The paper by T. Moriya and Y. Takahashi discusses a general theory of spin fluctuations and the thermodynamic properties of itinerant electron magnets, bridging the gap between weak and strong ferromagnetic limits. The authors present a unified expression for the Curie temperature and explore the physical meaning of the Curie-Weiss magnetic susceptibility. They introduce a model functional for the dynamical susceptibility, which can describe both weakly ferromagnetic and local moment cases. This model allows for an interpolation between these limits, providing a comprehensive understanding of itinerant electron ferromagnetism.
Key points include:
1. **Curie-Weiss Magnetic Susceptibility**: The authors address the long-standing controversy over the nature of magnetic susceptibility in ferromagnetic metals, particularly the Curie-Weiss law. They show that the local moment picture is not sufficient to explain the behavior of ferromagnetic metals, and their theory provides a more general framework.
2. **Model Functional**: A model functional is introduced to describe both weakly ferromagnetic and local moment cases. This functional is derived from a Hubbard-type Hamiltonian and is shown to be applicable to a wide range of itinerant electron magnets.
3. **Temperature-Induced Local Magnetic Moments**: The authors discuss the phenomenon of temperature-induced local magnetic moments, observed in materials like CoS₂ and CoSe₂. These moments are significant when the longitudinal stiffness constant is small or the coupling among spin fluctuation modes is weak.
4. **Nearly Ferromagnetic Semiconductors**: The theory is applied to explain the peculiar magnetic and thermal properties of nearly ferromagnetic semiconductors, such as FeSi. The magnetic susceptibility and specific heat of FeSi are explained using the theory, providing a qualitative explanation for these properties.
Overall, the paper offers a unified and comprehensive approach to understanding the complex behavior of itinerant electron magnets, including the role of spin fluctuations and their temperature dependence.The paper by T. Moriya and Y. Takahashi discusses a general theory of spin fluctuations and the thermodynamic properties of itinerant electron magnets, bridging the gap between weak and strong ferromagnetic limits. The authors present a unified expression for the Curie temperature and explore the physical meaning of the Curie-Weiss magnetic susceptibility. They introduce a model functional for the dynamical susceptibility, which can describe both weakly ferromagnetic and local moment cases. This model allows for an interpolation between these limits, providing a comprehensive understanding of itinerant electron ferromagnetism.
Key points include:
1. **Curie-Weiss Magnetic Susceptibility**: The authors address the long-standing controversy over the nature of magnetic susceptibility in ferromagnetic metals, particularly the Curie-Weiss law. They show that the local moment picture is not sufficient to explain the behavior of ferromagnetic metals, and their theory provides a more general framework.
2. **Model Functional**: A model functional is introduced to describe both weakly ferromagnetic and local moment cases. This functional is derived from a Hubbard-type Hamiltonian and is shown to be applicable to a wide range of itinerant electron magnets.
3. **Temperature-Induced Local Magnetic Moments**: The authors discuss the phenomenon of temperature-induced local magnetic moments, observed in materials like CoS₂ and CoSe₂. These moments are significant when the longitudinal stiffness constant is small or the coupling among spin fluctuation modes is weak.
4. **Nearly Ferromagnetic Semiconductors**: The theory is applied to explain the peculiar magnetic and thermal properties of nearly ferromagnetic semiconductors, such as FeSi. The magnetic susceptibility and specific heat of FeSi are explained using the theory, providing a qualitative explanation for these properties.
Overall, the paper offers a unified and comprehensive approach to understanding the complex behavior of itinerant electron magnets, including the role of spin fluctuations and their temperature dependence.