Anomalous magnetic response in the Au-Al-Gd 1/1 quasicrystal approximant

The magnetic response of the Tsai-type 1/1 Au-Al-Gd approximant crystals (ACs) was quantitatively investigated in terms of the magnetic entropy change ($\Delta S_{M}$) for different magnetic ground states. A comprehensive $\Delta S_{M}$ map over a wide electron concentration range has been established, demonstrating the detailed variation of $\Delta S_{M}$ across the entire magnetic phase diagram in the Tsai-type 1/1 ACs. Near the boundaries of the ferromagnetic (FM) phase, a clear deviation from the mean-field theory (MFT) was observed in both the Curie temperature ($T_{C}$) and magnetic field ($H$) dependences of the maximum magnetic entropy change ($\Delta S_{M}^{max}$). Contrary to general expectations, a high $\Delta S_{M}^{max}$ (7.2 J/K mol-Gd under a 5 T field variation), even comparable to those of candidate materials for low-temperature magnetic refrigeration, was obtained within the antiferromagnetic (AFM) region near the FM / AFM phase boundary. The unexpected enhancement of $\Delta S_{M}^{max}$ toward the AFM region under high magnetic fields indicates an anomalous magnetic response in the present Tsai-type 1/1 AC, which is presumably associated with the breakdown of the MFT. The present finding suggests that tuning the magnetic ground state across the phase boundary is an effective strategy to enhance $\Delta S_{M}$, even in general rare-earth intermetallic compounds.