Non saturating linear resistivity up to 900 K in MgB2

The thermal dependence of the linear and nonlinear ac susceptibilities of the ferromagnetic Y2Fe17 and of the ferrimagnetic Er2Fe17 intermetallic compounds is investigated as a function of the amplitude and frequency of the applied ac field. In both compounds, the change of sign at T-C and the behavior below and above the ferromagnetic or ferrimagnetic transition temperature are well explained in terms of divergence of the nonlinear susceptibility. Low temperature transitions detected by real and imaginary components of the fundamental ac susceptibility are related to the coupling of domain-wall dynamics and mobility defects. The thermal behavior of the linear and third order susceptibilities has been tentatively divided in different regions with different relaxation phenomena. Moreover, nonlinear effects due to the annealing procedure have been analyzed and discussed.