Magnetic Properties of Solids: Classification & Domain Theory
Every substance has some magnetic properties associated with it. The origin of these properties lies in the electrons, which behave like tiny magnets due to their orbital motion and spin angular momentum.
1. Diamagnetism & Paramagnetism
These properties depend on the presence or absence of unpaired electrons in individual atoms/ions.
Diamagnetism
- Behavior: Weakly repelled by magnetic fields.
- Electronic Config: All electrons are paired ($\uparrow\downarrow$). Magnetic moments cancel out.
- Examples: $\text{H}_2\text{O}, \text{NaCl}, \text{C}_6\text{H}_6$.
Paramagnetism
- Behavior: Weakly attracted by magnetic fields. Lost on heating.
- Electronic Config: Contain one or more unpaired electrons ($\uparrow$).
- Examples: $\text{O}_2, \text{Cu}^{2+}, \text{Fe}^{3+}, \text{Cr}^{3+}$.
Figure: Alignment of Magnetic Domains in Different Substances
2. Collective Magnetic Effects (Domain Theory)
In certain substances, metal ions group together into small regions called "domains". The alignment of these domains determines the macroscopic magnetic property.
Ferromagnetism (Strong)
- Mechanism: Domains align in the same direction permanently, even after the field is removed ($\uparrow \uparrow \uparrow \uparrow$).
- Examples: Iron ($\text{Fe}$), Cobalt ($\text{Co}$), Nickel ($\text{Ni}$), Gadolinium ($\text{Gd}$), $\text{CrO}_2$.
- Use: $\text{CrO}_2$ is used in magnetic recording tapes.
Antiferromagnetism
- Mechanism: Domain structure is similar to ferromagnetic but domains are oppositely oriented and cancel each other out ($\uparrow \downarrow \uparrow \downarrow$). Net moment = 0.
- Example: $\text{MnO}$.
Ferrimagnetism
- Mechanism: Magnetic moments of domains are aligned in parallel and anti-parallel directions in unequal numbers ($\uparrow \uparrow \downarrow \uparrow \uparrow$). Weakly attracted.
- Examples: Magnetite ($\text{Fe}_3\text{O}_4$), Ferrites ($\text{MgFe}_2\text{O}_4, \text{ZnFe}_2\text{O}_4$).
Note: Ferrimagnetic substances lose ferrimagnetism on heating and become paramagnetic (e.g., $\text{Fe}_3\text{O}_4$ at $850\text{ K}$).
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