Dia / Para / Ferromagnetic
Dipole alignment under H — dia opposite, para weak parallel, ferro strong.
Key Notes
Three classes of magnetic materials, distinguished by χ (susceptibility):
DIAMAGNETIC (χ < 0, small): Bi, Cu, Ag, Au, water, most non-metals. Repelled weakly by magnetic fields. Property of ALL matter; usually masked by para- or ferro-magnetism.
PARAMAGNETIC (χ > 0, small ~10⁻³): Al, Pt, Mn, Cr, O₂. Attracted weakly. Follows Curie's law: χ = C/T.
FERROMAGNETIC (χ huge, ~10³-10⁵): Fe, Co, Ni, Gd, and many alloys. Spontaneous magnetization persists without external field. Exists below Curie temperature T_C.
ANTIFERROMAGNETIC: adjacent spins anti-align (Cr, MnO). Net M ≈ 0 below Néel temperature. Special class.
FERRIMAGNETIC (ferrites): Fe₃O₄, MnFe₂O₄. Sublattice spins anti-align but unequal ⇒ net M ≠ 0. Used in transformers due to high resistivity (low eddy currents).
Soft magnets: easy to magnetize/demagnetize (electromagnets, transformer cores).
Hard magnets: retain magnetization (permanent magnets: alnico, neodymium NdFeB).
Formulas
Curie's law (paramagnets)
C = Curie constant; valid above Néel temperature.
Curie-Weiss (ferromagnets above T_C)
Diverges as T → T_C from above.
Susceptibility magnitudes
Different magnitudes by ~8 orders.
Curie temperatures
Above T_C, ferromagnet → paramagnet.
Important Points
Only Fe, Co, Ni (+ Gd at low T) and their alloys are common ferromagnets at room temperature.
Diamagnetism exists in ALL atoms but is usually overwhelmed by para or ferro contributions.
Para: spins exist but disordered; B aligns them. Ferro: spins spontaneously align in domains.
Domains in ferromagnets: regions where all spins are aligned. External B causes domain walls to move ⇒ macroscopic M.
Heating ferromagnets above T_C makes them paramagnetic — used in temperature sensors and demagnetization.
Ferrites (insulating ferrimagnets) are crucial in high-frequency electronics (transformer cores, magnetic memory).
Dia / Para / Ferromagnetic notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.