Anomalous magnetocaloric effect in van vleck paramagnet HoVO4 near energy level crossing

The anomalous magnetocaloric effect in singlet rare-earth paramagnets near energy level crossing has been experimentally observed for the first time. The magnetization and differential magnetic susceptibility of Ho_{1 − x} Y _x VO₄ crystals (x = 0, 0.5) measured along the tetragonal axis in a pulsed magnetic field with various change rates near the crossover field, dH/dt, from 3 × 10³ to 2.5 × 10² T/s are compared to the data in static magnetic fields down to 0.1 K. These data indicate the large negative magnetocaloric effect in the pulsed magnetic field, so that the crystal with the initial temperature T ₀ = 4.2 K is cooled much lower than 1 K. The experimental data are qualitatively described in the crystal field model with various interactions including hyperfine interactions and with the known interaction parameters.