Magnetization measurements of reentrant ferromagnetic superconductors: The pseudoternary system $\left({\mathrm{Er}}_{1-x}{\mathrm{Ho}}_x\right){\mathrm{Rh}}_4{\mathrm{B}}_4$

The magnetization of cylindrical samples of ${\mathrm{Er}}_{1-x}{\mathrm{Ho}}_x{\mathrm{Rh}}_4{\mathrm{B}}_4$ has been measured in the temperature range from 1.4 K to $T_{c1\mathrm{}}\mathrm{}\left(x\right)\mathrm{}$. The low-temperature parts of the bell-shaped $H_{c2\mathrm{}}\mathrm{}\left(T\right)\mathrm{}$ curves show an increasingly pronounced anomaly with increasing $T_{c2\mathrm{}}$. The low-temperature magnetization curves and the observed behavior of $H_{c1\mathrm{}}$ vs $T$ suggests a transition from type-II to type-I superconductivity in the vicinity of $T_{c2\mathrm{}}$. The normal-state magnetic susceptibility follows a Curie-Weiss law with a Curie temperature significantly below $T_{c2\mathrm{}}$ for $x>~0.40$. $\frac{\left(\frac{d{H}_{c2\mathrm{}}}{\mathrm{dT}}\right)}{T=T_{c1\mathrm{}}}$ is found to be constant within experimental error for $x<~0.60$, but about a factor of 3 smaller for $x=0.70\mathrm{}$.