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

Abstract

Computational design of more efficient rare earth/transition metal (RE-TM) permanent magnets requires accurately calculating the magnetocrystalline anisotropy (MCA) at finite temperature, since this property places an upper bound on the coercivity. Here, we present a first-principles methodology to calculate the MCA of RE-TM magnets which fully accounts for the effects of temperature on the underlying electrons. The itinerant electron TM magnetism is described within the disordered local moment picture, and the localized RE-4f magnetism is described within crystal field theory. We use our model, which is free of adjustable parameters, to calculate the MCA of the RECo5 (RE = Y, La-Gd) magnet family for temperatures 0-600 K. We correctly find a huge uniaxial anisotropy for SmCo5 (21.3 MJm -3 at 300 K) and two finite temperature spin reorientation transitions for NdCo5. The calculations also demonstrate dramatic valency effects in CeCo5 and PrCo5. Our calculations provide quantitative, first-principles insight into several decades of RE-TM experimental studies. arXiv:1910.07436v1 [cond-mat.mtrl-sci]

Spin Orientation and Magnetostriction of Tb1−xDyxFe2 from First Principles

“…We have previously used the Y-analog method to calculate CF coefficients for various RE-transition-metal compounds [25], demonstrating its applicability to describe temperature-and pressure-induced spin-reorientation transitions in the RECo 5 compounds [37,42,43]. Substitution of Tb or Dy with Y to calculate the CF is consistent with the assumptions of the single-ion model [34], namely that the CF depends on the valence electronic structure and not on the RE-4f electrons themselves.…”

“…, −J [36]. Now, we should construct a Hamiltonian for the RE-4f electrons including the crystal, exchange, and external fields, and diagonalize it within the manifold of states with different M J [37]. Without the crystal and external fields, the ground state will be |L, S, J , −J , with the quantization axis (the magnetic moment direction) aligned with the exchange field.…”

“…This requirement is satisfied by using the Y-analog model, where the RE-4f electrons do not enter the calculation of the CF potential at all [25]. These same considerations have led us to adopt a similar scheme in the calculation of finite-temperature anisotropy of the RCo 5 compounds [37].…”

Caloric effects around phase transitions in magnetic materials described by ab initio theory: The electronic glue and fluctuating local moments

“…where Ω {m n }, H, P, σ αβ , T is a magnetic energy of the material, which can include the effect of an external magnetic field H, applied pressure P and mechanical stress σ αβ . Ω {m n }, H, P, σ αβ , T is obtained as an average over local moment configurations of the grand potential of the interacting electrons of a material with spin polarization constrained to {ê n } [4][5][6]11,12,14 and S mag = n S n is the total entropy of the local moments (S n are singlesite magnetic entropies). The equilibrium state of the system for specific values of the temperature, T , and applied field H and/or P and σ αβ , is given by the set of order parameters {m n } which minimizes the Gibbs free energy function G 1 4,6 .…”

“…Our recent applications of the theory include the firstorder ferromagnetic (FM) to antiferromagnetic (AFM) transition around room temperature and large magnetocaloric effect in the much studied FeRh ordered alloy 6 , metamagnetic critical fields in CoMnSi-based alloys 7 , FM, AFM and canted magnetic phases in lanthanide intermetallics 8 , the magnetic field and temperature induced transitions between long period helical AFM, fan and FM phases in the heavy lanthanide elements 9 , the frustrated magnetism and mechanocaloric effects in the Mn-based antiperovskite nitrides 10 , temperature dependent permanent magnetic properties 5 , and transitions between paramagnetic, ferrimagnetic, collinear AFM and non-collinear triangular AFM phases in the Mn 3 A class of materials together with the influence of strain and volume change 4 .…”