Unit-IV: Magnetic & Electrical Properties of Materials
Unit-IV: Magnetic & Electrical Properties of Materials
+ Superconductivity
1. Magnetic Properties of Materials
Types of Magnetism
| Type | Behavior in External Field | Magnetic Susceptibility (χ) | Examples |
|---|---|---|---|
| Diamagnetic | Weakly repelled | Small negative (–10⁻⁵) | Cu, Au, Ag, Water, NaCl, Superconductors |
| Paramagnetic | Weakly attracted | Small positive (+10⁻³ to 10⁻⁵) | Al, Mg, O₂, Pt |
| Ferromagnetic | Strongly attracted | Very large positive (>1000) | Fe, Co, Ni, Gd |
| Antiferromagnetic | No net magnetism | Small positive | MnO, FeO, Cr |
| Ferrimagnetic | Net magnetism (partial cancellation) | Large | Ferrites (Fe₃O₄), Magnetite |
Hysteresis Loop (B-H Curve)
- B = Magnetic induction, H = Magnetic field strength
- Key points:
- Retentivity (Br): Remaining magnetism after removing H
- Coercivity (Hc): Field needed to demagnetize
- Saturation magnetization: Maximum B
Soft vs Hard Magnetic Materials
| Property | Soft Magnetic | Hard Magnetic |
|---|---|---|
| Coercivity | Low | High |
| Hysteresis loss | Low | High |
| Area of loop | Narrow | Wide |
| Examples | Pure Fe, Si-steel, Permalloy | Alnico, NdFeB, SmCo, Hard ferrite |
| Applications | Transformers, motors, relays | Permanent magnets, speakers, MRI |
Magnetic Storage
- Hard disk → Magnetic domains store bits
- Magnetic tape, MRAM, etc.
2. Electrical & Electronic Properties
Energy Band Theory
| Material | Band Gap (Eg) | Conduction | Examples |
|---|---|---|---|
| Conductor | 0 eV (overlapping bands) | High (10⁷ S/m) | Cu, Ag, Al |
| Semiconductor | 0.1 – 3 eV | Moderate | Si (1.1 eV), Ge (0.67 eV), GaAs |
| Insulator | > 5 eV | Very low (<10⁻¹⁰ S/m) | Diamond (5.5 eV), Glass |
Intrinsic vs Extrinsic Semiconductors
- Intrinsic: Pure Si/Ge → n = p = nᵢ (electrons = holes)
- Extrinsic (Doping):
- n-type: Group V (P, As) → Extra electrons (majority carriers)
- p-type: Group III (B, Ga) → Holes (majority carriers)
P-N Junction & Devices
- Forward bias → Current flows (LED, Diode)
- Reverse bias → No current (except leakage)
- Transistor:
- BJT (NPN/PNP) → Current amplifier
- MOSFET → Voltage-controlled switch
- Applications: Rectifiers, Amplifiers, Switches, Solar cells, ICs
Diffusion in Solids
- Fick’s 1st law: J = –D (dc/dx)
- Fick’s 2nd law: For non-steady state
- Important in doping, carburizing, oxidation, etc.
3. Superconductivity
Key Features
- Zero electrical resistance below Tc (critical temperature)
- Perfect diamagnetism → Meissner Effect (expulsion of magnetic field)
Type-I vs Type-II Superconductors
| Property | Type-I | Type-II |
|---|---|---|
| Meissner effect | Complete up to Hc | Complete up to Hc1, partial (vortex state) till Hc2 |
| Transition | Abrupt | Gradual |
| Examples | Pure metals (Pb, Hg, Sn) | Alloys & high-Tc (NbTi, Nb₃Sn, YBCO) |
| Applications | Limited | MRI magnets, power cables, Maglev |
High-Tc Superconductors
- Discovered 1986 (Bednorz & Müller – Nobel 1987)
- Ceramic cuprates: YBa₂Cu₃O₇ (YBCO) → Tc = 92 K (liquid N₂ cooling)
- BSCCO, TBCCO → Tc > 100 K
- Advantages: Can be cooled with liquid nitrogen (77 K) instead of liquid He (4.2 K)
Applications of Superconductivity
- MRI machines (NbTi magnets)
- Maglev trains
- Power transmission cables (zero loss)
- SQUID (Superconducting Quantum Interference Device) → Ultra-sensitive magnetic sensors
- Particle accelerators (LHC)
Quick Revision Table – Unit IV
| Topic | Key Points |
|---|---|
| Dia / Para / Ferro | χ negative / small +ve / large +ve |
| Soft Magnet | Low Hc → Transformers |
| Hard Magnet | High Hc → Permanent magnets |
| Conductor / SC / Insulator | Eg = 0 / 0.1–3 / >5 eV |
| n-type / p-type | Pentavalent / Trivalent dopant |
| Superconductivity | R = 0, Meissner effect |
| Type-II SC | Used in high-field magnets (MRI) |
| High Tc | YBCO (92K), liquid N₂ cooled |
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