Magnetic clusters on the verge of long range ferromagnetism in Lu(Fe_0.75Al_0.25)₂and Lu(Fe_0.50Al_0.50)₂alloys

Lu(Fe_0.75Al_0.25)₂and Lu(Fe_0.50Al_0.50)₂alloys 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 T_SROand T_f, and magnetic irreversibility. The peak at T_SROvanishes and the one at T_fsmears out when high fields (H ≥ 1 kOe) are applied. The thermal variation of AC susceptibility shows that both transitions at T_SROand T_fare frequency dependent. Neither magnetic splitting nor broadening is observed in the Mössbauer spectra of both compounds in the T_f< T < T_SROthermal range. A magnetically split spectrum is only observed in Lu(Fe_0.75Al_0.25)₂for T < T_f.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 T_SROa short range ferromagnetic order takes place within the clusters, which become frustrated at T_f, when the random anisotropy of the clusters enables their freezing.