Specific heat investigation on the magnetic phase diagrams of 𝑅Ni₂B₂C ( 𝑅 = Gd, Er)

Abstract

The borocarbides RNi2B2C (R=magnetic rare earth) exhibit rich H-T magnetic phase diagrams. Using field-dependent specific heat measurements on single-crystals of RNi2B2C (R=Gd, Er), this work investigated the magnetic contribution to the specific heat when T and H are varied across these H-T phase diagrams. These measurements, together with the ones reported on HoNi2B2C, confirm that the overall evolution of each Cmag(T,H) curve is a faithful reflection of the features observed in the corresponding phase diagram: in particular the successive field-induced metamagnetic modes, appearing in the reported magnetization M(T,H) curves, are also manifested in these Cmag(T,H) curves, just as required by the Maxwell identity (∂Cmag/∂H)T=T(∂2M/∂T2)H. Within the lower ranges of temperature and fields, the calculations based on linearized field-dependent spin-wave theory are found to reproduce satisfactorily the measured Cmag(T,H) curves: accordingly, within these ranges, the thermodynamical properties of these compounds can be rationalized in terms of only two parameters, namely, the spin-wave energy gap and the stiffness coefficient. Based on the satisfactory agreement between theory and experiment, we are able to provide an explanation for the plateau like behavior observed in, say, the M(T,H) isotherms. Finally, for the particular case of GdNi2B2C wherein the anisotropy is dictated solely by the classical dipole interactions, the main features of its Cmag(T,H) are found to be reproduced by numerical calculations based on the model of Jensen and Rotter [Phys. Rev. B 77 (2008) 134408].