Balancing structural distortions via competing 4f and itinerant interactions: a case of polymorphism in magnetocaloric HoCo₂

Abstract

The ground state of HoCo2is governed by 4f electrons while the role of itinerant electrons in magnetism is reduced.

Role of4felectrons in crystallographic and magnetic complexity

“…5d), which is typical for first order ferromagnetic/ferrimagnetic to paramagnetic transitions, while the magnetic field dependence of T 1 is virtually nonexistent. This behavior is different from HoCo 2 , [16] where temperatures of both transitions are increasing with rising magnetic field. The maxima of the magnetocaloric effect -property that is critical for the emerging solid-state magnetocaloric refrigeration technology -calculated from the heat capacity data are: ΔS M = -15.5 J kg -1 K -1 at T C and -5.2 J kg -1 K -1 at T 1 , and ΔT = 5.8 K and 2.2 K, respectively, for ΔH = 50 kOe.…”

“…The total energy lowers further by 11.1 meV/cell in the Imma-type orthorhombic structure, confirming the ground state structure observed experimentally. A similar cubic to tetragonal to orthorhombic phase transformation sequence occurs in HoCo 2 , [16] while a direct cubic to orthorhombic transformation has been observed in HoAl 2 . [25] The symmetry of the orthorhombic structure in both cases is higher and corresponds to Fddd group.…”

“…These transformations are basically identical to that observed in the binary DyCo 2 [14] and the volume changes at the first order transition in both compounds are practically the same (ΔV/V = 0.29 %). other RCo 2 systems which adopt ground state Fddd symmetry, [16] where the Co atoms remain symmetrically equivalent just like in both the cubic and tetragonal polymorphs.…”

“…[13] HoCo 2 is a special case, where the compound exhibits two successive transformations: a first-order cubic to tetragonal at T C ; and a second-order distortion coupled with spin re-orientation that lowers symmetry further from the tetragonal to orthorhombic leading to Fddd space group symmetry via a rearrangement of both 3d states of Co and 5d states of Ho. [16] Although times flipping over after each melting to ensure homogeneity. The weight losses after the melting were less than 1 %, thus the nominal compositions were accepted as the actual ones.…”

Negative Thermal Expansion in the Materials With Giant Magnetocaloric Effect

“…Figure 14B shows the crystal and magnetic structure of the TbCo 1.9 Fe 0.1 compound at 10 K, the moments of Tb and Co atoms are antiparallel alignment along the c axis, which is similar to that of ErCo 2 and TbCo 2 . However, NdCo 2 and HoCo 2 (Ouyang et al, 2005b; Mudryk et al, 2016) undergo two crystallographic transitions: the room temperature cubic lattice (space group Fd$\overline{3}$m) transforms into the tetragonal one (I4 1 /amd) at T C , and then becomes orthorhombic (space group: Fddd) below spin orientation T SR .…”