Anomalous magnetization cycle of UFe₄Al₈ single crystals: A Mössbauer effect study

An ⁵⁷Fe Mössbauer spectroscopy study was performed on an UFe₄Al₈ single crystal, in an external magnetic field B_ext, subtending an angle of 11° at the b axis. It shows that, even in low fields, the configuration of the Fe magnetic moments, μ_Fe, is consistent with the canted antiferromagnetic order of μ_Fe, established by single-crystal neutron diffraction in a field of 4.6 T. The canting of μ_Fe is always observed towards the direction of the U magnetic moments μ_U, clearly showing that it is induced by μ_U rather than by B_ext. At a certain value of B_ext during the magnetic domain rotation, the direction of μ_Fe is found to be temporarily frozen approximately along the direction of B_ext. This confirms previous magnetization and magnetoresistance data, which show that for the same values of B_ext,μ_U is blocked perpendicularly to B_ext. Furthermore, the present study reveals that this blocking is a metastable state with a relaxation time of a few hours, at 4.2 K.