Enhanced spin pumping near a magnetic ordering transition

We study the temperature-dependent static and dynamic magnetic properties of polycrystalline bilayers of permalloy (Ni80Fe20, 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 (γ) and effective Gilbert damping parameter (αeff) of the neighboring Py layers. The increase of γ 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 αeff, on the other hand, can be explained by spin pumping into the rare-earth layer when taking into account the increase of γ, 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.