Controlling magnetic structure in extended solids using targeted chemical compositions

Brgoch, Jakoah
Major Professor
Gordon J Miller
Committee Member
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The ability to combine experiment and theory provides the framework for targeting compositions that may exhibit a fascinating magnetic response such as ferromagnetism, antiferromagnetism, or ferrimagnetism. Using solid-state synthesis techniques, structural characterization, and theoretical analysis, two intermetallic borides series were analyzed for their magnetic properties. In M2M′(T1-xT′x)5B2 (M = Sc, Ti, Zr; M′ = 3d element; T/T′ = Ru, Rh, Ir), the M′ atom forms chains that when occupied by magnetic atoms, i.e., Mn, Fe, Co, Ni, have interatomic bond distances short enough for one-dimensional, long-range magnetic ordering. The prototypical series, Sc2Fe(Ru1-xRhx)5B2 (0 ≤ x ≤ 1), was previously identified to change from antiferromagnetic in the Ru-rich structures to ferromagnetic in the Rh-containing compounds. The change in magnetic ordering as a function of composition stems from the occupation of antibonding states at the Fermi level. As a result, theoretical techniques were utilized to identify additional compositions that may form this structure type and show this same unique trend in magnetism. The discovery of a Zr series, by directed synthesis, provided further unique magnetic response by being the first intermetallic boride to order ferrimagnetically.

Additionally, the structures of Ti9−yM2+yRu18B8, contains M atoms that form dumbbells of Fe atoms in the ab-plane that condense along the c-direction to form ladders. When Ti atoms are substituted by the M atoms (y = ca. 1-2) the resulting structure contains one-dimensional, single-atom chains (as in the M2M′T5B2 series) and one-dimensional ladders (as in the Ti9M′2T18B8 series) in the same compound. The synthesis of Ti8Fe3Ru18B8 was the first compound to show both of these subunits in the same structure. Since the bond distances between the chain and ladder sites is only ca. 3.00 Å, the magnetic atoms form a linear tetramer that we have termed a "magnetic scaffold". Furthermore, Ti8Fe3Ru18B8 contains two separate, one-dimensional chain sites allowing independent local magnetic ordering ultimately providing a system to discover new intermetallic ferrimagnets. In fact, experimental investigations indicate Ti8Fe3Ru18B8 and the isotypic Ti7Fe4Ru18B8 order ferrimagnetically. Computational results identified complex magnetic exchange in the magnetic scaffold as the origin of the ferrimagnetism in these structures.

The composition-property relationship was extended to investigate non-stoichiometry in tetragonal iron sulfide (Fe1+δS). A delicate balance between the Madelung energy and the occupation of antibonding orbitals drives the inclusion of interstitial Fe in this structure. The additional Fe atoms change the Fermi surface, as well as create a spin density wave. These predicted changes in properties have implications for identifying potential superconductivity in the new Fe-based compounds.