We report the anomalous characteristics of magnetostructural phases in the vicinity of the orbital/charge-ordered state ($T_{\mathrm{OO}/\mathrm{CO}}\sim 220\mathrm{K}$) below the antiferromagnetic (AFM) Néel temperature ($T_{\mathrm{N}}\sim 130\mathrm{K}$) of the correlated electron system ${\mathrm{Ca}}_{0.75}{\mathrm{Sm}}_{0.25}{\mathrm{MnO}}_3$ by means of neutron diffraction, field-dependent heat-capacity, and magnetometry studies. Our neutron diffraction study reveals an apparent coexistence of two commensurate magnetic phases with the propagation vectors ${\mathit{k}}_{\mathit{C}}=(1/2,1/2,0)$ and ${\mathit{k}}_{\mathit{E}}=(1/2,\epsilon ,0)$ forming probably a $CE$ phase, with $\epsilon \sim 0.267\left(3\right)$ close to 1/4, indicating that the magnetic unit cell coincides with the enlarged superstructure of dimensions $a\times 4b\times c$. It was found that the moments of both magnetic phases are AFM-coupled along all directions forming an $A$- or a $G$-type ordering, where the magnetic moments of the $C$- and $E$-type phases are aligned within the $ab$ plane and along the $c$ axis, respectively. In the noncollinear magnetic structure, the $C$-type phase can be described with the irreducible representations of ${\mathrm{\Gamma }}_6(A_x,G_y)$ and ${\mathrm{\Gamma }}_8(G_x,A_y)$, where the Mn moments are oriented parallel to the directions [110] and $\left[1\overline{1}0\right]$. Moreover, phenomena such as field-induced metamagnetic transitions ($H_{\mathrm{T}}\sim 120\mathrm{kOe}$ at 3 K), critical behavior of specific heat ($C_P=A{|T-T_{\mathrm{N}}|}^{-\alpha }$), negative exchange bias effect ($H_{\mathrm{EB}}$), and a zero-crossover of magnetic entropy change ($\mathrm{\Delta }{S}_{\mathrm{M}}$) are newly observed in this unique composition.

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• Magnetic interactions