The order of the magnetic phase transitions in RCo₂(R = rare earth) intermetallic compounds

Abstract

It has been found experimentally that the order of the magnetic phase transitions in RCo 2 compounds (R standing for rare-earth metals) at T c changes from second order for the lightrare-earth series up to TbCo 2 to first order for the heavier-rare-earth compounds DyCo 2 , HoCo 2 and ErCo 2 . On the basis of results of fixed-spin-moment band-structure calculations for the isostructural compound YCo 2 at different lattice constants, we propose an explanation for this behaviour. In contrast to the widely accepted Inoue-Shimizu theory for this class of compounds, our explanation also includes Pr, Nd which were thought to behave differently due to the influence of crystal-field effects. We show that an itinerant-electron metamagnetic transition in these compounds can occur only over a certain range of lattice constants and that the possibility of a first-order phase transition is connected to features of the electronic structure rather than to the magnitude of the transition temperature as conjectured earlier. The influence of the latter is only important if the transition takes place at elevated temperatures, where effects of spin fluctuations can suppress a first-order transition.

Itinerant electron metamagnetism and magnetocaloric effect in Dy(Co,Si)2

“…These refined models have been found to be in good agreement with the experimental results in many compounds [6,7]. In the case of RCo 2 compounds, the model proposed by Khmelevskyi and Mohn [5] appears to be the unified model to describe IEM and the order of transition for both light and heavy rare earths.…”

“…1 that the magnetization changes abruptly at T C in DyCo 2 and the transition smears gradually with Si concentration. It has already been reported that DyCo 2 shows FOT and this is attributed to the IEM [3,5]. According to the model by Khmelevskyi and Mohn, the lattice parameter, 'a' of DyCo 2 is well within the range 7:05 , a , 7:22 A; thereby satisfying the condition for IEM.…”

“…Table 1 The values of lattice parameter ðaÞ; ordering temperature ðT C Þ; saturation magnetization ðM S Þ and 3d magnetic moment ðm 3d Þ in DyðCo 12x Si x Þ 2 compounds sublattice has a permanent moment above the ordering temperature, the Co sublattice would always be spontaneously magnetic and the transition at T C would be of second order, as seen in many intermetallics. This is the case in RCo 2 compounds containing light rare earths in which the lattice parameters are more than the critical value of 7.22 Å [5]. It is interesting to see that Duc et al [9] have reported the critical lattice parameter to be 7.27 Å for the enhancement of T C as a result of Al substitution in R(Co,Al) 2 compounds.…”

“…However, the second order transitions (SOT) observed in RCo 2 compounds containing rare earths other than Ho, Dy and Er could not be satisfactorily explained using these theories, though they predicted FOT in DyCo 2 , HoCo 2 and ErCo 2 [5]. Recently, these models have been corrected by incorporating the spin fluctuation and magnetovoulme effects [5][6][7]. These refined models have been found to be in good agreement with the experimental results in many compounds [6,7].…”

“…According to the earlier theories of IEM, the critical parameter that determines the possibility of IEM and thereby the order of transition is the molecular field or the ordering temperature. However, the second order transitions (SOT) observed in RCo 2 compounds containing rare earths other than Ho, Dy and Er could not be satisfactorily explained using these theories, though they predicted FOT in DyCo 2 , HoCo 2 and ErCo 2 [5]. Recently, these models have been corrected by incorporating the spin fluctuation and magnetovoulme effects [5][6][7].…”