Unique interplay between magnetic irreversibility and vortex behaviour in the ferromagnetic Fe-based superconductor EuFe₂(As_1-xP_x)₂

The EuFe₂(As_1-xP_x)₂ system features a complex phase diagram characterised by various magnetically-ordered phases. Intriguingly, within the range of approximately 0.15 < x < 0.3, a region of superconductivity emerges, reaching a maximum transition temperature of T_c ≈ 25 K at x ≈ 0.21. Notably, this superconducting dome overlaps with a ferromagnetic phase that onsets at T_FM ∼ 19 K.

Conventionally, ferromagnetism is perceived as antagonistic to singlet superconductivity, given the substantial internal exchange energy that typically exceeds the condensation energy. However, in EuFe₂(As_1-xP_x)₂, the spatial separation of the superconducting FeAs layers and magnetically ordered Eu ions within the crystal structure enables the coexistence of these two distinct orders. This unique situation presents an exciting opportunity to investigate the impact of intrinsic magnetic ordering on the superconducting state as the magnetic order is "switched on".

To explore this scenario, we systematically examine the magnetic hysteresis and magnetic relaxation properties of single crystals of EuFe₂(As_1-xP_x)₂, encompassing both T_FM < T_c and T_c < T_FM. Our findings indicate that the observed magnetic irreversibility is strongly dependent on the presence of both the magnetic and superconducting orders. Furthermore, our investigation reveals the significant impact on vortex dynamics stemming from both the magnitude of magnetic irreversibility and the underlying magnetic microstructure.