Tuning Spin and Magnetocrystalline Anisotropy of L10-ordered MnAl with Transition Metal Elements (TM = Mn, Fe, Co, Ni)*

Abstract

L10-ordered MnAl shows a high magnetic moment of 2.4 μB/f.u. and magnetocrystalline anisotropy constant (K) of 1.5 MJ/m3 [1,2]. The magnetocrystalline anisotropy and spin direction can be tuned by either the axial ratio c/a or substituting the Mn of MnAl by transition metals. Sakuma calculated Ku of L10-ordered equiatomic MnAl as a function of the axial ratio c/a ranging from 0.60 to 1.2 by first-principles calculation [1]. It was found that the spin direction changes to the out-of-plane direction (uniaxial) from the in-plane direction at c/a = 0.7071 by changing the magnetocrystalline anisotropy sign to positive from negative. The calculated magnetocrystalline anisotropy constant is 1.5 MJ/m3 at c/a = 0.9058. Sakuma used the lattice constants of a =3.93 Å and c =3.56 Å measured at 300 K in his first-principles calculations.On the other hand, Manchanda et al. have performed the first-principles calculation on Fe-substituted L10-ordered MnAl to investigate magnetocrystalline anisotropy constant and found that Fe increases the magnetocrystalline anisotropy constant to 2.5 MJ/m3 from 1.5 MJ/m3 of non-substituted MnAl. However, the total magnetic moment decreased, and the spin direction did not change [3]. Neither Sakuma nor Manchanda reported the Curie temperature for various c/a MnAl and Fe-substituted MnAl and relaxed the studied MnAl system.In this paper, we have systematically investigated the effects of transition metals (Fe, Co, Ni) on magnetization, magnetocrystalline anisotropy, and Curie temperature of L10-ordered MnAl.We used the WIEN2k package based on density functional theory (DFT) and the full-potential linearized augmented plane wave (FPLAPW) method to conduct the first-principles calculations [4]. All calculations used 19 × 19 × 27 k-point mesh generating 1400 k-points in the irreducible part of the Brillouin zone to obtain electronic structures of Mn0.5TM0.5Al (TM = Mn, Fe, Co, and Ni). Each TM has the following valence electrons (n): 7 for Mn (3d5 4s2), 8 for Fe (3d6 4s2), 9 for Co (3d7 4s2), and 10 for Ni (3d8 4s2). After having relaxed L10-ordered Mn0.5TM0.5Al (TM = Mn, Fe, Co, and Ni), we obtained the lattice constants, volumes, and c/a ratios in Table 1. It is found that the c/a ratio and volume decrease with an increase in n from 7 (Mn) to 10 (Ni) in Table 1.Figure 1 shows the density of states (DOS) for Mn0.5TM0.5Al (TM = Mn, Fe, Co, and Ni) with spin configuration. DOS significantly changes with the TM substitution. It is striking that Ni-substituted MnAl shows the ferrimagnetic spin configuration, as seen in Fig. 1(d). However, the ferromagnetic spin configuration is set in Mn0.5TM0.5Al (TM = Mn, Fe, and Co). As summarized in Table 1, the total magnetic moment decreases as the valence electron increases in the order of Ni, Co, and Fe.In order to calculate the K of the Mn0.5TM0.5Al system, the magnetocrystalline anisotropy energy (MAE) was calculated using the total energy difference between <001> and <100> spin configurations (ΔE = E<001> - E<100>). The Fe substitution leads to a significant increase in K from 1.34 MJ/m3 to 2.98 MJ/m3. When either Co or Ni replaces Fe, the K becomes negative and is -0.3 MJ/m3 for Co or -0.18 MJ/m3 for Ni in Table 1. This implies the magnetization (Co, Ni) direction changes to the in-plane direction from the out-of-plane direction, i.e., uniaxial (Mn and Fe). Curie temperature (TC) was calculated by the mean-field approximation (MFA). The TC dramatically decreases from 685 K(Mn) to 20 K(Ni). Alloying the MnBi with transition elements could tune the spin direction and magnetocrystalline anisotropy of L10-ordered MnAl. However, magnetization and Curie temperature are negatively affected by the alloying. We will discuss all these findings in detail in this presentation.* This work was supported in part by the E. A. “Larry” Drummond Endowment at the University of Alabama. **