When photoexcited by an ultrafast laser pulse, antiferromagnets allow direct angular momentum transfer between opposing spins, promising faster functionality than ferromagnets, which are intrinsically limited because their net angular momentum must dissipate to the lattice. The process of angular momentum transfer is closely linked to the nature of magnetic coupling in the system. In lanthanides, 4f magnetic exchange is mediated indirectly through the conduction electrons (the Ruderman–Kittel–Kasuya–Yosida interaction, RKKY), and the effect of such conditions on the antiferromagnetic direct spin transfer is largely unexplored.

In our study we used resonant ultrafast X-ray diffraction to study ultrafast magnetization dynamics in a series of 4f antiferromagnets, and systematically varied the 4f occupation- thereby altering the magnitude of RKKY. Combined with with ab-initio calculations, we find that the rate of angular momentum transfer between opposing moments is directly determined by the magnitude of RKKY. Given the direct relationship between RKKY and the conduction electrons, our results offer a novel approach for controlling the speed of magnetic devices.