Physics · Magnetism And Matter · NEET
It is positive and very large (χ >> 1), often in the hundreds or thousands. Diamagnetic χ is small and negative (0 > χ ≥ −1), paramagnetic χ is small and positive, but ferromagnetic χ is huge and positive. This large positive value is why iron is gripped so strongly. NCERT: ferromagnetic materials have large χ.
In a ferromagnetic solid the atomic magnetic moments line up together inside small regions called domains. Once the field aligns these domains, the material develops a large net magnetisation, so the pull is strong. In a paramagnetic material each atom acts alone with no domain teamwork, so alignment is weak and easily broken by heat. That is why ferro = strong, para = weak.
A domain is a tiny region inside a ferromagnet where a huge number of atomic magnetic moments already point the same way, even with no external field. Unmagnetised iron has many domains pointing in random directions, so their moments cancel. When you apply a field, favourable domains grow and align, giving strong net magnetisation. This domain teamwork is unique to ferromagnetism.
Above the Curie temperature (Curie point), heat destroys the domain alignment and the ferromagnetic material turns into an ordinary paramagnetic material with only small positive χ. For example, iron loses ferromagnetism above about 1043 K. So heating a magnet enough makes it stop behaving like a strong magnet. This one-way switch (ferro → para on heating) is a common NEET fact.
Yes, hard ferromagnetic materials keep some magnetisation even after the external field is switched off; this leftover magnetisation is called retentivity (remanence) and is why permanent magnets exist. Soft ferromagnets (like soft iron) lose most of it quickly. This retained magnetisation, plus hysteresis, separates ferromagnets from para/dia materials, which lose magnetisation instantly.
Match List-I with List-II. List-I (Material): A. Diamagnetic, B. Ferromagnetic, C. Paramagnetic, D. Non-magnetic. List-II (Susceptibility χ): I. χ = 0, II. 0 > χ ≥ −1, III. χ >> 1, IV. 0 < χ < ε (a small positive number). Choose the correct answer:
The magnetic susceptibility is negative for:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Iron (Fe), cobalt (Co), nickel (Ni), gadolinium (Gd) and alloys like alnico. Remember the core three as Fe-Co-Ni.
μ_r is very large, typically thousands, because μ_r = 1 + χ and χ >> 1 for ferromagnets. This large μ_r is why iron cores are used in transformers and electromagnets.
It moves strongly from the weak-field region toward the strong-field region, i.e. it is strongly attracted to the pole. Paramagnets do the same but weakly; diamagnets move the opposite way (toward weak field).
Yes. As temperature rises, alignment weakens; above the Curie temperature the material becomes paramagnetic. For a ferromagnet turned paramagnet, susceptibility above the Curie point follows the Curie-Weiss law.
Soft iron magnetises and demagnetises easily (low retentivity), ideal for temporary electromagnets. Steel has high retentivity, so it keeps its magnetism, making it good for permanent magnets.
