Magnetic Properties of Matter — Core Principles

Core Principles

The magnetic properties of matter stem from the orbital and spin motions of electrons within atoms, creating tiny magnetic dipole moments. Materials are categorized based on their response to an external magnetic field.

Diamagnetic materials, with all paired electrons, are weakly repelled as the field induces an opposing moment; their susceptibility ($chi_m$) is small and negative, and relative permeability ($mu_r$) is slightly less than 1.

Paramagnetic materials, possessing unpaired electrons and thus permanent atomic moments, are weakly attracted as these moments partially align with the field; their $chi_m$ is small and positive, $mu_r$ is slightly greater than 1, and $chi_m$ follows Curie's Law ($chi_m propto 1/T$).

Ferromagnetic materials exhibit strong attraction due to spontaneous alignment of atomic moments within 'magnetic domains' via exchange coupling; they have very large positive $chi_m$ and $mu_r$, show hysteresis, and lose ferromagnetism above a critical Curie temperature ($T_C$), becoming paramagnetic.

Key parameters include magnetic intensity (H), magnetization (M), magnetic induction (B), susceptibility ($chi_m$), and permeability ($mu$).

Important Differences

vs Paramagnetic, Ferromagnetic

Aspect	This Topic	Paramagnetic, Ferromagnetic
Origin of Magnetism	Diamagnetic: Induced opposing moments due to paired electrons (Lenz's Law). No permanent atomic moments.	Paramagnetic: Permanent atomic moments due to unpaired electrons. Randomly oriented in absence of field.
Behavior in External Field	Weakly repelled; moves from stronger to weaker field regions.	Weakly attracted; moves from weaker to stronger field regions.
Magnetic Susceptibility ($chi_m$)	Small and negative (e.g., $-10^{-5}$ to $-10^{-6}$).	Small and positive (e.g., $10^{-3}$ to $10^{-5}$).
Relative Permeability ($mu_r$)	Slightly less than 1 (e.g., $0.9999$).	Slightly greater than 1 (e.g., $1.0001$).
Effect of Temperature	Largely independent of temperature.	Decreases with increasing temperature (Curie's Law: $chi_m propto 1/T$).
Hysteresis	Does not exhibit hysteresis.	Does not exhibit hysteresis.
Examples	Copper, Bismuth, Water, Gold, Nitrogen, Diamond.	Aluminum, Sodium, Platinum, Oxygen, Copper Chloride.

The three main types of magnetic materials—diamagnetic, paramagnetic, and ferromagnetic—are distinguished by their fundamental atomic structure and their macroscopic response to an external magnetic field. Diamagnets are weakly repelled due to induced opposing moments from paired electrons, showing negative susceptibility and $mu_r < 1$. Paramagnets are weakly attracted due to partial alignment of permanent atomic moments from unpaired electrons, exhibiting positive but small susceptibility and $mu_r > 1$, with susceptibility decreasing with temperature. Ferromagnets are strongly attracted due to cooperative alignment within magnetic domains, possessing very large positive susceptibility and $mu_r gg 1$, exhibiting hysteresis, and losing their strong magnetism above a characteristic Curie temperature.