Magnetic clusters on the verge of long range ferromagnetism in Lu(Fe0.75Al0.25)2 and Lu(Fe0.50Al0.50)2 alloys

Lu(Fe0.75Al0.25)2 and Lu(Fe0.50Al0.50)2 alloys have been produced by arc-melting. DC and AC magnetization, Mössbauer spectroscopy, specific heat and neutron diffraction experiments have been performed to determine their magnetic ground state. DC magnetization curves measured at low fields (H < 1 kOe) evidence two peaks, at TSRO and Tf, and magnetic irreversibility. The peak at TSRO vanishes and the one at Tf smears out when high fields (H ≥ 1 kOe) are applied. The thermal variation of AC susceptibility shows that both transitions at TSRO and Tf are frequency dependent. Neither magnetic splitting nor broadening is observed in the Mössbauer spectra of both compounds in the Tf < T < TSRO thermal range. A magnetically split spectrum is only observed in Lu(Fe0.75Al0.25)2 for T < Tf. The absence of long range ferromagnetic order in both alloys is confirmed by specific heat and neutron diffraction measurements. The overall interpretation points to a clustered magnetic environment due to the random positions of Fe atoms in the crystallographic cell. At TSRO a short range ferromagnetic order takes place within the clusters, which become frustrated at Tf, when the random anisotropy of the clusters enables their freezing.