Magnetocrystalline anisotropy of YCo5 and related RECo5 compounds

Thermodynamics and Magnetism of YCo5 Compound Doped with Fe and Ni: An Ab Initio Study

YCo5 permanent magnet exhibits high uniaxial magnetocrystalline anisotropy energy and has a high Curie temperature. These are good properties for a permanent magnet, but YCo5 has a low energy product, which is notably insufficient for a permanent magnet. In order to improve the energy product in YCo5, we suggest replacing cobalt with iron, which has a much bigger magnetic moment. With a combination of density-functional-theory calculations and thermodynamic CALculation of PHAse Diagrams (CALPHAD) modeling, we show that a new magnet, YFe3(Ni1-xCox)2, is thermodynamically stable and exhibits an improved energy product without significant detrimental effects on the magnetocrystalline anisotropy energy or the Curie temperature.

Crystal structure prediction of magnetic materials

We present a methodology to predict magnetic systems using ab initio methods. By employing crystal structure method and spin-polarized calculations, we explore the relation between crystalline structures and their magnetic properties. In this work, testbed cases of transition metal alloys (FeCr, FeMn, FeCo and FeNi) are study in the ferromagnetic case. We find soft-magnetic properties for FeCr, FeMn while for FeCo and FeNi hard-magnetic are predicted. In particular, for the family of FeNi, a candidate structure with energy lower than the tetrataenite was found. The structure has a saturation magnetization (M _s) of 1.2 MA m^-1, magnetic anisotropy energy (MAE) above 1200 kJ m^-3 and hardness value close to 1. Theoretically, this system made of abundant elements could be the right candidate for permanent magnet applications. Comparing with the state-of-the-art (Nd₂Fe₁₄B) hard-magnet, (M _s of 1.28 MA m^-1 and MAE of 4900 kJ m^-3) is appealing to explore this low energy polymorph of FeNi further. Considering the relatively limited number of magnets, predicting a new system may open routes for free rare-earth magnets. Furthermore, the use of the computational algorithm as the one presented in this work, hold promises in this field for which in near future improvements will allow to study numerous complex systems, larger simulations cells and tackled long-range antiferromagnetic cases.

Torque magnetometry study of the spin reorientation transition and temperature-dependent magnetocrystalline anisotropy in NdCo5

We present the results of torque magnetometry and magnetic susceptibility measurements to study in detail the spin reorientation transition (SRT) and magnetic anisotropy in the permanent magnet NdCo₅. We further show simulations of the measurements using first-principles calculations based on density-functional theory and the disordered local moment picture of magnetism at finite temperatures. The good agreement between theory and experimental data leads to a detailed description of the physics underpinning the SRT. In particular we are able to resolve the magnetization of, and to reveal a canting between, the Nd and Co sublattices. The torque measurements carried out in the ac and ab planes near the easy direction allow us to estimate the anisotropy constants, K ₁, K ₂ and K ₄ and their temperature dependences. Torque curves, τ(γ) recorded by varying the direction of a constant magnetic field in the crystallographic ac plane show a reversal in the polarity as the temperature is changed across the SRT (240 < T < 285 K). Within this domain, τ(γ) exhibits unusual features different to those observed above and below the transition. The single crystals of NdCo₅ were grown using the optical floating zone technique.

Magnetocrystalline anisotropy in cobalt based magnets: a choice of correlation parameters and the relativistic effects

The dependence of the magnetocrystalline anisotropy energy (MAE) in MCo₅ (M  =  Y, La, Ce, Gd) and CoPt on the Coulomb correlations and strength of spin orbit (SO) interaction within the GGA  +  U scheme is investigated. A range of parameters suitable for the satisfactory description of key magnetic properties is determined. We show that for a large variation of SO interaction the MAE in these materials can be well described by the traditional second order perturbation theory. We also show that in these materials the MAE can be both proportional and negatively proportional to the orbital moment anisotropy (OMA) of Co atoms. Dependence of relativistic effects on Coulomb correlations, applicability of the second order perturbation theory for the description of MAE, and effective screening of the SO interaction in these systems are discussed using a generalized virial theorem. Such determined sets of parameters of Coulomb correlations can be used in much needed large scale atomistic simulations.

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

Magnetocrystalline anisotropy, the microscopic origin of permanent magnetism, is often explained in terms of ferromagnets. However, the best performing permanent magnets based on rare earths and transition metals (RE-TM) are in fact ferrimagnets, consisting of a number of magnetic sublattices. Here we show how a naive calculation of the magnetocrystalline anisotropy of the classic RE-TM ferrimagnet GdCo_{5} gives numbers that are too large at 0 K and exhibit the wrong temperature dependence. We solve this problem by introducing a first-principles approach to calculate temperature-dependent magnetization versus field (FPMVB) curves, mirroring the experiments actually used to determine the anisotropy. We pair our calculations with measurements on a recently grown single crystal of GdCo_{5}, and find excellent agreement. The FPMVB approach demonstrates a new level of sophistication in the use of first-principles calculations to understand RE-TM magnets.