Magnetic Properties of Materials: Diamagnetism and Paramagnetism
Initial Observations & Core Question:
An experiment by Irvin the Gupta shows water in a beaker, floating on a tub, repelling a strong magnet brought near it. This seems counterintuitive.
A demonstration by Professor Walter Lewin at MIT shows liquid oxygen poured into an electromagnet, where it sticks to the magnet (attraction).
The central question is: Why do most materials, which are not typically thought of as magnetic, either attract or repel magnets?
It turns out most materials are slightly attracted or repelled by magnets, detectable with careful experiments.
Atomic Structure and Magnetic Moments:
Atoms possess magnetic moments due to electrons orbiting the nucleus, forming current loops.
These current loops act as tiny magnets.
Paired Electrons: When electrons pair up, they often form current loops in opposite directions, causing their magnetic moments to cancel out. Atoms with all electrons paired do not behave like tiny magnets.
Unpaired Electrons: If an atom has at least one unpaired electron, it will generate a net magnetic moment and behave like a tiny magnet.
Diamagnetism: Repulsion due to Induction (Paired Electrons):
The Mechanism:
Atoms with all paired electrons initially have no net magnetic moment and should seemingly be unaffected by magnetic fields.
However, electromagnetic induction occurs when an external magnet is brought close.
According to Faraday's Law, a changing magnetic flux through the electron current loops induces an electromotive force (EMF).
By Lenz's Law, this induced EMF creates an induced current that opposes the change in magnetic flux.
For electrons forming current loops, this means the induced EMF strengthens one loop's current and weakens the opposing loop's current.
The balance is disrupted, creating a small, temporary, induced magnetic moment in the atom.
This induced magnetic moment is oriented in the opposite direction to the external magnetic field.
Result: The atom, and thus the material, starts behaving like a tiny magnet that repels the external magnet.
Examples: Water, organic substances (humans, wood – all living beings), and some metals.
Properties of Diamagnetism:
Very weak property, requiring strong external fields or careful experiments to observe.
Levitation: Strong enough diamagnetic repulsion can be used to levitate objects (e.g., a frog levitating over a powerful magnet).
Temporary Nature: If the external magnet is removed, the magnetic flux returns to normal, the induced currents cease, and the material loses its induced magnetism. Diamagnetism is not permanent.
Temperature Independence: Diamagnetism depends on induced currents, which are not affected by atomic vibrations, making it largely independent of temperature.
Paramagnetism: Attraction due to Alignment (Unpaired Electrons):
The Mechanism:
Materials with unpaired electrons have atoms that possess permanent magnetic dipoles, acting like tiny magnets.
In the absence of an external magnetic field, these atomic magnets are typically randomly oriented, so the material as a whole does not exhibit magnetism.
When an external magnetic field is applied, the field exerts a torque on these tiny magnetic dipoles, causing them to tend to align with the external field.
This alignment results in the material becoming slightly magnetized in the same direction as the external field.
Result: The material is attracted to the external magnet.
Examples: Liquid oxygen (explaining why it stuck to the electromagnet), aluminum, calcium.
Properties of Paramagnetism:
Also a very weak property, requiring careful experiments.
Temporary Nature: When the external magnet is removed, thermal agitation (random jiggling and vibrations of atoms) causes the aligned dipoles to return to their random orientations, and the material loses its magnetism. Paramagnetism is not permanent.
Temperature Dependence: Thermal vibrations oppose the alignment of dipoles.
Higher temperatures (T) lead to more thermal agitation, making alignment harder and resulting in less paramagnetism.
Lower temperatures (T) lead to less thermal agitation, allowing for more alignment and stronger paramagnetism (e.g., liquid oxygen is more paramagnetic than gaseous oxygen due to lower temperature and higher density).
Comparison of Diamagnetism and Paramagnetism:
Differences:
Effect on Magnet: Diamagnets repel; Paramagnets attract.
Electron Configuration: Diamagnetism occurs in materials with paired electrons; Paramagnetism occurs in materials with unpaired electrons (permanent dipoles).
Origin of Magnetism: Diamagnetism involves temporary, induced dipoles in the opposite direction; Paramagnetism involves permanent dipoles turning (aligning) with the field.
Temperature Dependence: Diamagnetism is largely temperature-independent; Paramagnetism is temperature-dependent (stronger at lower temperatures).
Similarities: Both are generally weak and temporary phenomena.
Ferromagnetism:
Brief mention that ferromagnetism allows materials to retain magnetism even after the external field is removed.
It is distinct from diamagnetism and paramagnetism due to its permanence and strength, deserving a separate discussion.
Universality of Diamagnetism and Material Classification:
All materials exhibit diamagnetism: Diamagnetism is a universal phenomenon because all atoms contain paired inner electrons, which undergo induction when exposed to a magnetic field.
Dominant Effect Determines Classification:
In materials with unpaired electrons, both diamagnetism (repulsion) and paramagnetism (attraction) occur simultaneously.
The overall magnetic behavior of a material is determined by whichever phenomenon is stronger.
If paramagnetism is stronger than diamagnetism, the material is classified as a paramagnet (e.g., oxygen, aluminum, calcium).
If diamagnetism is stronger than paramagnetism (or if there are no unpaired electrons), the material is classified as a diamagnet (e.g., water, wood, and surprisingly, metals like copper, gold, and silver, which do have unpaired electrons but where their diamagnetic repulsion dominates over their paramagnetic attraction).