Question

Upon cooling from room temperature, the magnetic susceptibility of MnO slowly increases until 118 K, and then it decreases. This phenomenon is known as:

• A. ferromagnetism
• B. paramagnetism
• C. antiferromagnetism
• D. ferrimagnetism

Hint:

Antiferromagnetism is identified by a peak in magnetic susceptibility at the Neel temperature, followed by a decrease as temperature drops further. MnO is a classic example of such behavior.

Answer 1

The Correct Option is C

Manganese(II) oxide (MnO) is known to undergo a magnetic transition as it is cooled. The key observations are:

• As MnO is cooled from room temperature, its magnetic susceptibility increases, indicating paramagnetic behavior.
• At \( 118 \, \text{K} \), the susceptibility reaches a maximum—this temperature is called the Néel temperature (\( T_N \)).
• Below \( T_N \), the susceptibility begins to decrease, which is characteristic of antiferromagnetic materials.

In an antiferromagnetic material, adjacent magnetic moments align in opposite directions, effectively canceling out the overall magnetization. This results in a drop in magnetic susceptibility below the Néel temperature.

\[ \boxed{\text{MnO exhibits antiferromagnetism with a Néel temperature of } 118 \, \text{K}} \]