Question

(b) A material is found to order antiferromagnetically with a Néel temperature (Tn) of 240 K. Illustrate with a graph how the magnetic susceptibility varies between 4 K and 600 K. Explain the change in magnetic properties which occurs at TN. [4 MARKS] (c) At low temperatures, spinel type lattices demonstrate cooperative magnetic behaviour. Use crystal field theory to predict the magnetic properties of zinc ferrite (ZnFe2O4) and copper ferrite (CuFe2O4), which both crystallise in an inverse spinel type structure. [8 MARKS]

Answer 1

(b)

EX hn 19 glossns Anti- ferromagnetic Paramagnetic 1 KI TN = 240K ok 300K Goog Temperature T

This the the required graph.

Below the Neel temperature the material becomes antiferromagnetic. Magnetic moments arrange in such a way so as to cancel the overall magnetic field. Susceptibility increases with temperature and is positive for antiferromagnetic material.

After the Neel temperature TN, the material becomes paramagnetic, magnetic moments lose their anti-parallel orientation and get randomly oriented due to thermal energy. The susceptibility after TN decreases with increase in temperature.

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