Metamagnetism is a physical phenomenon that determines a sudden (often, dramatic) increase in the magnetization of a material; the transition for which some antiferromagnetic substances (FeCl2, MnAu2 etc.) become ferromagnetic. The metamagnetism occurs in the presence of a strong magnetic field and at a certain temperature.
The metamagnetic behavior may have quite different physical causes for different types of metamagnets. Some examples of physical mechanisms leading to metamagnetic behavior are:
- itinerant metamagnetism: exchange splitting of the Fermi surface in a paramagnetic system of itinerant electrons causes an energetically favorable transition to bulk magnetization near the transition to a ferromagnet or other magnetically ordered state.
- antiferromagnetic transition: field-induced spin flips in antiferromagnets cascade at a critical energy determined by the applied magnetic field.
Depending on the material and experimental conditions, metamagnetism may be associated with a first-order phase transition, a continuous phase transition at a critical point (classical or quantum), or crossovers beyond a critical point that do not involve a phase transition at all. These wildly different physical explanations sometimes lead to confusion as to what the term “metamagnetic” is referring in specific cases.
Many antiferromagnetic lanthanides exhibit the phenomenon of metamagnetism. They change to a ferromagnetic state in an appropriate high-intensity magnetic field. For example, dysprosium (Dy) is ferromagnetic below 85 K, but antiferromagnetic at higher temperatures. Application of a magnetic field maintains the ferromagnetic state above 85 K, up to the Néel temperature of 179 K.