Magnetic Properties of Solids
Understand Domains, Ferromagnetism, and the origin of magnetic moments.
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 due to two types of motion: its orbital motion around the nucleus, and its spin around its own axis.
Based on their response to an external magnetic field, solids are classified into five major categories.
1. Basic Magnetism
A. Paramagnetism
Paramagnetic substances are weakly attracted by a magnetic field. They are magnetized in the same direction as the magnetic field. They lose their magnetism in the absence of a magnetic field.
- Cause: The presence of one or more unpaired electrons which are attracted by the magnetic field.
- Examples: $O_2$, $Cu^{2+}$, $Fe^{3+}$, $Cr^{3+}$.
B. Diamagnetism
Diamagnetic substances are weakly repelled by a magnetic field. They are weakly magnetized in the opposite direction to the magnetic field.
- Cause: Shown by those substances in which all the electrons are paired. The pairing of electrons cancels their magnetic moments.
- Examples: $H_2O$, $NaCl$, Benzene ($C_6H_6$).
2. The Domain Theory (Advanced Magnetism)
In solid states, the metal ions of certain substances are grouped together into small regions called domains. Each domain acts as a tiny magnet. In an unmagnetized piece of metal, these domains are randomly oriented, and their magnetic moments cancel out.
How these domains align when placed in an external magnetic field defines the next three types of magnetism.
C. Ferromagnetism
These substances are strongly attracted by a magnetic field. More importantly, they can be permanently magnetized.
- Domain Alignment: When placed in a magnetic field, ALL domains get oriented in the same direction, producing a strong magnetic effect. This ordering persists even when the magnetic field is removed.
- Examples: Iron ($Fe$), Cobalt ($Co$), Nickel ($Ni$), Gadolinium ($Gd$), $CrO_2$ (used in magnetic tapes).
D. Antiferromagnetism
Substances like $MnO$ exhibit antiferromagnetism. Despite having unpaired electrons, their net magnetic moment is zero.
- Domain Alignment: Their domain structure is similar to ferromagnetic substances, but their domains are oppositely oriented in equal numbers. They exactly cancel out each other's magnetic moment.
- Examples: $MnO$.
E. Ferrimagnetism
These substances are weakly attracted by a magnetic field as compared to ferromagnetic substances. They lose their ferrimagnetism on heating and become paramagnetic.
- Domain Alignment: The magnetic moments of the domains are aligned in parallel and anti-parallel directions in unequal numbers. Thus, there is a small net magnetic moment.
- Examples: Magnetite ($Fe_3O_4$), and ferrites like $MgFe_2O_4$ and $ZnFe_2O_4$.
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