Orienting spins in a thin magnetic layer thanks to the interface with a 2D insulator

Interactions emerging at the interface of an ultrathin magnetic layer, such as the Dzyaloshinskii-Moriya (DMI) antisymmetric exchange interaction, can lead to the appearance of particular spin orientations within the layer, notably the vortex orientations of skyrmions. These are envisionned as possible carriers of information storage in spintronics. By succeeding in preparing an ultra-clean interface between a layer of a few atomic planes of cobalt and deposited flakes of a two-dimensional material namely hexagonal boron nitride (h-BN), physicists were able to measure the intensity of the DMI interaction observed for the first time between these two materials, which is as strong as the current record in the Pt-Co interface.

Behaviors observed in ultrathin magnetic layers, such as the anisotropy that gives magnetization a preferential orientation, or the exchange interaction that tends to align magnetic moments with one another, are based on interface effects. A few years ago, a so-called “Dzyaloshinskii-Moriya” (DMI) antisymmetric exchange interaction was revealed at the interface between a ferromagnetic layer and a “heavy” metal layer with strong spin-orbit coupling (Pt, Au, Ir…). Theoretical predictions indicated that this same interaction could be observed by replacing the heavy metal with a 2D material.

For the first time, researchers at the Laboratoire de Physique des Solides (LPS, CNRS / Université Paris-Saclay) have fabricated an ultra-clean Co/h-BN interface by mastering the growth of ultra-thin cobalt films in ultra-high vacuum and depositing 2D crystals – flakes – of h-BN by in-situ mechanical exfoliation. They reported the experimental observation of strong DMI and perpendicular anisotropy induced by h-BN flakes on a ferromagnetic cobalt ultrathin film.

The DMI arising from the sole Co/h-BN interface has been measured with different buffer layers using inelastic light-scattering spin-wave spectroscopy (Brillouin effect). In addition to being comparable to the record values obtained in Pt/Co metal heterostructures, the DMI of Co/h-BN is of a sign that enhances that of Pt/Co when they are combined, favoring the formation of skyrmions (Figure). Surprisingly, these strong effects originate in the absence of spin-orbit coupling in the otherwise insulating h-BN layer, raising theoretical questions, as DMI has hitherto been associated with a heavy metal exhibiting strong spin-orbit coupling. In addition to fundamental questions, these results open up new possibilities for integrating two-dimensional materials ten of microns wide but ultrathin (a single atomic plane in thickness) into spin-orbitronic devices.

Figure. (top) Schematic diagram of the multilayers. (bottom) Magnetic Force Microscopy observations of skyrmions stabilized by a weak magnetic field observed at room temperature in Pt/Co/h-BN.

Evidence of strong Dzyaloshinskii–Moriya interaction at the cobalt/hexagonal boron nitride interface
B. El-Kerdi, A. Thiaville, S. Rohart, S. Panigrahy, N. Bras, J. Sampaio, A. Mougin
Nano Letters, 2023, 23, 3202-3208
doi: 10.1021/acs.nanolett.2c04985
Available on ArXiv

Highlight from the CNRS Institut de Physique

Banan El-Kerdi