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ChemistrySolid State
Statement I: Ferromagnetic substances can be permanently magnetised.
Statement II: The ferromagnetic property of substances decreases with an increase in temperature and completely disappears at a temperature called the Neel temperature.
Choose the correct option:
Options
1
Statement I is true, Statement II is true
2
Statement I is true, Statement II is false
3
Statement I is false, Statement II is true
4
Both statements are false
Correct Answer
Statement I is true, Statement II is false
Solution
1

Statement I: Ferromagnetic substances (Fe, Ni, Co) CAN be permanently magnetised — domains align and remain aligned even after removing external field.

Statement I is TRUE ✓

2

Statement II: The temperature at which ferromagnetism disappears is the Curie temperature, NOT Neel temperature.

Neel temperature = antiferromagnetism → paramagnetism transition.

Statement II is FALSE ✗

Statement I: TRUE (ferromagnets can be permanently magnetised)
Statement II: FALSE (Curie temperature, not Neel temperature)
Theory: Solid State
1. Types of Magnetic Materials

All matter responds to magnetic fields. The five main types: (1) Diamagnetic: all electrons paired, no permanent dipole. Weakly repelled by magnetic field. Magnetic susceptibility $\chi < 0$ (small negative). Examples: Bi, Cu, Ag, Au, H2O, N2, noble gases, NaCl, benzene. (2) Paramagnetic: one or more unpaired electrons, permanent dipole. Weakly attracted to magnetic field. $\chi > 0$ (small positive). Alignment lost when field removed. Examples: O2, NO, Fe2+, Cu2+, Cr3+, Al, Na, K. (3) Ferromagnetic: unpaired electrons with strong parallel alignment of magnetic moments within domains. Very strongly attracted to field. Can be permanently magnetised. $\chi >> 0$. Examples: Fe, Co, Ni, Gd (at low T), permalloy. (4) Antiferromagnetic: adjacent magnetic dipoles aligned antiparallel (equal and opposite) → zero net magnetic moment. Weakly repelled or attracted. Examples: MnO, FeO, NiO, Cr, MnF2. (5) Ferrimagnetic: adjacent moments antiparallel but unequal magnitudes → net magnetic moment. Strongly attracted. Examples: Fe3O4 (magnetite), ferrites (MFe2O4, M = Mn, Zn, Ni, Cu).

2. Magnetic Domains and Ferromagnetism

In ferromagnetic materials, electron spins are aligned in small regions called magnetic domains (typically 0.001 to 1 mm in size). Within each domain: all spins parallel → domain has large magnetic moment. In an unmagnetised ferromagnet: domains are randomly oriented → no net magnetisation. In external magnetic field: (1) Domains favourably oriented grow at expense of unfavourably oriented (domain wall motion). (2) Domains rotate to align with field. Net magnetisation increases. When field removed: domains may remain aligned (hard magnet) or randomise (soft magnet). Hysteresis loop: plot of B (magnetisation) vs H (applied field). Area of loop = energy dissipated per cycle. Hard magnets (large coercivity): permanent magnets (Alnico, neodymium magnets NdFeB — strongest permanent magnets). Soft magnets (small coercivity, small hysteresis loop): transformer cores, electromagnets (Fe-Si alloys). Saturation magnetisation: maximum magnetisation when all domains aligned.

3. Curie and Neel Temperatures

Curie temperature ($T_C$): critical temperature above which ferromagnetic material becomes paramagnetic. At $T > T_C$: thermal energy ($k_BT$) overcomes exchange interaction that aligns spins → random orientation → paramagnetic. Values: Fe: $T_C = 1043$ K (770°C). Ni: $T_C = 631$ K (358°C). Co: $T_C = 1388$ K (1115°C). Gadolinium: $T_C = 292$ K (just below room temperature). Neel temperature ($T_N$): critical temperature above which antiferromagnetic material becomes paramagnetic. At $T > T_N$: antiparallel alignment disorder → paramagnetic. Values: MnO: $T_N = 122$ K. FeO: $T_N = 198$ K. Cr: $T_N = 311$ K. NiO: $T_N = 523$ K. The distinction: Curie $T$ = ferromagnetic→paramagnetic. Neel $T$ = antiferromagnetic→paramagnetic. NEET question specifically tests this distinction.

4. Crystal Defects in Solids

Point defects: Schottky defect: equal number of cation and anion vacancies. Density decreases. Examples: NaCl, KCl, CsCl, AgBr. Frenkel defect: one ion (usually smaller cation) moves to interstitial site, leaving a vacancy. Density unchanged. Examples: AgCl, AgBr, ZnS. AgBr has both Schottky and Frenkel — unique property used in photography. F-centres: anion vacancy filled by electron. Gives colour to alkali halides (NaCl becomes yellow when excess Na vapour treated — F-centres absorb visible light). Line defects (dislocations): edge dislocation (extra half-plane of atoms), screw dislocation. Affect mechanical properties. Planar defects: grain boundaries, stacking faults. Affects strength, corrosion resistance. Impurity defects: substitutional (same size) or interstitial (small atom in holes). Doping semiconductors: Si + B (p-type), Si + P or As (n-type). Carbon in steel (interstitial): makes harder than pure Fe.

5. Crystal Structures and Packing

Close packing: hexagonal close packing (hcp) ABAB — 74% packing efficiency. Cubic close packing (ccp/fcc) ABCABC — 74%. Body-centred cubic (bcc) — 68%. Simple cubic — 52.4%. Radius ratios for ionic crystals: cation/anion ratio determines coordination number. 0.155-0.225: CN=3 (triangular). 0.225-0.414: CN=4 (tetrahedral). 0.414-0.732: CN=6 (octahedral). 0.732-1.0: CN=8 (cubic). NaCl structure (rock salt): $r^+/r^- = 0.524$, CN=6 for both. CsCl: $r^+/r^- = 0.93$, CN=8. ZnS (zinc blende): CN=4 tetrahedral. ZnS (wurtzite): CN=4 tetrahedral, hexagonal. Diamond cubic: tetrahedral, CN=4, 34% packing. Fluorite (CaF2): Ca²⁺ in ccp, F⁻ in all tetrahedral holes. Antifluorite (Na2O): oxide in ccp, Na⁺ in all tetrahedral holes.

6. Electrical Properties of Solids

Conductors (metals): band gap = 0, partially filled conduction band. Resistivity $10^{-8}$ to $10^{-6}$ Ω·m. Increases with temperature (more phonon scattering). Insulators: large band gap (>3 eV). Diamond: 5.4 eV. SiO2: 9 eV. Resistivity $10^{10}$ to $10^{20}$ Ω·m. Semiconductors: small band gap (0.1-3 eV). Si: 1.1 eV. Ge: 0.67 eV. Resistivity decreases with temperature (more electrons promoted to conduction band). Intrinsic semiconductor: pure Si or Ge. Conductivity increases with T. n-type semiconductor: Si doped with P or As (Group 15) → extra electron in conduction band. p-type: Si doped with B or Al (Group 13) → "hole" in valence band. Holes act as positive charge carriers. p-n junction: basis of diodes, transistors, solar cells, LEDs. Superconductors: zero resistance below critical temperature $T_c$. Meissner effect (diamagnetic, expels magnetic field). Applications: MRI magnets, particle accelerators. High-Tc superconductors (YBCO: $T_c = 93$ K) are ceramic copper oxides.

7. Colligative Properties

Properties that depend on number of solute particles, not their nature. (1) Relative lowering of vapour pressure: $\Delta P/P_0 = x_{solute}$ (Raoults law). (2) Elevation of boiling point: $\Delta T_b = K_b \times m$ where $K_b$ = ebullioscopic constant (0.512°C·kg/mol for water), $m$ = molality. (3) Depression of freezing point: $\Delta T_f = K_f \times m$ where $K_f$ = cryoscopic constant (1.86°C·kg/mol for water). (4) Osmotic pressure: $\pi = MRT$ (van't Hoff equation). For electrolytes: multiply by van't Hoff factor $i = 1 + \alpha(n-1)$ where $\alpha$ = degree of dissociation, $n$ = number of ions. Example: 0.01 M NaCl: $i \approx 2$ (fully dissociated) → $\Delta T_f = 2 \times 1.86 \times 0.01 = 0.0372°C$. Applications: anti-freeze (ethylene glycol), osmosis in biological cells, molecular weight determination by osmotic pressure.

8. Bragg Equation and X-ray Crystallography

Bragg's law: $n\lambda = 2d\sin\theta$ where $n$ = order of diffraction, $\lambda$ = X-ray wavelength, $d$ = interplanar spacing, $\theta$ = angle of incidence. Used to determine: crystal structure, interplanar spacings, unit cell dimensions, atomic positions. X-ray diffraction (XRD): powerful technique for: identifying crystalline materials (each compound has unique diffraction pattern = fingerprint), determining unit cell parameters, determining atomic arrangement in crystals. Rosalind Franklin's X-ray diffraction of DNA (Photo 51) provided key data for Watson-Crick double helix model (1953). Modern applications: pharmaceutical crystal form analysis (polymorphs), protein structure determination (over 200,000 protein structures in PDB), material characterisation, forensic analysis. Powder XRD: used for polycrystalline samples, gives characteristic d-spacings for identification.

