PHD → Inorganic chemistry → Solid state chemistry ↓
Magnetic and optical properties of solids
Introduction
The study of magnetic and optical properties in solids is a fascinating aspect of solid state chemistry. These properties determine how materials interact with magnetic fields and light, which affects their practical applications in a variety of technologies.
Magnetic properties of solids
Magnetic properties in materials arise from the motion of electrons. This is mainly due to two forms of electron motion: the orbital motion of electrons around the nucleus, and the rotation of electrons on their own axis. When these motions interact with external magnetic fields, magnetic properties emerge.
Paramagnetism and diamagnetism
Paramagnetism and diamagnetism are fundamental magnetic properties found in solids.
Paramagnetism occurs in materials with unpaired electrons. These unpaired electrons have a net magnetic moment, which causes the material to be attracted by external magnetic fields. In a paramagnetic material, the magnetization increases linearly with the intensity of the applied magnetic field but disappears when the field is removed. Examples of paramagnetic materials include aluminum, platinum, and some transition metal complexes.
Diamagnetism arises from changes in the orbital motion induced in atoms when exposed to an external magnetic field. All electrons in diamagnetic materials are paired, so they have no net magnetic moment in the absence of an external field. When a magnetic field is applied, these materials generate a small, opposing magnetic field, causing slight repulsion. Examples of diamagnetic materials include bismuth, copper, and lead.
Ferromagnetism, antiferromagnetism and ferrimagnetism
Some materials exhibit more complex forms of magnetism:
Ferromagnetism is observed in materials such as iron, cobalt, and nickel. In these materials, the magnetic moments of the atoms are aligned in the same direction even in the absence of an external magnetic field. This alignment arises from an exchange interaction mechanism. The material exhibits a permanent magnetic moment, which can be very strong.
Antiferromagnetism involves materials in which adjacent magnetic moments point in opposite directions. This results in a net magnetic moment of zero. These materials, such as manganese oxide (MnO), exhibit this behavior due to specific electron exchange interactions that prefer opposite alignments.
Ferrimagnetism occurs in materials where the magnetic moments are aligned in opposite directions, which is similar to antiferromagnetism, but the magnitudes of the opposing magnetic moments are unequal. This results in a net magnetization. An example of a ferrimagnetic material is magnetite (Fe 3 O 4
).
Visual example: alignment of magnetic moments
+++++ ----- +++--- North South Mixed Ferromagnetic Antiferrow Ferrimagnetism
Optical properties of solids
Optical properties deal with how solid materials interact with electromagnetic radiation, particularly visible light. When light interacts with a solid material, it can be absorbed, reflected, or transmitted, leading to various optical phenomena.
Absorption, reflection, and transmission
Absorption of light occurs when photons are absorbed by electrons in a solid material, causing the electrons to move to higher energy levels. The specific wavelengths that are absorbed depend on the energy levels available in the material. Strong absorption in the visible spectrum gives materials their color.
Reflection occurs when light waves bounce off the surface of a material, without penetrating the material. Metals usually reflect a large amount of light, which is why they appear shiny.
Transmission is the passage of light through a substance. Transparent substances, such as glass, transmit most of the light, allowing objects to be seen through them.
Refraction and refractive index
Refraction is the bending of light when it passes from one medium to another with a different refractive index. The refractive index is a measure of how much the speed of light is reduced inside a medium.
n = c / v
Where n
is the refractive index, c
is the speed of light in vacuum, and v
is the speed of light in the material.
Examples of optical phenomena
A classic optical phenomenon is the spectrum of colours that appears when light passes through a prism. This is due to the different refractive indices for different wavelengths, causing the light to spread out into its component colours.
White Light -------> Prism -------> Spectrum of Colors
Band theory and electronic transitions
To understand magnetic and optical properties it is necessary to have a good understanding of band theory, which describes the energy levels of electrons in solids.
Energy bands: valence and conduction
In solids, closely packed atoms form bands of energy rather than distinct levels. The two most important bands are the valence band, which is filled with valence electrons, and the conduction band, which is higher in energy and normally empty. The difference between these two bands is known as the band gap.
Electronic transitions and band gap
When an electron absorbs energy, which can be in the form of a photon, it can move from the valence band to the conduction band. This transition depends on the band gap energy. Materials with a small band gap absorb in the infrared region, while large band gaps require UV radiation for electronic excitation.
Optical materials: applications
Understanding optical properties is important in designing materials for lenses, lasers, and fiber optics.
Glasses and crystals: Used in lenses, prisms, and other optical components due to their transparency and ability to bend light at desired angles.
Photonic crystals: Structurally alter the light path, important in the development of modern optical devices.
Magnetic materials: applications
Magnetic properties are integral to the development of data storage and electronic devices.
Magnetic storage: Uses ferromagnetic materials in hard drives and magnetic tape.
Magnetic sensors and detectors: Used in a variety of applications, including navigation systems and medical devices such as MRI scanners.
Conclusion
Magnetic and optical properties play a critical role in the functionality of many materials and devices. From the fundamentals of electron interactions with magnetic fields and light to complex applications in modern technology, understanding these properties is critical for advancing materials science and engineering new devices to meet emerging needs.