Valence fluctuation in Ce$_2$Re$_3$Si$_5$ and Ising-type magnetic ordering in Pr$_2$Re$_3$Si$_5$ single crystals

Single crystals of ${\rm Ce_2Re_3Si_5}$ and ${\rm Pr_2Re_3Si_5}$ have been grown by Czochralski method in a tetra-arc furnace. Powder x-ray diffraction confirmed that these compounds crystallize in the ${\rm U_2Mn_3Si_5}$-type tetragonal crystal structure with space group $P4/mnc$ (No. 128). The anisotropic physical properties have been studied comprehensively by measuring the magnetic susceptibility, isothermal magnetization, electrical transport and specific heat. The low value of magnetic susceptibility together with no magnetic transition down to $2$~K gives evidence that the Ce-ions are in the intermediate valence state in ${\rm Ce_2Re_3Si_5}$. On the other hand ${\rm Pr_2Re_3Si_5}$ revealed a magnetic ordering at $9$~K. The sharp drop in the magnetic susceptibility and a spin flip like metamagnetic transition, for $H~\parallel~[001]$ in the magnetization plot of ${\rm Pr_2Re_3Si_5}$ suggest an Ising-type antiferromagnetic ordering. Based on magnetic susceptibility and isothermal magnetization data, a detailed crystal electric field (CEF) analysis shows that degenerate ${J} = 4$ Hund's rule derived ground state of ${\rm Pr^{3+}}$ ion splits into nine singlets with an overall splitting of $1179$~K. The magnetic ordering in ${\rm Pr_2Re_3Si_5}$ is due to the exchange-generated admixture of the lowest lying CEF energy levels. Heat capacity data reveal a sharp peak at $9$~K, that confirms the bulk nature of the magnetic ordering in ${\rm Pr_2Re_3Si_5}$.