Which is ferromagnetic?

Options

A. Co
B. Mg
C. Ag
D. Cu

Answer

To determine which material is ferromagnetic, we need to understand the definitions and characteristics of different types of magnetic materials: ferromagnetic, paramagnetic, and diamagnetic.

### Step-by-Step Solution:

1. **Understanding Ferromagnetism**:
- Ferromagnetic materials are those that exhibit permanent magnetism. This means that they can be magnetized and retain their magnetic properties even after the external magnetic field is removed.

2. **Understanding Diamagnetism**:
- Diamagnetic materials are characterized by the presence of paired electrons. When placed in a magnetic field, they are repelled by it. Examples include materials like silver and copper.

3. **Understanding Paramagnetism**:
- Paramagnetic materials contain unpaired electrons and are attracted to a magnetic field. They do not retain magnetism when the external field is removed. An example of a paramagnetic material is magnesium.

4. **Identifying Ferromagnetic Materials**:
- Common examples of ferromagnetic materials include iron, cobalt, and nickel. These materials can be magnetized and will retain their magnetism.

5. **Analyzing the Given Options**:
- From the information provided, cobalt is identified as a ferromagnetic substance. The question asks to identify which material is ferromagnetic.

6. **Conclusion**:
- Based on the definitions and examples discussed, the correct answer to the question "Which is ferromagnetic?" is **Cobalt**.