A nonequilibrium rapid-quenching method has been used to fabricate NiMnIn and NiMnGa alloys that are chemically and morphologically similar but crystallographically and physically very different. NiMnGa crystallizes in a Ni 2 In-type hexagonal structure, whereas NiMnIn is a cubic Heusler alloy. Both alloys yield a topological Hall effect contribution corresponding to bubble-type skyrmion spin structures, but it occurs in much lower magnetic fields in NiMnIn as compared to NiMnGa. The effect is unrelated to net Dzyaloshinskii-Moriya interactions, which are absent in both alloys due to their inversion-symmetric crystal structures. Based on magnetic-force microscopy, we explain the difference between the two alloys by magnetocrystalline anisotropy and uniaxial and cubic anisotropies yielding full-fledged and reduced topological Hall effects, respectively. Since NiMnIn involves small magnetic fields (0.02-0.3 kOe) at and above room temperature, it is of potential interest in spin electronics.

Magnets with chiral crystal structures and helical spin structures have recently attracted much attention as potential spin-electronics materials, but their relatively low magnetic-ordering temperatures are a disadvantage. While cobalt has long been recognized as an element that promotes high-temperature magnetic ordering, most Co-rich alloys are achiral and exhibit collinear rather than helimagnetic order. Crystallographically, the B20-ordered compound CoSi is an exception due to its chiral structure, but it does not exhibit any kind of magnetic order. Here, we use nonequilibrium processing to produce B20-ordered Co 1þx Si 1−x with a maximum Co solubility of x ¼ 0.043. Above a critical excess-Co content (x c ¼ 0.028), the alloys are magnetically ordered, and for x ¼ 0.043, a critical temperature T c ¼ 328 K is obtained, the highest among all B20-type magnets. The crystal structure of the alloy supports spin spirals caused by Dzyaloshinskii-Moriya interactions, and from magnetic measurements we estimate that the spirals have a periodicity of about 17 nm. Our density-functional calculations explain the combination of high magneticordering temperature and short periodicity in terms of a quantum phase transition where excess-cobalt spins are coupled through the host matrix.

Permanent-magnet materials are one cornerstone of today’s technology, abundant in disk drives, motors, medical equipment, wind generators, and cars. A continuing challenge has been to reconcile high permanent-magnet performance with low raw-material costs. This work reports a Mn-Bi-Sn alloy exclusively made from inexpensive elements, exhibiting high values of Curie-temperature, magnetization, anisotropy, coercivity, and energy product. The samples are produced by field annealing of rapidly quenched Sn-containing MnBi alloys, where the improvement of the magnetic properties is caused by the substitutional occupancy of the 2c sites in the hexagonal NiAs structure by Sn. The substitution modifies the electronic structure of the compound and enhances the magnetocrystalline anisotropy, thereby improving the coercivity of the compound. The energy product reaches 114 kJ/m3 (14.3 MGOe) at room temperature and 86 kJ/m3 (10.8 MGOe) at 200 °C; this value is similar to that of the Dy-free Nd2Fe14B and exceeds that of other rare-earth-free permanent-magnet bulk alloys, as encountered in automotive applications.

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The relationship between atomic-scale and micromagnetic Dzyaloshinski-Moriya (DM) interactions has been investigated. By analyzing the Lifshitz invariants for different point groups, we have found that there is no unique link between the absence of inversion symmetry and DM interactions. The absence of inversion symmetry is a necessary condition for a net DM interaction in crystals, but several noncentrosymmetric point groups have zero DM interactions. In many cases, the key consideration is whether the crystals are polar and/or chiral. For example, MnSi-type spin spirals, which violate helical spin symmetry, are caused by the insertion of chiral atomic-scale building blocks into an achiral cubic lattice, and the scalar interaction parameter D used to describe the spirals is only loosely related to the DM vector D. It contains, in fact, magnetostatic and magnetocrystalline contributions of unknown magnitude. Finally, we discuss some aspects of the micromagnetism of the skyrmionics of nanoparticles and granular nanostructures.