Question

Example of anti-ferro-magnetic materials:
A. Salts of transition elements
B. Rare earth elements
C. Transition metals
D. Ferrites

Answer 1

We have to know that when the magnetic moments of atoms or molecules, generally associated with the electrons spin, aligned during a regular pattern but in opposite directions, the type of magnetism is called antiferromagnetism. Depending upon the temperature antiferromagnetic solids show unique behavior in presence of an applied magnetic field.

Complete step by step answer:
In simple words, we can say antiferromagnets are similar to ferromagnets but their magnetic moments align antiparallel to adjacent moments. This arrangement takes place suddenly below the critical temperature called the Neel temperature.
For an antiferromagnet, the magnetization remains fixed below the critical temperature and when the external field is removed, the material retains the antiparallel alignment. The magnetic moments appear from the spin of the outermost electrons. Without a magnetic field applied to the material, the magnetic moments of each orbiting electron are randomly oriented. When a magnetic field is applied to the material, the magnetic moments of the electrons react and align antiparallel to each nearby moment.
In the presence of a strong magnetic field, the materials that are weakly magnetized in the direction of the field are called antiferromagnetic materials and the magnetism is known as anti-ferromagnetism.
We must remember that the salts of transition elements exhibit antiferromagnetism.

So, the correct answer is Option A.

Note:
We also remember anti-ferromagnets are typically ceramics of transition metals and either oxygen or sulfur. The transition metal is the sub lattice that has magnetic moments which respond to magnetic fields below Neel temperature. The oxide sub lattice magnetic moments respond suddenly to the magnetization of the first lattice by aligning in the opposite direction with the same magnitude. The oxygen ion alignments influence the other manganese ions and effectively tell the metal ions how to order their magnetic spins.
Some of the antiferromagnetic materials are $MnO$, $FeO$, $NiO$, $Mn{F_2}$ etc.