Pseudo-easy-axis anisotropy in antiferromagnetic $S=1$ diamond-lattice systems Ni$X_{2}$(pym)$_{2}$

TL;DR

This study reveals a unique pseudo-easy-axis anisotropy in $S=1$ antiferromagnetic diamond-lattice systems, demonstrated through neutron diffraction, magnetization measurements, and Monte Carlo simulations, highlighting unconventional magnetic behavior.

Contribution

The paper uncovers the existence of a pseudo-easy-axis in antiferromagnetic $S=1$ diamond-lattice systems and characterizes its effects using combined experimental and computational methods.

Findings

Identification of a pseudo-easy-axis in Ni$X_{2}$(pym)$_{2}$ compounds.

Observation of spin-flop transitions aligned with the pseudo-easy-axis.

Asymptotic approach to saturation without symmetry-breaking for perpendicular fields.

Abstract

We investigate the magnetic properties of $S=1$ antiferromagnetic diamond lattice, Ni$X_{2}$(pyrimidine)$_{2}$ ($X$ = Cl, Br), hosting a single-ion anisotropy (SIA) orientation which alternates between neighbouring sites. Through neutron diffraction measurements of the $X$ = Cl compound, the ordered state spins are found to align collinearly along a pseudo-easy-axis, a unique direction created by the intersection of two easy planes. Similarities in the magnetization, exhibiting spin-flop transitions, and the magnetic susceptibility in the two compounds imply that the same magnetic structure and a pseudo-easy-axis is also present for $X$ = Br. We estimate the Hamiltonian parameters by combining analytical calculations and Monte-Carlo (MC) simulations of the spin-flop and saturation field. The MC simulations also reveal that the spin-flop transition occurs when the applied field is parallel to the pseudo-easy-axis. Contrary to conventional easy-axis systems, there exist field directions perpendicular to the pseudo-easy-axis for which the magnetic saturation is approached asymptotically and no symmetry-breaking phase transition is observed at finite fields.