At the Curie temperature, a ferromagnetic material undergoes a transition and becomes which type of magnetic material?

Introduction / Context:
This question comes from the topic of magnetism and material science. Ferromagnetic materials such as iron, cobalt and nickel exhibit strong magnetisation and can form permanent magnets at ordinary temperatures. However, their magnetic behaviour changes with temperature. There is a characteristic temperature, called the Curie temperature, above which ferromagnetic order is lost. Understanding what happens at this temperature is important in physics, electrical engineering and materials technology, and it is a popular concept in multiple choice questions.

Given Data / Assumptions:

• We start with a ferromagnetic material, meaning it shows strong spontaneous magnetisation below a certain temperature.
• At the Curie temperature, a change in magnetic ordering occurs.
• Options include diamagnetic, non-magnetic, paramagnetic and none of the others.
• We assume the material structure remains intact; only magnetic ordering changes.

Concept / Approach:
Ferromagnetism arises because atomic magnetic moments in certain materials tend to align parallel to each other in regions called domains, leading to a net magnetisation. As temperature increases, thermal agitation tends to randomise these moments. At a specific temperature, known as the Curie temperature (or Curie point), thermal energy becomes strong enough to break the long range alignment of domains. Above this temperature, the material no longer shows strong spontaneous magnetisation and behaves like a paramagnetic substance, where magnetic moments are more or less randomly oriented but can align weakly with an external field. The material does not become diamagnetic or entirely non-magnetic; rather, it becomes paramagnetic.

Step-by-Step Solution:
Step 1: Identify that below the Curie temperature, ferromagnetic materials have aligned domains that produce strong magnetisation. Step 2: Recognise that increasing temperature increases random thermal motion of atoms and magnetic moments. Step 3: At the Curie temperature, thermal energy disrupts the cooperative alignment responsible for ferromagnetism. Step 4: Above this temperature, the material still has atomic magnetic moments, but they no longer align spontaneously to form strong domains. Step 5: This new state matches the behaviour of paramagnetic materials, which show weak magnetisation only in the presence of an external magnetic field. Step 6: Therefore, we conclude that the ferromagnetic material becomes paramagnetic at the Curie temperature.

Verification / Alternative check:
One way to verify is to recall that magnetic susceptibility of ferromagnets is very high below the Curie temperature but drops sharply and follows a paramagnetic like Curie law above that temperature. Experimental plots of magnetisation versus temperature show a steep fall at the Curie point, beyond which the behaviour is similar to paramagnets. These standard graphs in physics textbooks confirm that ferromagnets do not become diamagnetic or fully non-magnetic; instead, they become paramagnetic when heated beyond the Curie temperature.

Why Other Options Are Wrong:
Diamagnetic materials are those in which induced magnetisation is opposite to the applied field and very weak; ferromagnets do not convert to purely diamagnetic state at the Curie temperature. Non-magnetic material is an imprecise term; ferromagnets heated above the Curie temperature still respond weakly to magnetic fields, so describing them as non-magnetic is inaccurate. None of the others cannot be correct because paramagnetic material is explicitly listed and is the correct description of their behaviour above the Curie temperature.

Common Pitfalls:
Students sometimes misinterpret the phrase non-magnetic to mean that the material loses all magnetic properties at high temperature, which is not true. Others confuse diamagnetism, which is present to some extent in all materials, with the change from ferromagnetic to paramagnetic ordering. To avoid mistakes, remember the clear sequence: ferromagnetic below the Curie point, paramagnetic above it, with the Curie temperature marking a magnetic phase transition between these two states.

Final Answer:
At the Curie temperature, a ferromagnetic material loses its strong ordered magnetisation and becomes paramagnetic.