Upper critical magnetic field of the mixed ternary system $\left({\mathrm{Sm}}_{1-x}{\mathrm{Y}}_x\right){\mathrm{Rh}}_4{\mathrm{B}}_4$

The upper critical magnetic field as a function of temperature $H_{c2}\mathrm{}\left(T\right)\mathrm{}$ has been determined for many compositions in the $\left({\mathrm{Sm}}_{1-x}{\mathrm{Y}}_x\right){\mathrm{Rh}}_4{\mathrm{B}}_4$ mixed ternary system. For $0\le x\le 0.30$, antiferromagnetic ordering of the ${\mathrm{Sm}}^{3+}$ ions at a temperature $T_M$ enhances $H_{c2}$ above an extrapolation from $T>\mathrm{}{T}_M$. A strong suppression of $H_{c2}$ at low temperature is observed for $0.35\le x\le 0.90$. Calculations of $H_{c2}\mathrm{}\left(T\right)\mathrm{}$ using a multiple-pair-breaking formulation reproduce the trends of the data well, although quantitative agreement is not found for some compositions. This analysis shows that paramagnetic limitation by the uniform component of the exchange field $H_J$ is a very important factor in describing $H_{c2}\mathrm{}\left(T\right)\mathrm{}$ for most compositions. In addition, enhancement of $H_{c2}$ in the antiferromagnetic state due to reductions of spin fluctuations are important for $x\sim 0$.