Large spin fluctuation in the magnetization process of S=1/2 frustrated square lattice model

In the frustrated spin systems, a lot of interesting phenomena including non-collinear magnetic orders, magnetization plateaus and spin liquids occur. Such frustrated interactions often appear in geometrically frustrated lattices, such as triangular, kagome or pyrochlore lattices. In addition to these lattice, when we consider further neighbor interactions or combination of ferromagnetic and antiferromagnetic interactions, frustrations can happen even in the square lattice.
Motivated by several compounds, we discuss the nature of S=1/2 square lattice Heisenberg magnets with ferromagnetic and antiferromagnetic nearest-neighbor interactions. By means of the infinite tensor product states (iTPS) ansatz, we investigated magnetization process of the models numerically. We show that in the model corresponding to the compound [1], a part of spins reveals very strong spin fluctuation around the half of saturation magnetization. This is because spins connected by strong antiferromagnetic interaction form spin singlet. By varying the interaction strength, we show that such spin fluctuation is related to the 1/2 magnetization plateau stabilized in the vicinity of the model.

[1] H. Yamaguchi, Y. Sasaki, T. Okubo, et al, Phys. Rev. B 98, 094402 (2018).