First- and second-order magnetic phase transitions in (R-Y)Co₂and R(Co-Al)₂(R=heavy rare-earth element) compounds

The types of magnetic phase transitions (first- or second-order) in (R-Y)Co2 and RCo2 (R=heavy rare-earth element) under high pressure are studied by making use of a phenomenological model where the coexistence of localised moments on R atoms and itinerant d electrons is assumed. The concentration dependence of the Curie temperature TC of R(Co-Al)2 is calculated in the same model. The calculated results of the boundary between the first- and second-order transitions are consistent with the observed results for (Er-Y)Co2 and (Ho-Y)Co2. The calculated values of TC for R(Co-Al)2 agree well with the experimental ones.