Magnetic order and superconductivity in Ho(Ir/sub x/Rh/sub 1-x/)4B4 compounds

Magnetization data for Ho(Ir/sub x/Rh/sub 1-x/)4B4 compounds in the temperature range 1.4 to 4.2 K and in fields up to 68 kOe are presented. Evidence of antiferromagnetic order below T/sub m/approx. =1.75 K, is found for x = 0.5. This modifies the phase diagram drawn previously and suggests that the boundary T/sub m/(x) might be continuous for 0< or =x< or =1. The order seems to be complex and to change from ferromagneticlike to antiferromagneticlike near x = 0.17, the only concentration at which chi is isotropic (an ''easy'' axis at x = 0 becomes ''hard'' for x>0.17). Antiferromagnetic interactions between ions appears to play a role at all concentrations. The higher is x, the larger is the difference between the ordering temperature T/sub m/ and the Curie-Weiss intercept theta. In Ho(Ir/sub 0.17/Rh/sub 0.83/)4B4, T/sub m/ still coincides, within +- 0.05 K, with the lower superconducting critical temperature, T/sub c/2, and the value of the upper critical field, H/sub c/2, drops, by cooling, in a narrow interval above T/sub c/2. For the antiferromagnetic Ho(Ir/sub 0.7/Rh/sub 0.3/)4B4 compound (T/sub m/ = 2.7 K) spin flop or distortion of the magnetic structure is detectable, at 1.4 K, for low values of the external field