Intriguing behavior of UCo1-xRhxGe ferromagnets in magnetic field along the b axis

Single crystals of solid solutions of ferromagnetic superconductors UCoGe and URhGe were investigated by magnetization and specific-heat measurements. These compounds behave as anisotropic ferromagnets with the easy-magnetization direction along the orthorhombic c axis. The maximum Curie temperature T-C = 20K has been observed in UCo1-xRhxGe for x = 0.4. The main interest of the study was the intriguing behavior in magnetic fields applied along the b axis. The temperature dependence of the b-axis magnetic susceptibility exhibits a maximum at a characteristic temperature T-max. For low Rh concentrations (x < 0.4)T-max > T-C and the magnetic entropy S-mag at T-C is very low, whereas T-max = T-C and the S-mag at T-C is much higher for x >= 0.4. A metamagneticlike anomaly in magnetization is observed in all compounds, at a critical field which appears on a similar energy scale with the value of the corresponding Tmax. The character of three paramagnetic regimes which can be distinguished in the T-x and H-x phase diagrams, normal, correlated, and field polarized, is discussed in relation to the field-induced reentrant superconductivity in the parent compounds.