Enhanced spin pumping near a magnetic ordering transition

Abstract

We study the temperature-dependent static and dynamic magnetic properties of polycrystalline bilayers of permalloy ($\mathrm{N}{\mathrm{i}}_{80}\mathrm{F}{\mathrm{e}}_{20}$, or Py) and gadolinium (Gd) bilayers using DC magnetometry and broadband ferromagnetic resonance. Magnetometry measurements reveal that the 3-nm-thick Gd layers undergo a magnetic ordering transition below 100 K, consistent with finite size suppression of their Curie temperature. Upon cooling below this Gd ordering temperature, ferromagnetic resonance spectroscopy reveals a sharp increase in both the gyromagnetic ratio (\ensuremath{\gamma}) and effective Gilbert damping parameter (${\ensuremath{\alpha}}_{\mathrm{eff}}$) of the neighboring Py layers. The increase of \ensuremath{\gamma} is attributed to the onset of strong antiferromagnetic coupling between the Gd and Py layers as the Gd orders magnetically. We argue that the increase of ${\ensuremath{\alpha}}_{\mathrm{eff}}$, on the other hand, can be explained by spin pumping into the rare-earth layer when taking into account the increase of \ensuremath{\gamma}, the decrease of the Gd spin diffusion length as it orders, and, most significantly, the corresponding increase of the Py/Gd interfacial spin mixing conductance in the vicinity of the magnetic ordering transition. We propose that these observations constitute a qualitative confirmation of a recent theoretical prediction of spin sinking enhancement in this situation.