Problem 48 - A Complete Resource Book in Chemistry for JEE Main

Question

If the alignment of magnetic moments in a substance is in a compensatory way so as to give zero net magnetic moment, then the substance is said to be (a) Anti-ferromagntic (b) Diamagnetic (c) Ferrimagnetic (d) Ferromagnetic

Step-by-Step Solution

Answer

The substance is antiferromagnetic.

Step 1: Understanding the Question

The problem asks to identify a type of magnetic ordering in a substance where magnetic moments are aligned in such a way that they cancel each other out, resulting in zero net magnetic moment.

Step 2: Analyzing the Options

We have four options: (a) Anti-ferromagnetic, (b) Diamagnetic, (c) Ferrimagnetic, (d) Ferromagnetic. We need to determine which of these describes a situation where magnetic moments cancel each other out.

Step 3: Exploring Antiferromagnetism

In an antiferromagnetic material, magnetic moments of atoms or ions align in opposite directions in a regular pattern, cancelling each other out, which leads to a zero net magnetic moment.

Step 4: Comparing Other Options

Diamagnetic materials generate a very weak magnetic field in opposition to an external magnetic field and do not have intrinsic magnetic moment cancellation. Ferrimagnetic materials have moments that cancel partially but result in a net magnetic moment. Ferromagnetic materials have aligned moments leading to a strong net magnetic moment.

Step 5: Conclusion

Based on the descriptions, only anti-ferromagnetic materials exhibit alignment of magnetic moments in a manner that results in zero net magnetic moment due to their compensatory alignment.

Key Concepts

Magnetic Moments

Magnetic moments are a fundamental property of particles and materials that give rise to magnetism. Essentially, they are vectors that represent the magnetic strength and direction of a magnetic source. These moments originate from the spin and orbital angular momentum of electrons.

• Electron Spin: Electrons have an intrinsic property called spin, which produces a magnetic moment similar to that of a tiny bar magnet.
• Orbital Motion: As electrons move around the nucleus in their orbitals, they create another form of magnetic moment.

The net magnetic moment of a system is the vector sum of the individual moments. In different materials, these moments can be aligned in various patterns relative to each other, leading to different types of magnetism.

Ferromagnetism

Ferromagnetism is a type of magnetism where the magnetic moments of atoms align parallel to each other, resulting in a strong net magnetic moment. This alignment occurs spontaneously below a certain temperature, known as the Curie temperature. Key characteristics of ferromagnetism:

• Permanent Magnetism: Ferromagnetic materials can become permanent magnets even after the external magnetic field is removed.
• Curie Temperature: Above this temperature, the thermal energy overcomes the alignment of moments, and the material becomes paramagnetic.
• Strong Magnetism: The parallel alignment increases the magnetic field strength significantly, as seen in materials like iron, cobalt, and nickel.

Ferromagnetism is fundamental to many technologies, such as data storage, electric motors, and transformers, due to its strong and persistent magnetic properties.

Ferrimagnetism

Ferrimagnetism is similar to ferromagnetism, but with a key difference in the alignment of magnetic moments. In ferrimagnetic materials, moments are aligned in opposite directions within their crystal lattice. However, these opposing moments do not completely cancel out, resulting in a net magnetic moment. Ferrimagnetism can be understood as:

• Partial Cancellation: Due to unequal magnitudes of opposing moments, a net moment persists.
• Common in Oxides: Many ferrimagnetic materials are oxides such as magnetite (Fe₃O₄).
• Useful in Technology: Ferrites, a type of ferrimagnetic material, are widely used in electronics and magnetic core memory due to their magnetic properties and electrical insulation.

Ferrimagnetism is crucial in various applications, particularly when a moderate magnetic field is needed.

Diamagnetism

Diamagnetism is a form of magnetism that is characterized by an object's ability to create a weak magnetic field in opposition to an external magnetic field. Interestingly, all materials exhibit some degree of diamagnetism, but it is often very weak and easily overshadowed by other types of magnetism in materials like ferromagnetic or ferrimagnetic substances. Key aspects include:

• Weak and Temporary: The induced magnetic field is very weak and only persists while the external magnetic field is applied.
• Universal Phenomenon: Present in all materials; however, significant only in materials with no other magnetic orders.
• Non-Dependence on Temperature: Unlike ferromagnetism or ferrimagnetism, diamagnetism is not affected by temperature changes.

Diamagnetic materials are often used in magnetic levitation experiments, where their ability to repel magnetic fields can be visually demonstrated by floating objects above a magnet.