Electrical resistivity singularities and spin reorientation transitions in Pr2(Co1-xFex)17

The temperature- and field-induced spin reorientation transitions in uniaxial ferromagnetic Pr 2 (Co 1-x Fe x ) 17 pseudo-compounds are studied using high-accuracy measurements of the electrical resistivity derivatives d rho /dT and d rho /dH. This technique avoids the need for single crystal samples. For x=0.2, a first-order spin reorientation transition is precisely located at T sr =190K, where a sharp peak occurs in d rho /dT. The spontaneous magnetisation M s then rotates abruptly from the basal plane to the c axis. A minute discontinuity was found in the resistivity at T sr , Delta rho / rho approximately=0.003. For x=0, no spin reorientation transition is observed, confirming previous (single crystal) information that M s always sits in the basal plane. For x=0.2 and T=77K, a field-induced spin reorientation transition was observed for a critical value H. c =5.0+or-0.2 kG, with a pronounced anomaly in d rho /dH. This just corresponds to the first-order M s rotations inside those particular crystallites with c parallel to H (hard direction). Theoretical considerations are developed to characterise the anomalies associated with the spin reorientations in polycrystalline samples.