Fathoming the anisotropic magnetoelasticity and magnetocaloric effect in GdNi

Abstract

Intermetallic GdNi adopts a CrB type of crystal structure (space group $Cmcm$), and it orders ferromagnetically via a second-order phase transition at 70 K, exhibiting unusually strong spontaneous striction along the three independent crystallographic axes in the ferromagnetically ordered state. We introduce a microscopic model to describe anisotropic changes of lattice parameters and elastic contribution to magnetocaloric effect of GdNi. In the model, results of density functional theory (DFT) calculations are used as inputs into a Hamiltonian that includes elastic energy of an anisotropic crystal lattice, exchange interactions, and Zeeman effect. The magnetic and elastic Hamiltonians are coupled through an anisotropic Bean-Rodbell model of magnetoelastic interactions. This coupling gives rise to anisotropic changes in the lattice parameters observed experimentally, and the model reveals good to reasonable agreements between the current theoretical results and earlier experimental data, thus validating the model within the limits of assumptions made. We also show that DFT calculations with $4f$ electrons of Gd treated as core electrons lead to a more adequate estimate of elastic constants of GdNi in comparison with the $\mathrm{LDA}+U$ method where $4f$ electrons are treated as valence electrons.