  Magnetic correlations of two iso-structural quasi-one-dimensional (1D)
antiferromagnetic spin-chain compounds Sr$M_2$V$_2$O$_8$ ($M$ = Co, Mn) have
been investigated by magnetization and powder neutron diffraction. Two
different collinear antiferromagnetic (AFM) structures, characterized by the
propagation vectors, $k$ = (0 0 1) and $k$ = (0 0 0), have been found below
$\sim$ 5.2 K and $\sim$ 42.2 K for the Co- and Mn-compounds, respectively. For
the Mn-compound, AFM chains (along the $c$ axis) order ferromagnetically within
the $ab$ plane, whereas, for the Co-compound, AFM chains order
ferro-/antiferromagnetically along the $a/b$ direction. The critical exponent
study confirms that the Co- and Mn-compounds belong to the Ising and Heisenberg
universality classes, respectively. For both compounds, short-range spin-spin
correlations are present over a wide temperature range above $T_N$. The reduced
ordered moments at base temperature (1.5 K) indicate the presence of quantum
fluctuations in both compounds due to the quasi-1D magnetic interactions.
