Impedance and initial magnetic permeability of the Heusler compounds Pd2MnSn and Pd2MnSb near the Curie temperature

In this paper we report measurements of the complex impedance of the Heusler compounds Pd2MnSn and Pd2MnSb near the Curie temperature. From the experimental data, the complex initial magnetic permeability is obtained as a function of temperature and frequency. Both the real mu(') and the imaginary mu(') components of the permeability show a sharp maximum at a characteristic temperature T-peak which is located just below T-c. This property is known as the Hopkinson effect. In low frequencies this effect is more pronounced in Pd2MnSn. We attribute the high values of mu(') obtained in this compound to a high density of domain walls generated by antiphase boundaries (APBs). We show that mu(') behaves as a critical power law of the reduced temperature 1-T/T-c below T-peak in both compounds. Above T-peak, mu(') may also be described by a power law of 1-T/T-c with an unusually large exponent. We argue that a crossover between two different domain-wall fluctuation regimes occurs at this temperature. A Cole-Cole analysis allows the determination of the relaxation time distribution at several fixed temperatures in the ferromagnetic region of Pd2MnSn. These results were fitted to a modified Debye formula. The obtained fit parameters indicate that the dynamical properties of the permeability in this compound are governed by a broad and temperature-independent distribution of relaxation times. This behavior is characteristic of a system where magnetic domains are distributed within a large range of sizes that do not change significantly when the temperature approaches T-c.