Magnetization and magnetoacoustics of single-crystalline ErFe₅Al₇ in high magnetic fields

The magnetization and sound propagation in single-crystalline ErFe₅Al₇ (tetragonal crystal structure) have been studied in steady (up to 18 T) and pulsed magnetic fields (up to 60 T). The compound orders ferrimagnetically at a Curie temperature T _C=201 K and has a compensation point at T _comp=34 K. ErFe₅Al₇ displays a strong magnetic easy-plane anisotropy. A strong magnetic anisotropy is present as well within the basal plane; the [100] axis is the easy magnetization direction with a spontaneous magnetic moment M _s=1.3 µ_B/f.u. at 2 K. Field-induced magnetic transitions, two along the [100] axis and two along the [110] axis, have been found in the vicinity of T=T _comp. Changes in the magnetic state at the transitions result in significant alterations of the spin–phonon coupling, which is manifested by sharp anomalies in the sound velocity and sound attenuation. Along the easy [100] axis the forced ferromagnetic state is reached in a field of about 50 T at 2 K, whereas along the [110] direction saturation is expected only above 60 T. A magnetic field-temperature phase diagram has been extracted up to 60 T. From the experimental data a value of n _ErFe=3.3 T/µ_B for the inter-sublattice Er–Fe exchange interaction has been obtained.