Magnetic excitations from first principles

Quantum magnetism in condensed matter is an intriguing field of research, with promising applications for future technology. However, the progress towards a disruption of data center computing with spintronics or a large-scale employment of high-temperature superconductors in modern energy infrastructure has been slow at best. One of several reasons for the slow progress is the profound theoretical difficulty associated with the treatment of magnetic quantum effects in real materials, both from a modelling perspective and within a first principles approach. Beyond the contemporary theoretical barriers, there is a vast realm of uncharted territory, where one may drive forward the technological applications of tomorrow, but also discover new physics.
In this thesis, the theoretical background of linear response time-dependent density functional theory is presented in order to illustrate how the fundamental magnetic excitations of real materials can be computationally characterized in terms of the transverse magnetic susceptibility. A concrete computer implementation of the first principles methodology is developed, relying on the projector augmented-wave method to compute plane wave susceptibilities. Using an appropriate gap error correction scheme, the implementation is proven to be both accurate and effective, yielding converged magnon dispersion relations even for the challenging class of itinerant ferromagnets.
Studies of the transverse magnetic excitations in itinerant ferromagnets Fe, Ni, Co and MnBi are presented along with a study of the antiferromagnets Cr and Cr₂O₃. Using the adiabatic local density approximation for the exchange-correlation kernel, a parabolic magnon dispersion is obtained for the investigated ferromagnets and a linear dispersion is obtained for the antiferromagnets. For Fe, Co and MnBi, the studies reveal a satisfactory match to experiment, although for MnBi only when applying a Hubbard correction to the local density approximation. For Ni and Cr₂O₃, the magnon stiffness/velocity is overestimated by roughly a factor of two, whereas sufficient experimental data is lacking in order to benchmark Cr. The thesis also presents a discussion of the itinerant electron effects in the magnon spectra of Fe, Ni, Co and Cr, including the observation of a new optical collective mode in Cr, which seems to be unaffected by Landau damping. Also the magnetic frustration in MnBi is discussed in detail and it is proposed that a magnetic phase transition to helical order can be realized in Cr alloys, potentially in combination with externally applied strain.
Furthermore, the thesis presents a newly developed class of nonlocal exchange-correlation functionals designed to improve short-range correlations based on the correlation hole of the homogeneous electron gas. The functionals use an effective density in the functional form for the local density approximation, which is shown to consistently improve the functional performance, also in the atomic limit. Finally, the thesis covers a recent computational development towards streamlining material simulation recipes.