Magnetic Properties Of Solids

Introduction

The ability of solids to conduct electricity varies and is related to their internal structure. These properties can, broadly speaking, be classified according to the ability of the material to let the electric current pass through it. Generally, there are conductors, insulators, and semiconductors. Conductors are metals; they have free electrons that are rather mobile and thus permit the flow of electricity. Insulators, like rubber, have their electrons tightly bound to the atoms, which don't move freely around, hence their electrical flow is poor. Semiconductors are materials whose electric conducts lie in between that of conductors and insulators. Their conductivity can be changed by the introduction of impurities it or by external factors such as temperature conditions. All these properties are very vital in making electronic gadgets or devices and other associated technologies.

Magnetic Properties
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.
(ii) its spin around its own axis

An electron being a charged particle and undergoing these motions can be considered a small loop of current that possesses a magnetic moment. Thus, each electron has a permanent spin and an orbital magnetic moment associated with it. The magnitude of this magnetic moment is very small and is measured in the unit called Bohr magneton, μ_B. It is equal to 9.27 × 10^–24 A-m².

Classification of substances

• Paramagnetism:
  These substances are attracted by the magnetic field and have unpaired electrons. They lose magnetism in the absence of a magnetic field.
  
  For example, Transition metals like Cr, Mn, Ni, Co, Fe, etc. Metal oxides like CuO, VO_2, etc.
• Diamagnetism:
  These substances are weakly repelled by the magnetic field and do not have any unpaired electron. They act as Insulators.
  
  For example, NaCl, Zn, Cd, Cu⁺, TiO₂, etc.
• Ferromagnetism:
  • These substances are attracted by the magnetic field and show permanent magnetism even in the absence of a magnetic field.
    Examples, are Fe, Co, Ni, CrO₂ (used in audio and videotapes), etc.
  • This arises due to the spontaneous alignment of magnetic momenta in the same direction.
    
  • Above the curie temperature, there is no ferromagnetism
• Antiferromagnetism:
  • These are the substances that are expected to possess paramagnetism or ferromagnetism on the basis of unpaired electrons but actually have zero net magnetic moments.
    Examples, MnO, MnO₂, Mn₂O₃, FeO, Fe₂O_3, etc.
  • Anti-ferromagnetism is due to the equal number of parallel and antiparallel magnetic momenta which leads to zero resulting magnetic moment.
    
• Ferrimagnetism:
  In ferrimagnetic substances, there are unequal numbers of parallel and antiparallel magnetic momenta which leads to some resulting magnetic moment.
  
  For example, Fe₃O₄ , Ferrites
• Effect of Temperature:
  The ferromagnetic, anti-ferromagnetic, and ferrimagnetic solids change into paramagnetic at a particular temperature. For example, Ferrimagnetic Fe₃O₄ on heating to 850 K becomes paramagnetic this is due to the alignment of spins in one direction on heating.
• Curie Temperature:
  Each ferromagnetic substance has a characteristic temperature above which no ferromagnetism is observed this is called the curie temperature.
  
  

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Some Solved Examples

Example 1: Which of the following substances would make better permanent magnets?

1)Ferromagnetic materials

2)Ferrimagnetic materials

3)Diamagnetic materials

4)Paramagnetic materials

Solution

Ferromagnetic materials have higher curie temperatures than other types of magnetic materials. In ferromagnetic materials, metal ions are randomly oriented in small regions known as domains. When a magnetic field is applied to these domains, they get oriented in the direction of the magnetic field and thus these materials become permanent magnets.
Hence, the answer is the option (1).

Example 2: Domain structure similar to ferromagnetic materials but oppositely oriented thus overall magnetic moment is zero is observed in which of the following phenomenon?

1)Ferrimagnetism

2) Antiferromagnetism

3)Ferromagnetism

4)Diamagnetism

Solution

Antiferromagnetism are the substances that have a magnetic domain structure similar to the ferromagnetic materials but these domains are oppositely oriented to each other and thus cancel out each other’s magnetic moment thus, the overall magnetic moment of these materials is zero
Hence, the answer is the option (2).

Example 3: Which of the following substances show antiferromagnetism?

1) $\mathrm{MnO}_2$

2)CdO

3)$\mathrm{CrO}_2$

4)$\mathrm{ZnO}_2$

Solution

Antiferromagnetism substances are those whose domains are oppositely oriented and cancel out each other’s magnetic moment.

Hence, the answer is the option (1).

Example 3: Which of the following statements are correct?

1)Ferrimagnetic substances do not lose ferrimagnetism on heating and remain ferrimagnetic
2)In ferromagnetic substances, all the domains get oriented in the direction of the magnetic field and then again change in random directions after removing the magnetic field
3) Ferrimagnetic substances lose ferrimagnetism on heating and become paramagnetic
4)Antiferromagnetic substances have domain structures similar to ferromagnetic substances and their magnetic moments are not cancelled by each other

Solution

Ferrimagnetic substances have the property to lose ferrimagnetism on heating and become paramagnetic.
Hence, the answer is the option (3).

Example 4: Ferrimagnetic is converted into paramagnetic at (in K) :

1)500

2)400

3)700

4) 850

Solution

Ferrimagnetism:

In ferrimagnetic substances, there is an unequal number of parallel and antiparallel magnetic momenta which leads to some resulting magnetic moment.

For example, Fe₃O₄, Ferrites

Effect of Temperature:
The ferromagnetic, anti-ferromagnetic, and ferrimagnetic solids change into paramagnetic at a particular temperature. For example, Ferrimagnetic Fe₃O₄ on heating to 850 K becomes paramagnetic this is due to the alignment of spins in one direction on heating.Ferrimagnetic is converted into paramagnetic at 850 K because at high-temperature randomisation of spin changes.

Summary

The electrical properties of solids differ due to their various internal structures and composition. Conductors have free electrons that conduct electricity with ease. In insulators, the electrons are tightly bound to the nucleus, thereby restricting electrical flow in them. Semiconductors possess intermediate properties, and their conductivity may be altered by adding impurities or changing conditions. These characteristics form the backbone for the working of all electronic devices. Through manipulation of electrical properties in materials, engineers can realize very different sorts of electronic applications, from simple circuits to sophisticated computing systems.