Magnetic Properties of Materials 🧲

Every solid can be grouped as diamagnetic, paramagnetic, or ferromagnetic. The quick way to spot the category is to check the magnetic susceptibility \( \chi \) and the relative permeability \( \mu_r \) :contentReference[oaicite:0]{index=0}.

1  Diamagnetism 🙅‍♂️

• Diamagnetic bars drift away from strong fields toward weaker zones — they’re gently repelled by magnets :contentReference[oaicite:1]{index=1}.
• The external field nudges atomic electrons: orbits aligned with the field slow down, those opposite speed up. The induced current (Lenz’s law!) sets up a moment opposite to the applied field, so \( \chi \) is negative :contentReference[oaicite:2]{index=2}.
• Every substance shows diamagnetism, but it usually hides behind stronger effects.
• All-stars: bismuth, copper, lead, water, nitrogen. Superconductors are the champs with perfect diamagnetism (\( \chi=-1,\; \mu_r=0 \)). The total expulsion of field lines is called the Meissner effect 🚄 :contentReference[oaicite:3]{index=3}.

2 Paramagnetism 🧲➕

• Paramagnetic bars creep toward stronger field regions; the attraction is mild :contentReference[oaicite:4]{index=4}.
• Each atom already owns a tiny magnetic dipole. At room temperature those dipoles point everywhere, so there’s zero net magnetisation.
• An external field \( B_0 \) plus cooler temperatures coax the dipoles to line up, boosting the internal field a little (\(~1\) part in \(10^5\)) 😀 :contentReference[oaicite:5]{index=5}.
• \( \chi \) and \( \mu_r \) both rise as the field grows or the temperature falls, until the dipoles saturate.
• Examples: aluminium, sodium, calcium, oxygen, copper chloride.

3 Ferromagnetism 💪

• Ferromagnets race toward strong fields; their magnetisation is huge (\( \mu_r > 1000 \)) :contentReference[oaicite:6]{index=6}.
• Inside, billions of atoms form cooperative “domains” that are already magnetised. Without a field, neighboring domains point randomly so the lump looks neutral :contentReference[oaicite:7]{index=7}.
• Apply \( B_0 \) and two things happen together: domains pivot to face the field, and the ones already aligned swell. Soon nearly the whole sample acts like one giant domain 💥 :contentReference[oaicite:8]{index=8}.
• Remove the field:
  • Soft ferromagnets (e.g., soft iron) relax; magnetisation vanishes.
  • Hard ferromagnets (e.g., lodestone, Alnico) lock in the magnetisation and become permanent magnets.
• Heat shakes the domain order. Above a certain temperature a ferromagnet turns paramagnetic — the famous Curie point.

4 Worked Example 📐

(From a 1000-turn solenoid with an iron core where \( \mu_r = 400 \)) :contentReference[oaicite:9]{index=9}

• Applied current: \( I = 2.0\;\text{A} \)
• Magnetic field inside: \( H = n I = 1000 \times 2.0 = 2\times10^{3}\,\text{A m}^{-1} \)
• Flux density: \( B = \mu_r \mu_0 H = 400 \times 4\pi\times10^{-7}\,\text{N A}^{-2}\; \times 2\times10^{3}\,\text{A m}^{-1} = 1.0\;\text{T} \)
• Magnetisation: \( M = \dfrac{B-\mu_0 H}{\mu_0} = (\mu_r -1)H = 399 H \approx 8\times10^{5}\,\text{A m}^{-1} \)
• Extra current needed to create the same \( B \) without the core: \( I_M = 794\;\text{A} \)

High-Yield NEET Pointers 🎯

1. The sign and size of \( \chi \) (negative, small positive, large positive) instantly classify a material as dia-, para-, or ferro- :contentReference[oaicite:10]{index=10}.
2. The Meissner effect — perfect diamagnetism of superconductors — is a unique concept worth remembering 🚄 :contentReference[oaicite:11]{index=11}.
3. Domain theory explains why ferromagnets can become permanent magnets and introduces the idea of “soft” vs “hard” materials 📎 :contentReference[oaicite:12]{index=12}.
4. Temperature can flip a ferromagnet to a paramagnet (Curie point) — always a favorite conceptual question 🌡️ :contentReference[oaicite:13]{index=13}.
5. Diamagnets drift from strong to weak field while paramagnets drift the opposite way — easy to test and easy to trick students 😉 :contentReference[oaicite:14]{index=14}.