Understanding Magnetic Material Properties: Retaining Magnetism

When a magnetic material is placed in an external magnetic field, it becomes magnetized. This happens because the magnetic domains within the material align themselves with the direction of the external field.

The question asks about the property that allows a magnetic material to keep some or all of its magnetism even after the external magnetic field is removed. This retained magnetism is called residual magnetism.

Let's look at the given options to identify the correct property:

• Relative permeability: This property describes how easily a material can be magnetized when placed in a magnetic field compared to vacuum. It is a measure of how much the material concentrates magnetic flux lines. While important for initial magnetization, it doesn't directly measure the ability to retain magnetism after the field is gone.
• Reluctance: This is the opposition that a magnetic circuit offers to magnetic flux. It is analogous to resistance in an electrical circuit. Reluctance relates to the magnetic circuit's geometry and material properties, but not specifically the retention of magnetism after the field removal.
• Resistivity: This is a property of a material that measures how strongly it resists electrical current flow. It is related to electrical properties, not magnetic properties, and has no bearing on the ability of a material to retain magnetism.
• Retentivity: This property specifically describes the ability of a magnetic material to retain a certain amount of residual magnetism after the external magnetizing field is removed. Materials with high retentivity are used to make permanent magnets, as they can hold their magnetism well. Materials with low retentivity are used in applications where magnetization needs to disappear quickly when the field is removed, such as in electromagnets cores.

Based on the definitions, the property that describes the retention of magnetism in a magnetic material after the removal of the external magnetic field is retentivity.