Question

Which of the following is true for hard magnetic materials:

• A. both coercivity and retentivity are high
• B. coercivity is high and retentivity is low
• C. both coercivity and retentivity are low
• D. coercivity is low and retentivity is high

Hint:

Think of the terms literally: a "hard" magnet is hard to magnetize but also hard to demagnetize. This directly translates to high coercivity. To be a strong permanent magnet, it must also hold a lot of magnetism, which means high retentivity.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Magnetic materials are categorized as "hard" or "soft" based on their hysteresis loop characteristics. Hard magnets are ideal for creating permanent magnets.
Step 2: Detailed Explanation:
An effective permanent magnet material needs to meet these two main requirements:

High Retentivity (or Remanence): The material should maintain a strong level of magnetization after a strong external magnetic field is removed. This ability to retain magnetization is retentivity.
High Coercivity: The material should be difficult to demagnetize, resisting demagnetization from external magnetic fields, temperature changes, or physical impacts. Coercivity measures this resistance, indicating the strength of the opposing magnetic field needed to reduce the magnetization to zero.
Hard magnetic materials possess both high retentivity and high coercivity, resulting in a wide and tall hysteresis loop. Conversely, soft magnetic materials (found in transformers and electromagnets) have low coercivity, enabling easy magnetization and demagnetization.
Step 3: Final Answer:
Hard magnetic materials are defined by their high coercivity and high retentivity.