EuCo2P2: A model molecular-field helical Heisenberg antiferromagnet

Abstract

The metallic compound EuCo2P2 with the body-centered tetragonal ThCr2Si2 structure containing Eu spins-7/2 was previously shown from single-crystal neutron diffraction measurements to exhibit a helical antiferromagnetic (AFM) structure below TN = 66.5 K with the helix axis along the c axis and with the ordered moments aligned within the ab plane. Here we report crystallography, electrical resistivity, heat capacity, magnetization and magnetic susceptibility measurements on single crystals of this compound. We demonstrate that EuCo2P2 is a model molecular-field helical Heisenberg antiferromagnet from comparisons of the anisotropic magnetic susceptibility χ, highfield magnetization and magnetic heat capacity of EuCo2P2 single crystals at temperature T ≤ TN with the predictions of our recent formulation of molecular field theory. Values of the Heisenberg exchange interactions between the Eu spins are derived from the data. The low-T magnetic heat capacity ∼ T 3 arising from spin-wave excitations with no anisotropy gap is calculated and found to be comparable to the lattice heat capacity. The density of states at the Fermi energy of EuCo2P2 and the related compound BaCo2P2 are found from the heat capacity data to be large, 10 and 16 states/eV per formula unit for EuCo2P2 and BaCo2P2, respectively. These values are enhanced by a factor of ∼ 2.5 above those found from DFT electronic structure calculations for the two compounds. The calculations also find ferromagnetic Eu-Eu exchange interactions within the ab plane and AFM interactions between Eu spins in nearest-and next-nearest planes, in agreement with the MFT analysis of χ ab (T ≤ TN).

Helical antiferromagnetic ordering in EuNi1.95As2 single crystals

“…Within MFT, this turn angle indicates dominant AFM nearest-layer and also next-nearest-layer Eu-Eu interactions [42,43]. Similar incommensurate Eu helical spin structures along the c axis with almost the same propagation vector were found in the isostructural compounds EuCo 2 P 2 and EuCo 2 As 2 [17,19,21,22].…”

“…High-pressure studies on EuCo 2 P 2 showed that the system changes its magnetic character from Eu(4f )-sublattice ordering to Co(3d)-sublattice ordering coincident with a pressure-induced first-order ucT to collapsed tetragonal (cT) structural transition [18]. We recently showed that EuCo 2 P 2 at ambient pressure is a textbook example of a noncollinear helical AFM for which the thermodynamic properties in the AFM state are well described by our so-called unified molecular field theory (MFT) [19]. Similarly, single crystals of EuCo 2 As 2 , which is isostructural and isoelectronic to EuCo 2 P 2 , exhibit coplanar helical AFM ordering of Eu +2 spins below 47 K but with anomalously-enhanced effective and/or ordered moments [8,[20][21][22].…”

Anomalous Composition-Induced Crossover in the Magnetic Properties of the Itinerant-Electron AntiferromagnetCa1−xSrxCo2−yAs2Sangeetha1, Smetana2, Mudring3 et al. 2017Phys. Rev. Lett.213540View full textAdd to dashboardCite

“…The χ ab (T ) data for x = 0.40 and 0.45 in Fig. 1, which remain relatively large with respect to χ c (T ) at T ≪ T N , suggest that the AFM structure in this x range is either extrinsically or intrinsically noncollinear [37][38][39]. In either case, the high-field M ab (H) behavior is not straightforward to interpret.…”

Flat-band magnetism and helical magnetic order in Ni-doped SrCo2As2

“…In classical molecular-field theory 32 , the parameter f ≡ T θ /T c is usually found to be smaller than 1 with −∞ < f < −1 for antiferromagnets and 1 for ferromagnets. This is because the real transition (either FM or AF) has to arise before the divergence of χ(q) which describes the instability of the system towards a particular magnetic state.…”