Magnetic nanoparticles such as FePt in the L10 phase are the bedrock of our current data storage technology. As the grains become smaller to keep up with technological demands, the superparamagnetic limit calls for materials with higher magnetocrystalline anisotropy. This, in turn, reduces the magnetic exchange length to just a few nanometers, enabling magnetic structures to be induced within the nanoparticles. Here, we describe the existence of spin-wave solitons, dynamic localized bound states of spin-wave excitations, in FePt nanoparticles. We show with time-resolved x-ray diffraction and micromagnetic modeling that spin-wave solitons of sub-10 nm sizes form out of the demagnetized state following femtosecond laser excitation. The measured soliton spin precession frequency of 0.1 THz positions this system as a platform to develop novel miniature devices.

Center for Magnetism and Magnetic Materials University of Colorado Colorado Springs Colorado Springs CO 80918 USA

Center for Memory and Recording Research University of California San Diego 9500 Gilman Drive La Jolla CA 92093 0401 USA

Complex Matter Department Jožef Stefan Institute Ljubljana Slovenia

Department of Mathematics Physics and Electrical Engineering Northumbria University Newcastle upon Tyne NE1 8ST UK

Department of Molecular Sciences and Nanosystems Ca' Foscari University of Venice 30172 Venice Italy

Department of Physics and Astronomy Uppsala University 751 20 Uppsala Sweden

Department of Physics Stockholm University 106 91 Stockholm Sweden

Deutsches Elektronen Synchrotron 22607 Hamburg Germany

Dipartimento di Elettronica Informazione e Bioingegneria Politecnico di Milano Milano Italy

European XFEL GmbH Holzkoppel 4 22869 Schenefeld Germany

Faculty of Mathematics and Physics Department of Condensed Matter Physics Charles University Ke Karlovu 5 121 16 Prague Czech Republic

Institut für Optik und Atomare Physik Technische Universität Berlin Berlin Germany

Institute of Experimental Physics Technische Universität Bergakademie Freiberg 09599 Freiberg Germany

Istituto Nazionale di Fisica Nucleare Sezione di Milano Milano Italy

Magnet Materials Unit National Institute for Materials Science Tsukuba 305 0047 Japan

Max Born Institut Berlin Germany

SLAC National Accelerator Laboratory 2575 Sand Hill Road Menlo Park CA 94025 USA

Sorbonne Université CNRS Laboratoire de Chimie Physique Matière et Rayonnement 75005 Paris France

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