Which of the following arrangements shows the schematic alignment of magnetic moments of antiferromagnetic substance?

Magnetic behaviour of a crystalline solid is decided by the way its individual permanent dipole moments (shown schematically by arrows) are oriented with respect to one another.

1. Ferromagnetic substances (e.g. $$Fe$$, $$Ni$$) have all moments aligned parallel to each other, hence they show a very large net moment.

2. Ferrimagnetic substances (e.g. $$Fe_3O_4$$) possess two sets of dipoles that are antiparallel but unequal in magnitude, so the cancellation is incomplete and a small net moment remains.

3. Antiferromagnetic substances (e.g. $$MnO$$, $$NiO$$, $$FeO$$) have dipoles arranged strictly antiparallel and of equal magnitude. Every “up” moment is exactly cancelled by a neighbouring “down” moment, giving zero resultant magnetisation.

Therefore, the correct schematic for an antiferromagnet must contain alternating moments of equal length pointing in opposite directions (↑↓↑↓ …) so that the total becomes zero.

Among the given diagrams, Option D is the only one that shows such a perfect head-to-tail (antiparallel) alignment with equal arrow lengths.

Hence, the required arrangement is
Option D which is: schematic with alternating up and down moments of equal magnitude.