Give some examples of ferromagnetic substances.

Explanation

### Step-by-Step Solution:

1. **Understanding Ferromagnetic Substances**:
- Ferromagnetic substances are materials that can be magnetized; they have a high magnetic susceptibility, which means they can be easily magnetized and retain their magnetic properties even after the external magnetic field is removed.

2. **Properties of Ferromagnetic Substances**:
- The magnetic susceptibility (denoted as χm) of ferromagnetic substances is positive and large. This indicates that these materials can become strongly magnetized.
- Due to their strong magnetic properties, ferromagnetic substances are often used to create permanent magnets.
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