Lucas Caretta1 Maxwell Mann1 Felix Buettner1 Kohei Ueda1 Bastian Pfau2 Cristian Guenther2 Piet Hessing2 Alexandra Churikova1 Christpher Klose2 M Schneider2 D Engel2 Colin Marcus1 David Bono1 Kai Bagschik3 Stefan Eisebitt2 Geoffrey Beach1

1, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
2, Max-Born Institute, Berlin, , Germany
3, DESY, Hamburg, , Germany

Spintronics is a research field geared towards understanding and controlling spins on the nanoscale, enabling next-generation data storage and manipulation. Ultimately, the technological and scientific challenge is to create ultrasmall solid-state magnetic bits (<10 nm) and to control their motion efficiently with ultrahigh velocities (>1 km/s). Inspired by materials used for hard disk drives, research so far has focused on ferromagnetic materials. However, these materials show fundamental limits for speed and size making applications unlikely. Ferromagnetic materials have large stray fields, causing ferromagnetic spin textures to repel each other over long distances. Stray field interactions also lead to a preferred demagnetization of the material, i.e., skyrmions are large (>100 nm) if they are not assisted by external fields or strong pinning. In addition, the velocity of magnetic solitons is fundamentally limited by the precessional dynamics underlying any coherent spin texture displacements, ultimately making motion inefficient. For skyrmions, the velocity is also limited by stripe-out instabilities and by topological damping. In both cases, the observed skyrmion velocities in ferromagnetic materials have always been lower than ~100 m/s. Moreover, ferromagnetic skyrmions suffer from a large skyrmion Hall angle and from topological damping. These fundamental limitations of ferromagnets call for new materials systems.
Here, we demonstrate that compensated rare earth - transition metal ferrimagnets (FiM) are not affected by these limits. FiM, comprised of two antiferromagnetically coupled sublattices, have two compensation temperatures: the magnetization compensation temperature TM, defined by Ms(TM)=0, and the angular momentum compensation temperature TA with vanishing spin density S(TA)=0. Near TA, the spins align with the magnetic field without any precession and a driving force immediately leads to acceleration in the direction of force. Near TM, stray fields become negligible and spin textures are stabilized by the competition of local exchange, anisotropy, and Dzyaloshinskii-Moriya interaction (DMI). Thus, zero field skyrmions with less than 10 nm in diameter can be realized at room temperature and interactions skyrmions is completely suppressed. In other words, very efficient dynamics are expected to occur near TA, and very small spin textures can be realized at TM. Using these concepts in ferrimagnetic Pt/GdCo/Ta films, we realize a record-fast current-driven domain wall velocity of 1.3 km/s and record small room-temperature stable ~10 nm diameter skyrmions near TA and TM, respectively. Moreover, TA and TM are engineered to be near each other and near room temperature for both fast dynamics and small textures. Compensated FiM are a promising spintronics candidate, as a range of easily accessible knobs, such as interfaces, annealing, sample temperature, and composition can control their properties.