Metamagnetism is a blanket term used loosely in physics to describe a sudden (often, dramatic) increase in the magnetization

Magnetization

In classical electromagnetism, magnetization or magnetic polarization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material...

 of a material with a small change in an externally applied magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

. The metamagnetic behavior may have quite different physical causes for different types of metamagnets. Some examples of physical mechanisms leading to metamagnetic behavior are:

1. Itinerant Metamagnetism - Exchange splitting of the Fermi surface
   Fermi surface
   In condensed matter physics, the Fermi surface is an abstract boundary useful for predicting the thermal, electrical, magnetic, and optical properties of metals, semimetals, and doped semiconductors. The shape of the Fermi surface is derived from the periodicity and symmetry of the crystalline...
   
    in a paramagnetic system of itinerant
   Itinerant
   An itinerant is a person who travels from place to place with no fixed home. The term comes from the late 16th century: from late Latin itinerant , from the verb itinerari, from Latin iter, itiner ....
   
    electrons causes an energetically favorable transition to bulk magnetization near the transition to a ferromagnet or other magnetically ordered state.
2. 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

Phase transition

A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....

, a continuous phase transition at a critical point

Critical point (thermodynamics)

In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions at which a phase boundary ceases to exist...

(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.