12th chemistry solid state-Magnetic Properties

Magnetic Properties

• Each electron in an atom behaves like a tiny magnet.
• The magnetic moment of an electron originates from its two types of motion.

• Orbital motion around the nucleus
• Spin around its own axis

• Thus, an electron has a permanent spin and an orbital magnetic moment associated with it.

• An orbiting electron
• A spinning electron

• Based on magnetic properties, substances are classified into five categories −

• Paramagnetic
• Diamagnetic
• Ferromagnetic
• Ferrimagnetic
• Anti-ferromagnetic

Paramagnetism

• The substances that are attracted by a magnetic field are called paramagnetic substances.

• Some examples of paramagnetic substances are O₂, Cu²⁺, Fe³⁺ and Cr³⁺.

• Paramagnetic substances get magnetised in a magnetic field in the same direction, but lose magnetism when the magnetic field is removed.
• To undergo paramagnetism, a substance must have one or more unpaired electrons. This is because the unpaired electrons are attracted by a magnetic field, thereby causing paramagnetism.

Diamagnetism

• The substances which are weakly repelled by magnetic field are said to have diamagnetism.

• Example − H₂O, NaCl, C₆H₆

• Diamagnetic substances are weakly magnetised in a magnetic field in opposite direction.
• In diamagnetic substances, all the electrons are paired.
• Magnetic characters of these substances are lost due to the cancellation of moments by the pairing of electrons.

Ferromagnetism

• The substances that are strongly attracted by a magnetic field are called ferromagnetic substances.
• Ferromagnetic substances can be permanently magnetised even in the absence of a magnetic field.
• Some examples of ferromagnetic substances are iron, cobalt, nickel, gadolinium and
  CrO^­₂.
• In solid state, the metal ions of ferromagnetic substances are grouped together into small regions called domains, and each domain acts as a tiny magnet. In an un-magnetised piece of a ferromagnetic substance, the domains are randomly oriented, so their magnetic moments get cancelled. However, when the substance is placed in a magnetic field, all the domains get oriented in the direction of the magnetic field. As a result, a strong magnetic effect is produced. This ordering of domains persists even after the removal of the magnetic field. Thus, the ferromagnetic substance becomes a permanent magnet.
• Schematic alignment of magnetic moments in ferromagnetic substances is as follows:

Ferrimagnetism

• The substances in which the magnetic moments of the domains are aligned in parallel and anti-parallel directions, in unequal numbers, are said to have ferrimagnetism.

• Examples include Fe₃O₄ (magnetite), ferrites such as MgFe₂O₄ and ZnFe₂O₄.

• Ferrimagnetic substances are weakly attracted by a magnetic field as compared to ferromagnetic substances.
• On heating, these substances become paramagnetic.
• Schematic alignment of magnetic moments in ferrimagnetic substances is as follows:

Anti-ferromagnetism

• Antiferromagnetic substanceshave domain structures similar to ferromagnetic substances, but are oppositely oriented.
• The oppositely oriented domains cancel out each other’s magnetic moments.
• Schematic alignment of magnetic moments in anti-ferromagnetic substances is as follows: