A ferromagnetic interaction acts to align spins. If the energy is expressed as the sum of all pairs, i, j, over an interaction term J(i,j), times the spin of atom i times the spin of atom j, J<0 is a ferromagnetic interaction (aligning spins leads to a lower total energy and is thus favorable energetically, although unfavorable entropically). J>0 is an antiferromagnetic interaction acting to anti-align neighboring spins. The combination of both can lead to spin glass behavior.
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