Louis Néel's paper, "Propriétés magnétiques des ferrites ; ferrimagnétisme et antiferromagnétisme," published in the Annales de Physique in 1948, explores the magnetic properties of ferrites, including ferrimagnetism and antiferromagnetism. The paper begins with an introduction (§§ 1-5) and then delves into the first part of the memoir (§§ 6-15), where Néel develops and discusses the theory of magnetic properties of substances composed of identical magnetic moment carriers distributed on two different types of sites, A and B, with different crystallographic arrangements. He generalizes the molecular field approximation by characterizing magnetic interactions with three molecular field coefficients relative to carriers on sites of the same or different categories. Néel introduces the concept of ferrimagnetic materials, which exhibit magnetic properties similar to ordinary ferromagnets at low temperatures despite negative molecular field coefficients. These materials have a saturation magnetization at absolute zero lower than that of ordinary ferromagnets, a thermal variation of spontaneous magnetization that can differ significantly from ferromagnets, and a hyperbolic variation of the inverse susceptibility above the Curie point, differing from Weiss' law. The second part of the memoir (§§ 23-26) demonstrates how the theory can account for the paramagnetic and ferromagnetic properties of ferrites using experimental data. Néel calculates characteristic parameters for copper ferrite and interprets the abnormal properties of zinc and cadmium ferrites. He also explains the magnetic properties of magnetite and manganese sulfide Mn₂Sb, showing how the theory can predict their magnetic behavior. In the third part (§§ 34-43), Néel applies the theory to experimental data on ferrites and antiferromagnets, calculating the characteristic molecular field coefficients for each bond. He finds that these values do not agree with those derived from pure metals, suggesting the need for significant interactions between atoms at great distances. Néel proposes the possibility of indirect interactions through anions, following Kramers' mechanism of superexchange, which would explain the negative exchange integrals and positive ordinary exchange integrals observed in the studied materials. Néel's work provides a comprehensive theoretical framework for understanding the magnetic properties of ferrites and antiferromagnets, contributing significantly to the field of solid-state physics.Louis Néel's paper, "Propriétés magnétiques des ferrites ; ferrimagnétisme et antiferromagnétisme," published in the Annales de Physique in 1948, explores the magnetic properties of ferrites, including ferrimagnetism and antiferromagnetism. The paper begins with an introduction (§§ 1-5) and then delves into the first part of the memoir (§§ 6-15), where Néel develops and discusses the theory of magnetic properties of substances composed of identical magnetic moment carriers distributed on two different types of sites, A and B, with different crystallographic arrangements. He generalizes the molecular field approximation by characterizing magnetic interactions with three molecular field coefficients relative to carriers on sites of the same or different categories. Néel introduces the concept of ferrimagnetic materials, which exhibit magnetic properties similar to ordinary ferromagnets at low temperatures despite negative molecular field coefficients. These materials have a saturation magnetization at absolute zero lower than that of ordinary ferromagnets, a thermal variation of spontaneous magnetization that can differ significantly from ferromagnets, and a hyperbolic variation of the inverse susceptibility above the Curie point, differing from Weiss' law. The second part of the memoir (§§ 23-26) demonstrates how the theory can account for the paramagnetic and ferromagnetic properties of ferrites using experimental data. Néel calculates characteristic parameters for copper ferrite and interprets the abnormal properties of zinc and cadmium ferrites. He also explains the magnetic properties of magnetite and manganese sulfide Mn₂Sb, showing how the theory can predict their magnetic behavior. In the third part (§§ 34-43), Néel applies the theory to experimental data on ferrites and antiferromagnets, calculating the characteristic molecular field coefficients for each bond. He finds that these values do not agree with those derived from pure metals, suggesting the need for significant interactions between atoms at great distances. Néel proposes the possibility of indirect interactions through anions, following Kramers' mechanism of superexchange, which would explain the negative exchange integrals and positive ordinary exchange integrals observed in the studied materials. Néel's work provides a comprehensive theoretical framework for understanding the magnetic properties of ferrites and antiferromagnets, contributing significantly to the field of solid-state physics.