Study of magnetic transitions in Dy by means of reversible Villari effect

Application of a new method—high-sensitivity measurements of periodic stress-induced induction (reversible Villari effect, RVE)—allows us to uncover new effects in the low-field behaviour of different magnetic phases of polycrystalline Dy. A loss of magnetoelastic coupling (Villari critical point) is found around 166 K, close to the temperature which was supposed to separate the helical structure from the possible vortex one. We show that the low-field magnetic hysteresis emerges immediately below the Neel temperature and, below the Villari critical point, demonstrates a qualitative difference for cooling and heating from the ferromagnetic state. It has been found that, below the Villari critical point, in the helical phase, polycrystalline Dy is in essentially non-equilibrium state, revealed as a time-dependent relaxation of RVE. We relate the effects observed in the helical phase with thermal internal stresses and existence of lattice defects which inherit ferromagnetic order upon heating from the ferromagnetic state.