Main Difference – Diamagnetism, Paramagnetism and Ferromagnetism
Diamagnetism, paramagnetism, and ferromagnetism refer to how different materials respond to magnetic fields. The main difference between diamagnetism, paramagnetism, and ferromagnetism is that diamagnetism refers to a type of magnetism which forms in opposition to an external magnetic field and disappears when the external field is removed; paramagnetism refers to a type of magnetism that forms along the direction of an external magnetic field and disappears when the external magnetic field is removed; ferromagnetism refers to a type of magnetism in materials which forms along the direction of the external magnetic field and can remain when the external magnetic field is removed.
Origin of Magnetism
In quantum mechanics, electrons have angular momenta. The “angular momentum” referred to here is a quantum mechanical property, but it can be considered to be analogous to the angular momentum in classical physics, where objects have angular momentum if they are in rotational motion.
Electrons exhibit two types of angular momenta: spin angular momenta and orbital angular momenta. Spin angular momentum is an intrinsic property of electrons, like their charge or mass. Orbital angular momentum is a property that electrons have when they are in atoms. There is a magnetic moment associated with each of these angular momenta. The magnetic moment is a property that causes electrons to experience a force when they are placed in a magnetic field.
The magnetic moment () due to spin angular momentum () is given by:
where and are the charge and mass of an electron respectively.
Similarly, the magnetic moment () due to orbital angular momentum () is given by:
What is Diamagnetism
All materials are diamagnetic. Diamagnetism is the weakest of the three different types of magnetism. Therefore, if a material is paramagnetic or ferromagnetic, its diamagnetic effects are masked by these other two types of magnetism. In diamagnetic materials, magnetic moments of each of the individual electrons in the material gets cancelled out. When a diamagnetic material is placed under a magnetic field, the material produces a magnetic field that opposes the external magnetic field. As a result, the material gets repelled by the external field. For instance, the figure below shows a living frog that has been made to levitate using a strong magnetic field. Here, the frog’s body exhibits diamagnetism:
What is Paramagnetism
In materials whose atoms have unpaired electrons, the magnetic moments of individual electrons cannot fully cancel out, and so the atoms are left with a resultant magnetic moment. However, the magnetic moments of atoms are aligned in random directions, so the material as a whole does not exhibit magnetism. However, if such a material is placed in an external magnetic field, then the magnetic moments of individual atoms can then align with the external magnetic field, causing the material to become magnetised. The magnetic field produced by paramagnetic materials points along the same direction as the external magnetic field. The material exhibits magnetism only as long as it is inside an external magnetic field. If the external magnetic field is switched off, then the material loses its magnetisation. Paramagnetic materials include liquid oxygen and certain metals. The video below demonstrates the paramagnetic property of liquid oxygen:
What is Ferromagnetism
Atoms that make up ferromagnetic materials have unpaired electrons in their atoms so each atom has a net magnetic moment. The magnetic moments of nearby atoms tend to become aligned, creating different regions (called domains) in the material, where magnetic moments due to individual atoms are aligned. However, different domains may still have their magnetic moments pointing in different directions. When a ferromagnetic material is placed inside an external magnetic field, the different domains inside the magnetic fields all align with the external magnetic field.
Even if the external magnetic field is removed, the material can retain its magnetization. Ferromagnetic materials include iron, cobalt, nickel and their alloys.
Difference Between Diamagnetism, Paramagnetism, and Ferromagnetism
Magnetic Moments of Individual Atoms
In diamagnetic materials, the individual atoms do not have a net magnetic moment.
In paramagnetic and ferromagnetic materials, each atom has its own magnetic moment.
Behaviour in External Magnetic Fields
Diamagnetic materials align their magnetic fields in the opposite direction to the external magnetic fields.
Paramagnetic and ferromagnetic materials align their magnetic fields in the same direction as the external magnetic fields.
Retention of Magnetism
Diamagnetic and paramagnetic materials lose their magnetization when the external magneic field is removed.
Ferromagnetic materials can retain their magnetization even when the external magnetic field is removed.
“A live frog levitates inside the Ø32mm vertical bore of a Bitter solenoid in a magnetic field of about 16 tesla at the Nijmegen High Field Magnet Laboratory” by Lijnis Nelemans (English Wikipedia) [CC BY-SA 3.0], via Wikimedia Commons