Question

Associated with every ferromagnetic solid is a temperature known as its Curie temperature. When heated above its Curie temperature, the substance no longer exhibits ferromagnetism but rather becomes paramagnetic. Use the kinetic- molecular theory of solids to explain this observation.

Short answer

In summary, a ferromagnetic material becomes paramagnetic when heated above its Curie temperature due to the increased thermal energy associated with particle vibrations in the solid (explained by the kinetic-molecular theory of solids). This increase in thermal energy overcomes the interactions between neighboring magnetic moments that were responsible for keeping them aligned and maintaining the material's ferromagnetic properties. As a result, the magnetic moments become disordered, and the material displays paramagnetic behavior, where the magnetic moments only align in the presence of an external magnetic field.

Step-by-step solution

Understand Ferromagnetism and Paramagnetism

Ferromagnetism is a type of magnetism exhibited by certain materials, such as iron and nickel, in which atoms or ions possess a permanent magnetic moment due to the alignment of their spins. In a ferromagnetic material, these magnetic moments are aligned in parallel, which leads to strong magnetic properties. Paramagnetism is a type of magnetism where materials are attracted to a magnetic field but do not retain a permanent magnetism. In a paramagnetic material, the atoms or ions have a magnetic moment due to unpaired electrons, but these moments are not aligned in a parallel fashion. These materials become magnetized in a magnetic field but lose their magnetism when the field is removed.

Understand Curie Temperature

Curie temperature, named after the physicist Pierre Curie, refers to the critical temperature above which a ferromagnetic material loses its ferromagnetic properties and becomes paramagnetic. Below the Curie temperature, the material's magnetic moments are aligned due to interactions between neighboring moments, creating a strong magnetic field. Above the Curie temperature, the thermal energy overcomes these interactions, causing the magnetic moments to become disordered and the material's magnetism to be lost.

Understand Kinetic-Molecular Theory of Solids

The kinetic-molecular theory of solids describes the behavior of particles (atoms, molecules, or ions) in a solid. According to this theory, particles in a solid are in constant motion, vibrating around their equilibrium positions. As the temperature increases, the particles' vibrational energy also increases, causing them to vibrate more intensely.

Explain the transition from Ferromagnetism to Paramagnetism

Using the kinetic-molecular theory of solids, we can now explain why ferromagnetic materials become paramagnetic when heated above the Curie temperature. 1. Below the Curie temperature, the magnetic moments in a ferromagnetic material are aligned due to the interactions between neighboring moments. These interactions are strong enough to overcome the thermal energy associated with the vibrations of the particles, which maintains the material's ferromagnetic properties. 2. When the temperature is increased beyond the Curie temperature, the vibrational energy of the particles also increases. This increase in thermal energy overcomes the interactions between neighboring magnetic moments, causing them to become disordered. 3. As a result, the material loses its ferromagnetic properties and becomes paramagnetic, with individual magnetic moments now randomly oriented and only aligning in the presence of an external magnetic field. 4. This transition in magnetic behavior can, therefore, be understood by considering the balance between the interactions between magnetic moments and the thermal energy associated with particle vibrations in a solid, as described by the kinetic-molecular theory of solids.

Key concepts

Ferromagnetism

Ferromagnetism is a fascinating phenomenon where certain materials, like iron, cobalt, and nickel, exhibit strong magnetic properties. This kind of magnetism is the result of the alignment of electron spins in the material. Electrons have a property called spin, which gives them a magnetic moment, sort of like tiny magnets with north and south poles. In ferromagnetic materials, a large number of these electron spins point in the same direction, creating a strong, collective magnetic field.

Now, how does this alignment happen? It's all thanks to something called exchange interactions. These are quantum mechanical forces that 'encourage' the spins of neighboring electrons to align parallel to each other. Below a certain temperature—the Curie temperature—these interactions are strong enough to keep the electron spins aligned even without an external magnetic field. This coherent alignment results in the material being magnetized, capable of producing a magnetic field by itself. This property is utilized in various applications, from electric motors to data storage devices.

Paramagnetism

Paramagnetism is a type of magnetism that is weaker compared to ferromagnetism, and it is observed in materials with unpaired electrons. These unpaired electrons contribute to a magnetic moment that can align with an external magnetic field, but unlike ferromagnetic materials, paramagnetic ones do not have spontaneously aligned moments in the absence of a magnetic field. This is due to the fact that the exchange interactions in paramagnetic substances are not strong enough to cause intrinsic alignment of the magnetic moments.

When you apply an external magnetic field to a paramagnetic material, the magnetic moments attempt to align with the field, resulting in the material being weakly attracted to the magnet. Remove the magnetic field, and the alignment is lost, with the moments returning to their disordered state. This behavior is essential for certain medical applications, such as magnetic resonance imaging (MRI), where the paramagnetic properties of certain substances are used to enhance image contrast.

Kinetic-Molecular Theory of Solids

The kinetic-molecular theory of solids paints a picture of solid materials as composed of atoms, molecules, or ions locked in an organized structure—a lattice. However, these particles are not motionless; they vibrate about their fixed positions within the lattice structure. The theory provides valuable insights into how temperature affects the behavior of solid materials.

As temperature rises, the vibrational energy of the particles increases. This increase in thermal energy causes the particles to vibrate more vigorously, which can impact the material properties such as electrical conductivity, thermal expansion, and, as we've discussed, magnetism. Understanding the principles of this theory helps explain the effects of temperature changes on the magnetic behavior of ferromagnetic materials, shedding light on why they lose their strong magnetism and become merely paramagnetic when heated above their Curie temperature. This transition is a direct consequence of increased thermal vibrations disrupting the orderly alignment of magnetic moments, which is critical for ferromagnetism.