Phase diagram and transport properties of Y_1-xNd_xCo₂ pseudo-binary alloys

The electrical resistivity ρ and the thermopower S of pseudo-binary Y_1-xNd_xCo₂ alloys have been measured at temperatures from 2 to 300 K under pressures up to 3.5 GPa. NdCo₂ is a ferromagnet with a Curie temperature T _C of 98 K. In the ferromagnetic phase, it undergoes a 4f spin-reorientation transition at T _R = 40 K. At this transition, the easy direction of 4f magnetization is changed from [100] at temperatures above T _R to [110] below T _R. The system phase diagram T _C(x, P) and T _R(x, P) is inferred from the temperature dependencies of the electrical resistivity and the thermopower. The Curie temperature of the alloys, TC decreases with decreasing Nd concentration x and vanishes around x _c = 0.3. Above x _c, the Curie temperature decreases with increasing pressure. The spinreorientation temperature weakly depends on the composition and on the pressure until it merges with T _C. There is a large region of the phase diagram, partly overlapping with the ferromagnetic phase, where the cobalt 3d electron system is non-uniformly magnetized due to spatial fluctuations of the 4f-3d exchange field related to a random distribution of Nd 4f magnetic moments over the R sublattice. In this region, large static 3d magnetic fluctuations govern the electronic transport.