Coexistence of ferromagnetism and superconductivity in ${\mathrm{ErRh}}_4{\mathrm{B}}_4$ single crystals probed by dynamic magnetic susceptibility

High-resolution measurements of the dynamic magnetic susceptibility are reported for the ferromagnetic, reentrant superconductor ${\mathrm{ErRh}}_4{\mathrm{B}}_4$. A detailed investigation of the coexisting regime reveals unusual temperature-asymmetry on warming and cooling and magnetically anisotropic behavior. The superconducting phase appears via a series of discontinuous steps upon warming from the ferromagnetic normal phase, whereas the ferromagnetic phase develops via a more gradual transition upon cooling from the superconducting phase. A model based on local field inhomogeneity is proposed to explain these observations.

Figure 1

(Color online) The dynamic magnetic susceptibility of a ${\mathrm{ErRh}}_4{\mathrm{B}}_4$ single crystal measured along the magnetic easy axis (perpendicular to the needle-shaped sample). Each curve corresponds to a fixed value of the applied dc magnetic field in the range indicated in the figure. The red curves (gray in print) correspond to to measurements where $4\pi \chi \left(T\right)$ becomes nonmonotonic in the vincinity of the FM-SC transition.

Figure 2

(Color online) The dynamic magnetic susceptibility of the ferromagnet-superconductor transition in ${\mathrm{ErRh}}_4{\mathrm{B}}_4$ measured at different applied fields. Note the temperature scale and highly asymmetric character of the FM-SC and SC-FM transitions. Blue (dark gray): warming, red (lighter gray): cooling.

Figure 3

(Color online) Hysteresis of the transition in zero applied field, measured at different ramp rates. The temperature sweep profiles are shown in the inset.

Figure 4

(Color online) Transition region measured for magnetic field oriented along the needle and $c$ axis. Inset: summary phase diagram for two orientations measured by transport (symbols) and TDR.

Figure 5

(Color online) Details of the hysteresis with partial scans as explained in the text. The inset shows another partial loop on a cooling part of the curve.