Magnetism and magnetic anisotropy in UGa 2
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.314
Magnetism and magnetic anisotropy in UGa2 Banhi Chatterjee and Jindřich Kolorenč Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 182 21 Praha, Czech Republic
We investigate whether first-principles calculations with an improved description of electronic correlations can explain the large magnetic moments and the strong magnetocrystalline anisotropy in the ferromagnetic compound UGa2. The correlations are treated within a static mean-field approximation DFT+U combining the density functional theory (DFT) with an onsite Hubbard interaction U. We find that DFT+U improves the agreement of the magnetic moments with the experiment compared to DFT but worsens the theoretical description of the magnetocrystalline anisotropy.
INTRODUCTION The uranium-uranium distances in UGa2 are as large as 4.2 Å, that is, they are well above the Hill limit 3.5 Å [1]. Indeed, large magnetic moments are formed at uranium atoms and the compound shows a ferromagnetic order below the critical temperature TC = 125 K. Experimentally, the magnetic moments point along the (100) direction and their size was found to be larger than 2.7 μ B per uranium atom by measuring the bulk magnetization [2] as well as by using the neutron diffraction [3]. A large magnetocrystalline anisotropy energy (MAE) of about −17 meV (198 K) per formula unit has been deduced in a single crystal of UGa 2 from the anisotropy of the magnetic susceptibility in the high-temperature paramagnetic phase, MAEexp = θ(100) − θ(001), where θ is the paramagnetic Curie temperature entering the Curie–Weiss law [2]. A large MAE indicates a higher energy cost for rotating the magnetic moments and it is a critical requirement for applications of a material as a hard magnet.
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Figure 1. Density of states calculated in LSDA and LSDA+U including the effects of the spin-orbit coupling. The total DOS is shown in solid blue, the U 5f contribution in dashed red, and the experimental photoemission spectrum [4] in green. The DOS from the two directions of polarization, (001) in the left panel and (100) in the right panel, are similar. In LSDA+U we use U = 1 eV (middle panel), and U = 2 eV (lower panel) with J = 0.4 eV in both the cases.
In UGa2, the large magnetic moments and MAE are attributed to the interaction of the uranium 5f electrons with the ligand field and a spin-orbit coupling (SOC). It is still debated whether these 5f electrons are itinerant or localized, in part due to inconclusive spectroscopic data [4,5]. The simple approximations to the density functional theory (DFT) – LDA or GGA, which assume the 5f electrons to be itinerant, do not reproduce the large magnetic moments. They also predict a low value of MAE (9 meV) with an incorrect easy axis (001) [6]. A
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