Crystal-field splittings in rare-earth-based hard magnets: Anab initioapproach

Abstract

We apply the first-principles density functional theory + dynamical mean field theory framework to evaluate the crystal-field splitting on rare-earth sites in hard magnetic intermetallics. An atomic (Hubbard-I) approximation is employed for local correlations on the rare-earth 4f shell and selfconsistency in the charge density is implemented. We reduce the density functional theory selfinteraction contribution to the crystal-field splitting by properly averaging the 4f charge density before recalculating the one-electron Kohn-Sham potential. Our approach is shown to reproduce the experimental crystal-field splitting in the prototypical rare-earth hard magnet SmCo5. Applying it to RFe12 and RFe12X hard magnets (R =Nd, Sm and X =N, Li), we obtain in particular a large positive value of the crystal-field parameter A 0 2 r 2 in NdFe12N resulting in a strong out-of-plane anisotropy observed experimentally. The sign of A 0 2 r 2 is predicted to be reversed by substituting N with Li, leading to a strong out-of-plane anisotropy in SmFe12Li. We discuss the origin of this strong impact of N and Li interstitials on the crystal-field splitting on rare-earth sites.

Rare-earth/transition-metal magnets at finite temperature: Self-interaction-corrected relativistic density functional theory in the disordered local moment picture

“…Interestingly, a recent neutron diffraction experiment reported even larger local moments in SmCo 5 , which add up to give a resultant magnetization in excess of 12µ B /FU [73]. Studies employing DMFT and open-core calculations have reported smaller Sm total moments of approximately -0.3µ B , which would bring the total SmCo 5 moment closer to 8µ B /FU [49][50][51]. Earlier GGA+U calculations found a much larger total moment of 9.9µ B /FU due to a ferromagnetic alignment of Sm and Co spins.…”

Temperature-dependent magnetocrystalline anisotropy of rare earth/transition metal permanent magnets from first principles: The light RCo5 (R=Y, La-Gd) intermetallics

“…First, RE-TM magnetism originates from both itinerant electrons and more localized lanthanide 4f electrons [11]. Although the local spindensity approximation to density-functional theory [12] (DFT) satisfactorily describes the itinerant electrons, the 4f electrons require specialist techniques like dynamical mean field theory [13,14], the local self-interaction correction (LSIC) [15], the open-core approximation [16][17][18], or Hubbard +U models [19,20]. Second, DFT calculations are most easily performed at zero temperature, but under actual operating conditions the RE-TM magnetic moments are subject to a considerable level of thermal disorder [21].…”

Calculating the Magnetic Anisotropy of Rare-Earth–Transition-Metal Ferrimagnets

“…An impressive body of theoretical work based on crystal field theory has been built up over decades [10], where model parameters are determined from experiment (e.g. Ref.[11]) or electronic structure calculations [12][13][14]. An alternative and increasingly more common approach is to use these electronic structure calculations, usually based on density-functional theory (DFT), to calculate the material's magnetic properties directly without recourse to the crystal field picture [15][16][17][18][19].Calculating the MCA of RE-TM magnets presents a number of challenges to electronic structure theory.…”