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The magnetic alignment of spins is determined by the parameter known as exchange energy which is given by
Eex= - 2JSi.Sj where J is the exchange integral and Si and Sj are the spins.
If J is +ve, for minimum energy Si and Sj should be parallel and hence ferromagnetic
If J is -ve, for minimum energy Si and Sj should be antiparallel and hence antiferromagnetic.
The exchange energy term explains the ferro and antiferromagnetic nature of certain elements in the periodic table.
Bethe Slater Curve
The Bether Slater Curve tells us why certain elements are antiferromagnetic and ferromagnetic according to the exchange integral term.
In the figure shown above, the x-axis gives the ratio of interatomic distance (rab) to the radius of 3d shell of electron (rd). The elements with positve J value shows ferromagnetic nature such as Fe, Co and Ni and elements with -ve J shows antiferromagnetic nature.