The paper by Groot, Mueller, Engen, and Buschow, published in *Physical Review Letters* in 1983, introduces a new class of materials known as half-metallic ferromagnets. These materials, specifically Mn-based Heusler alloys with the C1_b crystal structure (MgAgAs type), exhibit unique electronic properties. The majority-spin electrons are metallic, while the minority-spin electrons are semiconducting. This asymmetric band structure is a result of the broken inversion symmetry in the C1_b structure compared to the L2_1 Heusler alloys, which have O_h symmetry. The authors used the augmented-spherical-wave method to calculate the band structure, showing that the minority band has a semiconducting gap straddling the Fermi level, while the majority band is metallic. This property leads to 100% spin polarization of the conduction electrons, a phenomenon not observed in other ferromagnets. The paper also discusses the magnetic properties of these materials and suggests that itinerant-electron band models can accurately describe their magnetic behavior. The existence of a gap in the minority-spin bands and the resulting unusual physical properties, such as magneto-optic Kerr rotation, are highlighted.The paper by Groot, Mueller, Engen, and Buschow, published in *Physical Review Letters* in 1983, introduces a new class of materials known as half-metallic ferromagnets. These materials, specifically Mn-based Heusler alloys with the C1_b crystal structure (MgAgAs type), exhibit unique electronic properties. The majority-spin electrons are metallic, while the minority-spin electrons are semiconducting. This asymmetric band structure is a result of the broken inversion symmetry in the C1_b structure compared to the L2_1 Heusler alloys, which have O_h symmetry. The authors used the augmented-spherical-wave method to calculate the band structure, showing that the minority band has a semiconducting gap straddling the Fermi level, while the majority band is metallic. This property leads to 100% spin polarization of the conduction electrons, a phenomenon not observed in other ferromagnets. The paper also discusses the magnetic properties of these materials and suggests that itinerant-electron band models can accurately describe their magnetic behavior. The existence of a gap in the minority-spin bands and the resulting unusual physical properties, such as magneto-optic Kerr rotation, are highlighted.