Pressure-induced change of magnetic order in Tb 1-x Y x NiAl and TbNi 1-x Cu x Al

We have studied the influence of hydrostatic pressure on a transition from antiferromagnetic (afm) to ferromagnetic (fm) order that occurs in the intermetallic series Tb1-xYxNiAl and TbNi1-xCuxAlbetween x=0 and x=0.05, using neutron powder diffraction. For all these compounds the ordering temperature is TC=47 K. The ternary compound TbNiAl shows afm order. A critical value of x exists in both series where both fm and afm types of domains coexist in parallel. We have already established earlier that applying hydrostatic pressure shifts the balance between the two types of domains to favour afm ones. New results are presented which suggest that the transition is actually more complicated than previously assumed: on increasing the hydrostatic pressure stepwise, fm domains first grow, but above p=2 kbar a gradual change to afm order is observed. At p=8 kbar the transition is complete: no fm domains are detected at this pressure.