Magnetic phase transitions in the UFe1-xCoxAl system

We present the results of x-ray-powder diffraction, magnetic susceptibility, magnetization, electrical resistivity, and Mossbauer effect measurements performed on the UFe1-xCoxAl system. The lattice parameters determined for the system show a sharp maximum for a(x) and minimum for c(x) approximately for x approximate to 0.3. From the point of view of magnetic properties this system appears to have two main concentration regions: (a) 0 less than or equal to x < 0.5 and (b) 0.5 less than or equal to x less than or equal to 1.0. The phases falling into the first region do not show a long-range magnetic ordering, while in the second region they become ferromagnetic with the maximum values of the Curie temperature and spontaneous magnetic moment at x approximate to 0.8. The ferromagnetic properties of the alloys at B = 0 T with 0.5 less than or equal to x less than or equal to 0.98 are also confirmed by the Mossbauer effect experiment. Furthermore, we compare the magnetic behavior of UFe1-xCoxAl with that of the analogous UFe1-xNixAl. We find that the maximum ferromagnetic response appears in both systems by introduction to them of approximately 0.8 or 1 more electron, i.e., for the x = 0.8 and 0.5 compositions, respectively. Moreover, we discuss the complex magnetic properties of pure U-Co-Al in more details and some view on its ground state is given.