Question

Curie temperature of ferrite is

• A. 768 $^\circ$C
• B. 910 $^\circ$C
• C. 1147 $^\circ$C
• D. 727 $^\circ$C

Hint:

At the Curie temperature of 768 $^\circ$C, ferrite undergoes a magnetic transition (ferromagnetic $\to$ paramagnetic) but its crystal structure remains Body-Centered Cubic (BCC).
The crystal structure change from BCC to FCC occurs later at 910 $^\circ$C.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks for the Curie temperature ($T_c$) of ferrite ($\alpha$-iron), which is the critical transition temperature at which the material changes from ferromagnetic to paramagnetic.

Step 2: Detailed Explanation:

• What is Curie Temperature?
The Curie temperature is the temperature above which a ferromagnetic material loses its permanent magnetic properties and becomes paramagnetic.
At this temperature, the thermal energy ($k_B T$) is sufficient to overcome the magnetic exchange coupling that aligns the individual atomic magnetic dipoles.

• Curie Temperature of Ferrite:
Alpha-ferrite ($\alpha$-Fe) is strongly ferromagnetic at room temperature.
As temperature increases, the magnetic alignment begins to randomize.
At exactly $768 \ ^\circ\text{C}$ (approximately 1043 K), ferrite undergoes a magnetic phase transition and becomes paramagnetic.
This is known as the Curie point of iron. Note that there is no change in crystal structure at $768 \ ^\circ\text{C}$; it remains BCC.

• Other Temperatures on the Iron-Carbon Diagram:
- $727 \ ^\circ\text{C}$ is the eutectoid temperature ($A_1$ line).
- $910 \ ^\circ\text{C}$ is the allotropic transformation temperature from BCC ($\alpha$-ferrite) to FCC ($\gamma$-austenite) ($A_3$ line).
- $1147 \ ^\circ\text{C}$ is the eutectic transformation temperature of cast iron.

Step 3: Final Answer:
The Curie temperature of ferrite is 768 $^\circ$C.