Internal (Molecular) Field in Ferromagnets In a ferromagnetic material, the magnetic dipole at a lattice site does not experience only the externally applied field H. Which description best represents the internal field sensed by a magnetic dipole inside a ferromagnet under magnetization?

Introduction / Context:
Ferromagnetism arises because atomic magnetic moments tend to align due to strong short-range exchange interactions. Inside a ferromagnet, each dipole (atomic moment) experiences not only the external magnetic field H, but also an additional internal or “molecular” field produced by the surrounding dipoles. Understanding this effective field is crucial for explaining spontaneous magnetization, hysteresis, and Curie temperature behavior.

Given Data / Assumptions:

• Material is ferromagnetic and homogeneous.
• Moments are localized on atoms or ions but coupled via exchange.
• Macroscopic magnetization M is nonzero below Curie temperature.

Concept / Approach:

Weiss’s molecular field theory introduces an internal field proportional to magnetization: H_int = H_applied + λ M, where λ is the molecular-field constant. This internal field strengthens alignment, explaining why ferromagnets can retain magnetization even with zero applied field. Microscopically, the quantum-mechanical exchange interaction between neighboring spins acts like a powerful aligning influence, effectively adding to the applied field at each site.

Step-by-Step Solution:

Verification / Alternative check:

Curie–Weiss law for susceptibility above the Curie point, χ ∝ 1/(T − θ), emerges naturally from assuming such an additive internal field, validating the concept against experimental temperature dependence of magnetization.

Why Other Options Are Wrong:

• Equal to applied field only: ignores exchange, cannot explain spontaneous magnetization.
• Applied minus neighbors: sign is wrong; exchange enhances, not opposes, alignment in ferromagnets.
• Always equal to or less: contradicts enhancement from exchange coupling.

Common Pitfalls:

Confusing demagnetizing fields (which oppose magnetization and depend on shape) with the microscopic exchange field (which enhances alignment). Both can exist simultaneously but play different roles.

Final Answer:

Equal to the applied field plus a contribution from neighboring dipoles