To satisfy the rapidly growing demand for permanent magnets, the search for rare-earth-free large magnetic anisotropy materials has become prevalent. Rare-earth-free metal borides exhibit outstanding magnetic properties, but they have been outpaced by the rare-earth metal borides, including the best permanent magnet, Nd2Fe14B. Recently, our group synthesized quinary Ti3Co5B2-type series of rare-earth-free metal borides, Ti2FeRu5−nRhnB2 (n = 1−5, VE = 63−67 e-) and Sc2FeRu5−xIrxB2 (x = 0−5, VE = 60−65 e-), with tunable magnetic properties ranging from soft magnetic Ti2FeRh5B2 (Hc = 0.9 kA/m), to semi-hard magnetic Ti2FeRu4RhB2 (Hc = 23.9 kA/m) and hard magnetic Sc2FeRu3Ir2B2 (Hc = 51.6 kA/m). The cooperation of spin exchange, magnetic dipole-dipole (MDD) interactions, and spin-orbit coupling effect (SOC) was investigated using density functional theory (DFT) calculations in order to understand their magnetic anisotropy and relate it to their various coercivities, with the aim to predict new materials with large magnetic anisotropy. The overall contribution of SOC and MDD interactions are assessed by the magnetocrystalline anisotropy energy (MAE), defined as the difference of the total energies with SOC and MDD in easy and hard spin orientations (MAE = E∥c − E⊥c). Our calculations show that strong Fe−Fe inter-chain spin exchange interactions are necessary for large magnetic anisotropy. In addition, the contribution of MDD interactions to MAE in Ti3Co5B2-type compounds is much weaker compared with the SOC effect. Sc2FeRu3Ir2B2 has by far the largest MAE and strong Fe−Fe inter-chain spin exchange interactions, thus confirming its hard magnetic properties. We then targeted materials containing the less expensive and abundant Co for the replacement of Rh, Ru or Ir, or high spin-orbit coupling 4d, 5d early transition metals to substitute Ti and Sc. So far, we examined potential quaternary phases with general formula “T2T′Co5B2” (T = Ti, Hf; T′ = Mn, Fe). We found that “Hf2MnCo5B2” has large MAE (+ 0.96 meV/f.u.) but relatively weak inter-chain Mn−Mn spin exchange interactions, whereas for “Hf2FeCo5B2” relatively smaller MAE (+ 0.17 meV/f.u.) but strong Fe−Fe inter-chain spin exchange interactions. Therefore, these two phases are good candidates for new rare-earth-free, semi-hard to hard magnetic materials.

Density functional investigation on the magnetic anisotropy in rare-earth-free metal borides with the Ti3Co5B2-type structure

253rd National Meeting (2017)

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