Strong anisotropy in the mixed antiferromagnetic system Mn$_{1-x}$Fe$_{x}$PSe$_3$

We report the magnetic phase diagram of Mn$_{1-x}$Fe$_{x}$PSe$_3$ which represents a random magnet system of two antiferromagnetic systems with mixed spin, mixed spin anisotropies, mixed nearest neighbor magnetic interactions and mixed periodicities in their respective antiferromagnetic structure. Bulk samples of Mn$_{1-x}$Fe$_{x}$PSe$_3$ have been prepared and characterized phase pure by powder X-ray and neutron diffraction and X-ray fluorescence. Nature and extent of magnetically ordered state has been established using powder neutron diffraction, dc magnetic susceptibility and heat capacity. Long-range magnetic ordering exists between $x = 0.0$ and 0.25 (MnPSe$_3$-type) and between $x = 0.875$ and $1$ (FePSe$_3$-type). A short-range magnetic order with existence of both MnPSe$_3$- and FePSe$_3$-type nano-clusters has been established between $x = 0.25$ and $0.875$. Irreversibility in dc magnetization measurements, also characterized by isothermal and thermoremanent magnetization measurements suggest similarities to magnetic nanoparticles where uncompensated surface spins result in a non-zero TRM and IRM response, further reinforcing existence of magnetic nano-clusters or domains. A spin glass state, observed in analogous Mn$_{1-x}$Fe$_x$PS$_3$, has been ruled out and formation of nano-clusters exhibiting both ordering types results from unusually high anisotropy values. The effect of ligand contributions to the spin-orbit interactions has been suggested as a possible explanation for high $D$ values in these compounds.

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