Frequently Asked Questions
1. Why is the Neel temperature for antiferromagnetism, not ferromagnetism? ⌄
In ferromagnetism: adjacent magnetic moments align PARALLEL (all pointing same direction within a domain) due to exchange interaction. Above Curie temperature $T_C$: thermal energy disrupts this parallel alignment → random orientation → paramagnetic. In antiferromagnetism: adjacent moments align ANTIPARALLEL (alternating up-down pattern) in equal numbers → zero net magnetic moment. Above Neel temperature $T_N$: this antiparallel arrangement breaks down → random → paramagnetic. Both transitions are paramagnetic at high temperature, but the intermediate state (below the transition temperature) is different: ferromagnetic (parallel) vs antiferromagnetic (antiparallel). Therefore, Curie T relates to ferromagnetism; Neel T relates to antiferromagnetism.
2. What makes a material a hard vs soft magnet? ⌄
Hard magnet (permanent magnet): high coercivity (large field needed to demagnetise), large hysteresis loop area, high remnant magnetisation. Once magnetised, stays magnetised. Used for: permanent magnets in motors, speakers, hard drives, MRI machines. Examples: Alnico (Fe-Al-Ni-Co), SmCo5, NdFeB (neodymium magnets — strongest: field up to 1.4 T). Soft magnet: low coercivity (easily magnetised and demagnetised), small hysteresis loop area, low energy loss per cycle. Used for: transformer cores, electromagnet cores, inductors. Examples: Fe-Si (electrical steel), soft ferrites, permalloy (Ni-Fe). The distinction is crucial in electrical engineering: transformer cores must be soft (minimise energy loss in AC cycles); permanent magnets must be hard (maintain magnetisation).
3. How does doping create semiconductors? ⌄
Silicon has 4 valence electrons in a tetrahedral diamond cubic lattice. All electrons paired in covalent bonds → insulator at absolute zero. n-type doping: replace Si atoms with Group 15 atoms (P, As, Sb — 5 valence electrons). 4 electrons form bonds with Si; 5th electron weakly bound → easily promoted to conduction band. "n" = negative charge carriers (electrons). Each P atom donates one electron. p-type doping: replace Si with Group 13 (B, Al, Ga — 3 valence electrons). Only 3 electrons for 4 bonds → one bond incomplete = "hole" (positive). Hole migrates through lattice as electrons fill it. "p" = positive charge carriers (holes). The concentration of free electrons ($n$) and holes ($p$) in pure Si at 300 K: $n = p \approx 1.5 \times 10^{10}$ cm$^{-3}$. With doping: one type dominates. At 300 K with $10^{17}$ P atoms/cm³: $n \approx 10^{17}$, $p \approx 2.25$ (mass action law: $np = n_i^2$).
4. What are F-centres and why do they cause colour? ⌄
F-centre (Farbe = colour in German): an anionic vacancy in an ionic crystal that has trapped an electron. Formed when alkali halide crystal is heated in vapour of the alkali metal: excess metal atoms enter the crystal, halide ions migrate to surface, electrons occupy the vacancies. NaCl in Na vapour → yellow-orange (F-centre absorbs visible blue-green light). KCl in K vapour → violet. LiF → pink. The trapped electron in F-centre has quantised energy levels. The energy gap between ground and first excited state corresponds to visible light photons → absorption of that colour → crystal appears complementary colour. F-centres are responsible for the colour in: irradiated glass (nuclear reactors), certain gemstone colours (blue topaz from irradiation), some laser crystals. Heavily F-centred crystals can be used as tunable lasers (colour centre lasers).
5. Explain the significance of Schottky and Frenkel defects? ⌄
Schottky defect: cation and anion vacancies in equal numbers (to maintain electrical neutrality). Density decreases. Ionic conductivity increases (vacancies facilitate ion migration). Occurs in: NaCl, KCl, CsCl (where cation and anion have similar sizes). Frenkel defect: ion (usually smaller, usually cation) displaced from normal lattice site to interstitial position. Creates a vacancy + an interstitial pair. Density unchanged. Ag⁺ displacement is common (AgCl, AgBr, ZnS — cation smaller than anion). AgBr has both Schottky AND Frenkel — this gives it exceptional ionic conductivity at room temperature → explains photographic sensitivity. In photography, AgBr grains in film: light causes Ag⁺ + e⁻ → Ag⁰ (photoelectron from bromide). Ag⁰ atoms accumulate at Frenkel defect sites → latent image. Development: Ag⁺ reduced to Ag by developer at those sites.
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