Every substance has some magnetic properties associated with it. The origin of these properties lies in the electrons. Each electron in an atom behaves like a tiny magnet. Its magnetic moment originates from two types of motions.

(i) Its orbital motion around the nucleus and

(ii) Its spin around its own axis.

Electron being a charged particle and undergoing these motions can be considered as a small loop of current which possesses a magnetic moment. Thus, each electron has a permanent spin and an orbital magnetic moment associated with it. Magnitude of this magnetic moment is very small and is measured in the unit called Bohr magneton, µB. It is equal to 9.27 × 10^–24 Am². On the basis of their magnetic properties, substances can be classified into five categories:

(i) Paramagnetic

(ii) Diamagnetic

(iii) Ferromagnetic

(iv) Antiferromagnetic and

(v) Ferrimagnetic.

Which of the following substances becomes paramagnetic on heating?

Ferrimagnetic

The correct answer is option 3. Ferrimagnetism.

Ferrimagnetic materials exhibit a fascinating change in their magnetic behavior when heated. Let us delve into this phenomenon.

Imagine tiny magnets within a ferrimagnetic material called magnetic domains. In these materials, unlike ferromagnets where all domains align in the same direction, the magnetic moments of these domains partially cancel each other due to an unequal antiparallel alignment. However, one set of domains has a stronger magnetic moment compared to the other. This results in a net magnetic attraction, making the entire material ferrimagnetic.

Now, when you heat a ferrimagnetic material, thermal energy vibrates the atoms within it. This increased thermal agitation disrupts the delicate balance between the aligned domains. The previously ordered structure loosens up, and the individual magnetic moments start behaving more independently.

With the random thermal motion, the magnetic moments within the domains no longer align in a specific direction. This lack of a preferred direction for the individual magnetic moments is a key characteristic of paramagnetic materials. In essence, the heating process randomizes the orientation of the magnetic moments, causing the ferrimagnetic behavior to vanish and the material to exhibit paramagnetism.

Paramagnetic materials are weakly attracted to a magnetic field. While they don't retain a permanent magnetic field like ferromagnetic materials, they do show a slight increase in magnetic susceptibility when placed in an external magnetic field. This means the randomly oriented magnetic moments tend to partially align with the applied field.

Think of a group of people wearing tiny magnets sewn onto their clothes. In the ferrimagnetic state, most people have their magnets facing a certain direction, resulting in a net attraction. Upon heating (increased activity), everyone starts moving around, and their magnets point in random directions, essentially losing the initial coordinated attraction. This random orientation is analogous to the paramagnetic state.