Effects of phase competition and frustration in itinerant and local-moment magnetic materials
Date
2023-08
Authors
Nedic, Ana-Marija
Major Professor
Advisor
Orth, Peter P.
McQueeney, Robert J.
Flint, Rebecca
Tuchin, Kirill
Rossini, Aaron J.
Committee Member
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Abstract
Magnetically frustrated systems, characterized by the inability to simultaneously satisfy competing exchange interactions, are extensively studied as platforms that can exhibit exotic emergent orders and tunable magnetic behavior. The competing magnetic tendencies in metals and the discovery of magnetically frustrated topological materials have made the investigation of competing magnetism and its interplay with other degrees of freedom in the system an active field of research with novel possibilities.
This thesis explores the effects of magnetic frustration in itinerant and local-moment magnetic models. We consider models that capture the essential degrees of freedom of studied materials and explore their magnetic phase diagrams.
Over different projects, we consider the role of orbital degrees of freedom, dimensionality, a frustrated geometry, and the symmetry of the magnetic order on the emergent behavior of the system. We often compare our results to the experimentally observed behavior in the studied materials.
Chapter~\ref{chap:intro} introduces the origin of the magnetic order in solids, the general mechanism for the appearance of the ordered phases of matter through spontaneous symmetry breaking, and the role of fluctuations.
Chapter~\ref{chap:Methods} summarizes the methods used in the calculations in this thesis.
In chapter~\ref{chap:SrCo2As2}, we revisit the intriguing magnetic behavior of the itinerant frustrated magnet $\rm{Sr}\rm{Co}_2\rm{As}_2$. We study the magnetic phase diagram of the multiorbital Hubbard model of $\rm{Sr}\rm{Co}_2\rm{As}_2$ and identify the proximity of several ferromagnetic (FM) and stripe antiferromagnetic (AFM) phases. While magnetic frustration is typically observed and studied in the local-moment systems, in this work we introduce a way to quantify the degree of magnetic frustration in itinerant models. We discuss the individual orbital contributions to the different magnetic instabilities, which are markedly
different for the FM and the AFM phases.
We also calculate the susceptibility
at finite frequencies and directly relate our findings to the inelastic neutron scattering (INS) observations.
ion is typically rationalized in local-moment models as the competition between magnetic orders that can originate from the magnetic interactions, the geometry of the lattice, etc. In this work, we will propose a way to quantify the degree of magnetic frustration in itinerant systems.
Motivated by a number of materials with stacked lattices, in Chapter~\ref{chap:PottsZ3} we identify and study a new family of continuous SO(3)-invariant bilayer spin models that host three-state Potts nematic order. We map out the zero and finite-temperature phase diagram of one of the lattice designs from the proposed family and by varying the ratio of exchange interactions in the Hamiltonian, we explore the role of several tuning knobs in the system: the effective fluctuating magnetic field, the rigidity of the fluctuating field, and the interpolation between the Heisenberg and Ising-type limits of the model, yet in a fully SO(3)-invariant setup. Our work provides a versatile platform to explore the Potts phases in models with continuous symmetry and extends the material space for the realization of the emergent three-state Potts nematic order in two dimensions.
In Chapter~\ref{chap:MTIs}, we model the ground state phases of the two intrinsic magnetic topological insulators, MnBi$_2$Te$_4$ and EuIn$_2$As$_2$, and study their magnetic phase diagrams. We capture the complex magnetic ground states of these systems from the competition of Heisenberg bilinear and biquadratic exchange interactions, and local magnetic anisotropies. We discuss the potential implications of the competing magnetic interactions in the magnetic crystalline topological insulators and outline the complex possibilities for the bulk-boundary correspondence in the topological insulators with low-symmetry magnetic ground states.
In Chapter~\ref{chap:conclusions}, we summarize our findings and discuss a few naturally emerging directions that would be interesting for future study.
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Type
dissertation