Which of the following properties is characteristic of ferromagnetic materials and is not found in ordinary paramagnetic or diamagnetic materials?

Introduction / Context:
Materials respond differently to external magnetic fields. Ferromagnetic materials such as iron, cobalt and nickel exhibit strong magnetisation and can become permanent magnets. Paramagnetic and diamagnetic materials respond only weakly and do not retain magnetisation when the field is removed. This question asks which property is unique or characteristic of ferromagnetic materials compared with other types.

Given Data / Assumptions:

• Ferromagnetic materials show strong attraction to magnetic fields and can be permanently magnetised.
• Paramagnetic materials show weak attraction, while diamagnetic materials show weak repulsion.
• We are comparing properties like temperature dependence, directionality and hysteresis.
• We assume basic knowledge of magnetic hysteresis loops.

Concept / Approach:
A key distinguishing feature of ferromagnetic materials is hysteresis: the ability to retain some magnetisation even after the external magnetic field has been reduced to zero. This is represented by a hysteresis loop on a graph of magnetisation versus applied field. Paramagnetic and diamagnetic materials do not show a significant hysteresis loop; their magnetisation follows the field almost reversibly, and they do not remain magnetised when the field is removed. While ferromagnets do point strongly in a direction and attract other magnetic substances, weak alignment and attraction can also occur in other types of materials, so hysteresis is the most distinctive property.

Step-by-Step Solution:
Step 1: Consider that ferromagnetic materials, once magnetised, can remain magnetised even after the external field is switched off. Step 2: This memory-like behaviour of magnetisation is described by a hysteresis loop, showing that the path of magnetisation depends on the history of the applied field. Step 3: Paramagnetic and diamagnetic materials, by contrast, do not retain magnetisation when the field is removed; their magnetisation curves return nearly to zero with little or no hysteresis. Step 4: Therefore, hysteresis is a property specifically associated with ferromagnetic (and related ferrimagnetic) materials. Step 5: Examine the other options and see that they are either incorrect statements or not unique to ferromagnets.

Verification / Alternative check:
If you magnetise a piece of iron using a strong external field, it becomes a magnet; when the external field is taken away, the iron remains magnetised to some extent. This retained magnetisation can be observed by its ability to attract small iron objects. The magnetic hysteresis loop, measured in laboratories, shows the lag of magnetisation behind the applied field and the presence of remanent magnetisation and coercivity, characteristic of ferromagnetic materials. Paramagnetic materials such as aluminium and diamagnetic materials such as bismuth do not show these features.

Why Other Options Are Wrong:
Magnetic susceptibility independent of temperature: In reality, the susceptibility of ferromagnets is strongly dependent on temperature and drops to zero above a certain Curie temperature; this statement is false.

The ability to point in a particular direction when placed in a weak field: Paramagnetic materials can also align weakly with a magnetic field; directional alignment is not unique to ferromagnets.
The ability to attract a magnetic substance weakly: Paramagnets also show weak attraction; ferromagnets show strong attraction plus hysteresis.

Common Pitfalls:
Students often focus on the fact that ferromagnets attract magnets strongly, but this can distract from the more scientifically distinctive property: the existence of a hysteresis loop and remanent magnetisation. Another confusion is with temperature effects; some may think ferromagnets are unaffected by temperature, when in fact temperature is crucial in determining their behaviour.

Final Answer:
The property found only in ferromagnetic materials among the options is Hysteresis (retaining magnetisation after the external field is removed).