Spontaneous magnetization and ferromagnetism Is spontaneous magnetization (nonzero magnetization without an external field below a critical temperature) a defining characteristic of ferromagnetic materials?

Introduction / Context:
Ferromagnets exhibit collective spin alignment due to exchange interactions, leading to a magnetized state even in zero applied field. This property—spontaneous magnetization—is central to magnetic materials used in motors, transformers, and data storage.

Given Data / Assumptions:

• Temperature below Curie temperature (Tc).
• Ferromagnetic ordering (no external H).
• Macroscopic domain formation allowed by microstructure.

Concept / Approach:

Below Tc, the free energy is minimized by finite magnetization due to exchange coupling, producing domains. Even if net sample magnetization averages to zero without field (due to randomly oriented domains), each domain has spontaneous magnetization. Above Tc, thermal agitation destroys long-range order and the material becomes paramagnetic.

Step-by-Step Reasoning:

Verification / Alternative check:

Hysteresis loops at room temperature for iron, nickel, cobalt show remanent magnetization, a direct signature of spontaneous magnetization and domain structure below Tc.

Why Other Options Are Wrong:

• “False”: contradicts the defining property.
• “Only above Tc”: above Tc, ferromagnets are paramagnetic and have no spontaneous magnetization.
• Paramagnets/diamagnets do not show spontaneous magnetization.

Common Pitfalls:

Confusing remanence (sample-level) with spontaneous magnetization (domain-level order); both reflect the same underlying ferromagnetic ordering.

Final Answer:

True