Louis Néel's article "Some Theoretical Aspects of Rock-Magnetism" provides a theoretical study of the magnetic properties of rocks, focusing on ferrimagnetism, single domain particles, and large multi-domain particles. The paper discusses the thermal variation of saturation magnetization, initial susceptibility, and the effects of applied fields on thermo-remanent magnetization (TRM). It also explores the ratio of TRM acquired in a given field to the induced magnetization and the additivity of partial TRM in small and large grains. The author delves into the magnetic 'viscosity' due to thermal agitation in small and large grains, and the irreversible decrease in isothermal remanent magnetization. The paper reviews different mechanisms that can cause negative TRM, including diffusion, anomalous thermal variation, and effects of demagnetizing fields. Néel also examines the thermal variation of spontaneous magnetization in ferrites and the inversion of spontaneous magnetization through diffusion. The article discusses the thermal variation of spontaneous magnetization in ferrimagnetic substances, including the Weiss molecular field theory and the N-type and P-type curves. It explores the distinction between compensation temperature and Curie point, and the role of triangular systems in ferrimagnetism. The paper also analyzes the magnetic properties of rhombohedral ferric oxide (hematite) and the ilmenite-hematite series, and proposes a possible reversal of spontaneous magnetization by change of composition. Finally, it discusses the theory of single domain particles, including wall thickness, grain size, and elementary magnetization cycles, and addresses objections to interpreting rock magnetic properties using single domain grain models.Louis Néel's article "Some Theoretical Aspects of Rock-Magnetism" provides a theoretical study of the magnetic properties of rocks, focusing on ferrimagnetism, single domain particles, and large multi-domain particles. The paper discusses the thermal variation of saturation magnetization, initial susceptibility, and the effects of applied fields on thermo-remanent magnetization (TRM). It also explores the ratio of TRM acquired in a given field to the induced magnetization and the additivity of partial TRM in small and large grains. The author delves into the magnetic 'viscosity' due to thermal agitation in small and large grains, and the irreversible decrease in isothermal remanent magnetization. The paper reviews different mechanisms that can cause negative TRM, including diffusion, anomalous thermal variation, and effects of demagnetizing fields. Néel also examines the thermal variation of spontaneous magnetization in ferrites and the inversion of spontaneous magnetization through diffusion. The article discusses the thermal variation of spontaneous magnetization in ferrimagnetic substances, including the Weiss molecular field theory and the N-type and P-type curves. It explores the distinction between compensation temperature and Curie point, and the role of triangular systems in ferrimagnetism. The paper also analyzes the magnetic properties of rhombohedral ferric oxide (hematite) and the ilmenite-hematite series, and proposes a possible reversal of spontaneous magnetization by change of composition. Finally, it discusses the theory of single domain particles, including wall thickness, grain size, and elementary magnetization cycles, and addresses objections to interpreting rock magnetic properties using single domain grain models.