Investigating the magnetism of Ni from a momentum space perspective

Abstract For more than a quarter of a century, clear discrepancies have existed between the spin density in momentum space of Ni revealed by magnetic Compton scattering experiments and theoretical calculations based on density functional theory (DFT). Here, by making a wide comparison between different theoretical methods, we discover how the magnetic Compton profiles (MCPs) of Ni can be predicted remarkably well. We find that the first essential ingredient beyond DFT is local dynamical spin fluctuations; from our magnetic DFT combined with magnetic dynamical mean field theory (DFT+DMFT) results, we see that these fluctuations suppress the spin moment, and for the spin density in momentum space, a suitable static approximation to these fluctuations is satisfactory. The second missing essential ingredient is a non-local treatment of electron correlations; our GW MCPs highlight the crucial role of non-local correlation effects in obtaining excellent experiment-theory agreement in the low-momentum region, hence correcting the dual-natured magnetic contributions. Furthermore, our results provide further insight into the so-called ‘negative spin polarization’ and the influence the many-body features of the electronic structure of Ni have on the MCPs. We also re-evaluate the strongly constrained and appropriately normed (SCAN) meta-GGA from this momentum-space perspective. We shed new light on its flaws, and we find that no suitable ad hoc correction can be made. Finally, our MCPs from alternative many-body implementations, such as the one-shot GW diagonal self-energy approximation and DFT(non-magnetic)+DMFT(magnetic), which only treats magnetism within DMFT, yield significantly poorer agreement with the experimental data. This work highlights the value of the momentum space perspective which Compton scattering provides.