momentum till -on larger times scales -the macroscopic Einstein-deHaas effect sets in [6].…”

Ultrafast Electron Diffraction: Visualizing Dynamic States of Matter

“…This was observed directly in silicon by Harb et al (2008). Spin-lattice coupling has also recently been interrogated from the lattice perspective using UED (Tauchert et al, 2022;Windsor et al, 2021).…”

Light-induced hexatic state in a layered quantum material

“…Over the past two decades, ultrafast electron diffraction (UED) 54,55 has evolved into a highly sensitive technique to probe structural phase transitions 23,[36][37][38][39][40][41][42][43][44][45]56 and transi- ent phonon populations 33,57,58 on femtosecond time scales. State-of-the-art time-resolved electron diffraction instrumentation typically uses large-diameter beams (>100 µm spot size) for averaged probing of CDW amplitudes, addressing a compromise between a small spot size and sufficient reciprocal-space resolution.…”