Synthesis and magnetic properties of spin-frac{3}{2} γ-phase of SrCo₂(PO₄)₂ antiferromagnet

We report on the synthesis of a new $\gamma$-phase of the spin $S$~=~$\frac{3}{2}$ compound SrCo$_2$(PO$_4$)$_2$ together with a detailed structural, magnetic and thermodynamic properties. The $\gamma$-phase of SrCo$_2$(PO$_4$)$_2$ crystallizes in a triclinic crystal structure with the space group $P\bar{1}$. Susceptibility and specific heat measurements reveal that SrCo$_2$(PO$_4$)$_2$ orders antiferromagnetically below $T_{\rm N}\simeq 8.5$\,K and the nature of ordering is three dimensional (3D). The magnetic isotherm at temperatures below $T_{\rm N}$ shows a field-induced spin-flop transition, related to the magnetocrystalline anisotropy, at an applied field of $\sim$~4.5~Tesla. Remarkably, heat capacity shows magnetic-field-induced transitions at $T_{\rm N1}$ = 3.6 K and $T_{\rm N2}$ = 7.4 K. The magnetic long range ordering (LRO) is also confirmed in both the Knight shift and spin-lattice relaxation rate ($1/T_{1}$) of the $^{31}$P-NMR measurements. However, below the LRO we have not detected any NMR signal due to faster relaxation. We have detected two structurally different phosphorous sites in $\gamma$-phase of SrCo$_{2}$(PO$_{4}$)$_{2}$ and they shift differently with temperature.