Synthesis and characterization of Heusler compounds with non-collinear magnetic structure. From spin glasses to spin reorientation

Heusler compounds form a large class of intermetallic materials, which attracted a lot of interest in recent years. The reason is their enormous flexibility, which makes it possible to observe almost every physical effect in one of the 1000 members known nowadays. Especially many magnetic Heusler compounds display promising properties, which offer potential application in fields like rare-earth free permanent magnets, magnetocalorics, spin transfer torque devices and tunnel junctions. Apart from that, magnetic Heusler systems are also interesting for fundamental research since some members host skyrmion lattices and other magnetically complex orders. The search for new Heusler compounds is therefore fruitful in many ways. Accordingly, the present thesis followed the approach of synthesizing and characterizing such Heusler compounds that were either entirely new or had unexplored magnetic properties. Exactly this second approach was demonstrated in Chapter 3, namely for IrMnGa. With help of combined neutron and x-ray diffraction experiments it was possible to correct the structural model from literature and show that the compound crystallizes indeed within the half-Heusler space group but with a substantial degree of Y -disorder. In contrast to older suggestions, the subsequent magnetic characterization revealed a robust canonical spin glass state instead of antiferromagnetic order. The magnetic phase diagram was found to be similar to Au${}_{1-x}$Fe${}_x$ and thus hinted on a Heisenberg-like spin glass with considerable anisotropy. Contrary to synthesis route and heat treatment, changing the composition allowed to tune the spin glass state extensively. Increasing the Mn content caused a transition from spin to cluster glass behavior and for Mn contents above 40 at%, it was even possible to introduce ferrimagnetic order. Notably, the composition dependence of spin glasses was only studied for binary systems before. It turned out that many trends are quite similar for the Ir-Mn-Ga ternary scenario with the exception of magnetic behavior near the percolation limit being more complex. Generally, spin glass order is rather rare in Heusler compounds and especially for half-Heusler systems a report remained elusive up to IrMnGa. Chapter 4 then summarized investigations on the Heusler series Fe${}_{3-x}$Mn${}_x$Si, which features a spin reorientation transition at low temperatures. Despite being one of the most studied Heusler systems, the magnetotransport properties were not yet covered systematically in literature. The presented investigations unveiled that the mechanisms of longitudinal as well as Hall resistivity change upon cooling through the spin reorientation transition. For the Hall effect, skew scattering dominates above TR whereas it is the intrinsic mechanism below. The finding emphasizes the dependency of the intrinsic Hall contribution on the magnetic structure and it should be possible to generalize this change of the Hall mechanism to all magnetic transitions, where the intrinsic contribution is affected. The subsequent Chapter 5 provided a reevaluation of the Fe-Mn-Si phase diagram. The approach seemed well justified since the obtained phase boundaries agreed better with theory than the old experimental studies. Furthermore, it was found that those compounds, which were previously identified as β-Mn, actually crystallize in a superstructure. The ordered version has a Mn${}_3$IrSi as prototype and derives from β-Mn by splitting of the 8c site into two 4a sites. Due to the close relation of both structures, this phase was named β'-Mn. Moreover, it turned out that the 'mysterious' secondary phase, which was mentioned for Mn-rich Fe${}_{3-x}$Mn${}_x$Si Heusler compounds but never specified, is given exactly by β'-Mn. The investigations of its magnetic properties indicated a transition to a canonical spin glass state at low temperatures. β'-Mn thus adds a further type of magnetic ordering to the Fe-Mn-Si system. Indeed, the latter comprised all kinds of solid state magnetism but no spin glass order was reported before. Finally, the spin glass state was demonstrated to exhibit a similar composition dependence as in Ir-Mn-Ga, which illustrated nicely the universal character of the spin glass concept. The last chapter dealt with the difficult search for entirely new Heusler compounds. It was explained that high-throughput studies struggle to predict phase stabilities, which is why they have to be treated with care. To overcome these issues, some design rules were suggested to evaluate whether a Heusler compound is likely to be experimentally stable or not. Usually, there are no reports for systems, which do not form as single phase. Since this is a highly inefficient habit, 26 multi-phase 'Heusler compounds' were listed. In the end of the chapter, the successful synthesis of three new compounds was presented, namely Ru${}_2$CrAl, Ru${}_2$CrGa and Ru${}_2$CrSb. Ru${}_2$CrGa was identified as Pauli paramagnet whereas Ru${}_2$CrSb exhibited an antiferromagnetic transition around 100 K. A second transition at 40 K was accompanied by a small increase of magnetization, which hinted on some more complex magnetic structure at low temperatures.