Question

In ferromagnetic materials, magnetic domains:

• A. Align in the direction of an external field
• B. Are always randomly oriented
• C. Disappear under cooling
• D. Neutralize each other in all cases

Hint:

Understand the magnetization loop (Hysteresis Curve):
- Application of \( H \) causes domain wall motion and domain rotation.
- Reaching saturation means all domains have aligned in the direction of the external field.
- Removal of \( H \) leaves a residual magnetization called remanence.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question asks about the behavior of magnetic domains within ferromagnetic materials when subjected to an external magnetic field.

Step 2: Key Formula or Approach:
A magnetic domain is a localized region within a ferromagnetic material where all atomic magnetic moments are aligned parallel to each other due to quantum mechanical exchange interactions.
In an unmagnetized bulk specimen, these domains point in different directions, yielding a net magnetization of zero:
\[ \vec{M}_{\text{net}} = \sum \vec{M}_{\text{domain}} = 0 \]

Step 3: Detailed Explanation:

• Effect of an External Field:
- When an external magnetic field (\( \vec{H} \)) is applied, a torque is exerted on the atomic magnetic moments.
- Domains that are favorably aligned with the direction of the field grow in size at the expense of unfavorably aligned domains through the movement of domain walls (Bloch walls).
- At higher field strengths, the magnetic moments within the remaining domains rotate to align perfectly parallel to the external field direction, reaching a state of magnetic saturation (\( M_s \)).

• Temperature Behavior:
- Magnetic domains disappear when heated above a critical temperature called the Curie temperature (\( T_c \)), where thermal agitation destroys the domain structure, converting the material to a paramagnetic state. They do not disappear under cooling.
- They only neutralize each other's fields in the demagnetized state, not under an external field.


Step 4: Final Answer:
Under an external magnetic field, magnetic domains grow and align in the direction of the field.
Thus, the correct option is (A).