Magnetic Compensation and Pinned Reversal in a Multisite Antiferromagnetic Ising Chain

In this work, we study an antiferromagnetic Ising chain with multiranged multisite interactions to determine whether magnetic compensation or negative magnetisation emerges spontaneously at an appropriate temperature. The chain consists of two sublattices, one with N number of S-spins separated by three lattice units and the other with N pairs of σ-spins. The σ-sublattice is sifted such that a pair of σ-spins appear between two S-spins and form an Sσσ spin chain with nearest neighbour separation. Both S and σ spins are spin-1/2 Ising type. The S-spins interact ferromagnetically with the nearest S-spins, and they interact antiferromagnetically with the nearest pairs of σ-spins. Whereas the σ-spins in a pair interact ferromagnetically without inter-pair interaction. The model is solved analytically using the transfer matrix method. Monte Carlo simulations are performed to obtain spin configurations under different conditions and to complement the analytical results. Competing antiferromagnetic and ferromagnetic interactions spanning multiple length scales give rise to fascinating magnetic behaviour, including magnetic compensation. At the compensation point, both the sublattices interchange their spin orientation in favour of the Zeeman energy corresponding to positive magnetisation instead of accessing the negative magnetisation state. However, pinning a single spin can drive the system into a negative magnetisation state right through the compensation point without the presence of a magnetic field. With such a simple model, we identify that although magnetic compensation is a natural feature of such an antiferromagnetic system, spin disorder is essential for negative magnetisation.