THE MAGNETIC PHASE-TRANSITIONS IN R(CO, AL)2 COMPOUNDS (R DY, HO, ER)

The type of the magnetic phase transition in the R(Co1-xAlx)2 (R = Dy, Ho, Er) compounds is studied by means of magnetization, thermal-expansion and electrical-resistivity measurements. With increasing Al content, a change from a first-order to a second-order transition is observed, at x congruent-to 0.075, 0.075 and 0.025 for Er, Ho and Dy, respectively. An explanation is offered in terms of the Inoue-Shimizu model by introducing a concentration dependence of T3, the temperature at which a3(T) changes sign (a3 is the coefficient of the M(d)4-term in the expansion of the free energy of the d-subsystem). In fact, T3, is taken proportional to T(m), the temperature at which the susceptibility of the corresponding Y(Co, Al)2 shows a maximum.