| Summary: | <p>In this thesis I present experimental investigations of four different quantum materials, each of which showcases the strongly correlated magnetism of 3<em>d</em> transition metal ions.</p>
<p>The topological semimetal candidate YbMnSb2 has a high magnetic ordering temperature of T<em>N</em> ≈ 345 K. I performed unpolarised and polarised neutron diffraction, as well as triple-axis spectroscopy on YbMnSb2. From these results, I refined the magnetic and crystal structure of YbMnSb2. Similar to other square net antimonides YbMnSb2 displays C-type antiferromagnetism with the moments oriented along the c axis. I interpreted the spin wave spectrum using linear spin wave theory, where the predominant exchange interactions are in-plane; however, the out-of-plane exchange along the c axis is surprisingly large. This interaction acts through the Sb square net and so may influence the charge transport properties of the material. Our magnetic model of YbMnSb2 is consistent with this compound being a gapped Dirac semimetal.</p>
<p>The topological insulator MnSb4Te7 is closely related to the much-studied van der Waals crystal MnBi2Te4. MnSb4Te7 had been identified as a possible axion insulator candidate. However, the ground state magnetic structure in the bulk had not been determined. I utilised single crystal neutron diffraction for this task and found that there is a strong degree of site-mixing between Mn and Sb sites. This leads to a magnetic structure where Mn2+ moments are coupled ferrimagnetically within the site-mixed magnetic layers but antiferromagnetically between magnetic layers with propagation vector q = (0, 0, 1/2). The application of an external magnetic field of 0.2 T drives a spin-flip transition, removing the interlayer antiferromagnetic coupling. The magnetism in MnSb4Te7 is increasingly 2D at low temperatures.</p>
<p>The charge-doped antiferromagnetic Mott insulator La(2–x)Ba(x)CoO4 is a structural analogue of the high temperature cuprate superconductors, which have properties that are also sensitive to dopant concentration x. The cuprates show a signature hour-glass shaped spin wave spectrum, which has also been observed in other similar insulators and explained using a disordered charge stripe model. I studied this material through magnetometry, time-of-flight neutron spectroscopy and single crystal diffraction. I discovered that the magnetic order in La(2-x)Ba(x)CoO4 is very short range with a correlation length of ≈ 15 Å and is described by the incommensurate propagation vector <strong>q</strong> = (0.5±0.23, 0.5±0.23) in the (<em>h, h</em>) plane for x = 0.5. The spin wave spectrum has the characteristic hour-glass shape of the cuprates. These results are in contrast to the equivalent Sr-doped compound La1.5Sr0.5CoO4, which has long-range collinear magnetic order and does not feature the hour-glass.</p>
<p>Finally, the rare earth iron garnets are of prime interest for spintronic applications due to their very coherent, spin-polarised magnons. I present a study of the novel cobalt-doped rare earth iron garnet Lu3Co0.5Si0.5Fe4O12, which utilised magnetometry and two different inelastic neutron scattering techniques. This material orders ferrimagnetically at <em>T</em>C ≈ 480 K. Introducing cobalt generates a high degree of magnetocrystalline anisotropy in the system at low temperatures, comparable with the magnetic rare earths. However, at room temperature, Lu3Co0.5Si0.5Fe4O12 is a soft magnet, similar to YIG and LuIG. This is an indication that the Co2+ moments are disordered by 300 K. The spin wave spectrum is gapped at low temperatures. The gap between the first acoustic and optical magnon modes is temperature-dependent and comparable with thermal energies at 300 K.</p>
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