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Magnetic states and magnetic transitions in RCo2 Laves phases

Magnetic states and the order of the magnetic transitions in RCo2 cubic Laves phases with R = Gd–Tm have been studied by a first-principle density functional theory (DFT) calculation using a linear muffin tin orbital (LMTO) method. The total energy, Et, and magnetic moments are calculated as a function of lattice constant, a. For R = Dy, Ho and Er, Et(a) shows two minima at a smaller and a larger lattice volume, corresponding to a low and a high spin state of Co, respectively. Further, the fixed spin moment calculations indicate that the magnetic moment dependence of the total energy, Et(M), also exhibits two minima, corresponding to a low and a high spin state of Co at the calculated equilibrium lattice constant, respectively, for R = Dy, Ho and Er. The double minimum features of Et(a) and Et(M) are responsible for the first-order magnetic transition at TC in RCo2 with R = Dy, Ho and Er. On the other hand, for R = Gd and Tb, both Et(a) and Et(M) show a single minimum at a stable high spin state of Co. For TmCo2, Et(a) shows a single minimum at a stable low spin state of Co, while the corresponding Et(M) also has two minima corresponding to a low and a high spin state of Co. The results are also discussed using the Landau–Ginzburg theory by analysing the Landau expansion coefficients derived from the fixed spin-moment calculation results. The calculated lattice constant and magnetic moments are in good agreement with the experimental results and successfully explain the change in the order of the magnetic transition.