Itinerant Ferromagnetism and Antiferromagnetism from a Chemical Bonding Perspective

When quantum-chemical tools such as the Crystal Orbital Hamilton Population (COHP) technique are used for the analysis of nonmagnetic periodic systems, ubiquitous chemical bonding phenomena known from the molecular fields (decrease of total, potential, and single-particle energy as well as increase in kinetic energy) are also found in the solid state; likewise, there is a clear preference for bonding interactions as well as avoidance of antibonding states by means of Jahn-Teller and Peierls distortions. Non-spin-polarized bonding analyses of systems with a tendency for ferromagnetic order (such as the archetypes Fe, Co, and Ni) exhibit antibonding interactions at the Fermi level which disappear upon spin polarization; within the elemental metals the onset of magnetism strengthens the metal-metal bond. A typical antiferromagnet (Cr) shows non-bonding metal-metal interactions in the highest bands which remain almost unaffected by spin polarization. The bond-theoretical classification of potential ferromagnets and antiferromagnets is easily generalized for intermetallic alloys and can be utilized for rational syntheses. The “chemical” strategy is exemplified for the case of multinary iron/manganese rhodium borides in which various adjustments of the valence electron concentration have been synthetically realized.