Neélpunkter

Neélpunkter, also known as Néel points or Néel temperatures, are the temperatures at which a material transitions from an antiferromagnetic state to a paramagnetic state. Unlike ferromagnetic materials which lose their spontaneous magnetization above the Curie temperature, antiferromagnetic materials exhibit a unique behavior where their ordered magnetic moments cancel each other out, resulting in no net magnetic moment at low temperatures. The Néel temperature signifies the point where thermal agitation overcomes the exchange interaction that aligns the magnetic moments in opposing directions, leading to a disordered paramagnetic state.

The Néel point is a critical parameter for understanding the magnetic properties of many inorganic compounds, minerals, and some alloys. For instance, certain metal oxides and halides are known to be antiferromagnetic below their respective Néel temperatures. The exact value of the Néel temperature varies significantly depending on the specific material and its crystal structure. For example, the Néel temperature of manganese oxide (MnO) is around 118 K, while that of chromium (Cr) is much higher, approximately 311 K.

Identifying the Néel point is typically achieved through measurements of magnetic susceptibility as a function of temperature. A characteristic peak or a sharp change in the magnetic susceptibility is observed at the Néel temperature, indicating the transition from the ordered antiferromagnetic state to the disordered paramagnetic state. This phenomenon is named after French physicist Louis Néel, who was awarded the Nobel Prize in Physics in 1970 for his pioneering work on the magnetic properties of solids, including the theoretical description of antiferromagnetism and ferrimagnetism.