Crystal Growth and Anisotropic Magnetic Properties of Quasi-2D (Fe$_{1-x}$Ni$_{x}$)$_{2}$P$_{2}$S$_{6}$

TL;DR

This study reports the synthesis, structural analysis, and magnetic characterization of (Fe$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ crystals, revealing a change in magnetic anisotropy behavior with Ni concentration and detailed magnetic properties of quasi-2D layered magnets.

Contribution

First comprehensive growth and magnetic study of (Fe$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ crystals, highlighting anisotropy transition at a critical Ni level.

Findings

All crystals exhibit antiferromagnetic order.

Magnetic anisotropy switches from Ising to different behavior at high Ni content.

Intermediate compounds show anisotropic magnetization in the paramagnetic state.

Abstract

We report the crystal growth by chemical vapor transport together with a thorough structural and magnetic characterization of the quasi-2D magnets (Fe$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ with $x_\textrm{Ni} = \textrm{0, 0.3, 0.5, 0.7, 0.9 & 1}$. As-grown crystals exhibit a layered morphology with weak van der Waals interactions between layers parallel to the crystallographic $ab$ plane of the monoclinic structure in the space group $C2/m$ (No. 12). Magnetization measurements reveal an antiferromagnetic ground state for all grown crystals. In the ordered state, the magnetization along different crystallographic directions is in agreement with an Ising anisotropy for $0 \leq x \leq 0.9$ and only for Ni$_2$P$_2$S$_6$ a different behavior is observed which is line with an anisotropic Heisenberg or XXZ model description. This striking abrupt change of anisotropy at a critical Ni concentration is coupled to a change in the width of the maximum in the magnetization around $T_\textrm{max}$. Furthermore, all intermediate compounds $0 < x \leq 0.9$ exhibit an anisotropic magnetization already in the paramagnetic state similar to the parent compound Fe$_2$P$_2$S$_6$.