Y.
Kinemuchi
*,
A.
Fujita
and
K.
Ozaki
National Institute of Advanced Industrial and Science and Technology (AIST), Nagoya 463-8560, Japan. E-mail: y.kinemuchi@aist.go.jp
First published on 2nd June 2016
L10 MnAl, which is a nonequilibrium ferromagnetic phase, is fabricated successfully with various compositions via high-pressure synthesis. The L10 phase is observed at pressures higher than 5 GPa, indicating that the volume effect is crucial for the stabilization of this phase. The employed synthesis route does not require a Mn-rich ε-phase, which has conventionally been used as the precursor compound. This allows for the synthesis of the L10 phase with a near-stoichiometric composition. In addition to variations in the composition in terms of the Mn/Al ratio, the axial ratio (c/a) as well as the ordering parameter (S) are modified systematically, with the maximum c/a and S values corresponding to the stoichiometric composition. With this structural change, the highest coercive force is also observed at the stoichiometric composition.
L10 MnAl can be considered as having a body-centered tetragonal structure and has been synthesized by using the ε-phase (hcp) as the mother phase. The ε-phase is a Mn-rich (typical composition of Mn0.55Al0.45) high-temperature phase that is stable at temperatures greater than 873 °C. The quenching of the ε-phase and its subsequent annealing (typically at approximately 500 °C) results in the formation of the L10 phase.9 L10 MnAl is known to decompose into nonmagnetic phases and is thus regarded as a metastable phase. In addition, synthesized MnAl compounds have a non-stoichiometric composition that deviates from the ideal one (Mn1Al1), depending on the synthesis route used. The excess Mn must occupy an antisite (Al site), which leads to antiferromagnetic (AFM) coupling with the Mn site and thus a decrease in the Ms value.
Historically, L10 MnAl has been considered a product formed by the displacive or martensitic transformation of the ε-phase.10 However, a detailed analysis performed using high-resolution transmission electron microscopy revealed that the growth front of the L10 phase is compositionally invariant with regard to the ε-phase that can be regarded as the mother phase. Furthermore, although the L10 phase nucleates heterogeneously at the previously existing ε-phase grain boundary, it forms incoherent interfaces.11 Hence, the proposed displacive or martensitic transformation must be discarded as the underlying mechanism. In other words, the formation of the ε-phase is not a necessary condition for the formation of the L10 phase.
The intrinsic properties of the L10 phase have been examined extensively using density functional theory (DFT). According to previous DFT studies, the Ms value of the L10 phase is 2.4μB per u.c., while its MAE value is 1.5–1.8 MJ m−3.12–14 It has also been predicted that extending the axial ratio (c/a) increases the MAE value.13 On the experimental side, however, pure MnAl has not yet been synthesized, owing to issues related to the synthesis method used, as mentioned earlier. Thus, the Ms value is lower than the theoretical value (1.9μB per u.c.).9 Therefore, there is a need to develop an alternative synthesis route that does not require the ε-phase.
It is interesting to note that the L10 MnAl phase possesses the smallest unit cell among all the phases near the stoichiometric composition in this system. Hence, the volume effect is thought to stabilize the L10 phase. In the present work, the high-pressure synthesis (HPS) of L10 MnAl was performed as an alternative synthesis route. Furthermore, the effects of the composition on the L10 structure as well as the resultant magnetic properties are discussed.
The HPS process was performed using a cubic anvil setup. The source materials were pelletized into cylinders with a diameter of 4.3 mm and a length of 6.2 mm. These small pellets were placed in a gold capsule, in order to prevent them from being contaminated during the synthesis process. Pyrophyllite was used as the pressure medium. Prior to the start of the experiments, the pressure acting on the sample was calibrated based on the phase transitions of Bi and Ba. The temperature was raised once the pressure had reached the desired level. It was then held at that level for 1 h and subsequently reduced while the pressure was maintained. Finally, the pressure was released once the temperature had dropped to room temperature. Typical samples after the HPS had dimensions of 4 mm in diameter and 4 mm in length.
The sample composition was analyzed using an energy-dispersive X-ray fluorescence spectrometer (EDX-8000, Shimadzu, Japan, for the average composition; and Quantax 70, Bruker AXS Japan, for the microanalysis). The X-ray powder diffraction pattern was obtained using parallel beam optics (RINT 2000, Rigaku, Japan). The pattern was then investigated using Rietveld analysis (Rietan 2000).16 The fraction of the product phases was estimated from the scale factors optimized in the Rietveld analysis. The ordering parameter (S) of the L10 phase was deduced from the relative intensity of the superlattice (h + k + l = odd) and fundamental (h + k + l = even):
The magnetic properties at room temperature were measured using a physical property measurement system (PPMS, Quantum Design Co.) equipped with a vibrating sample magnetometer.
Fig. 2 shows the strong effect of compositional variation on the constituent phases. With the increase in the Mn content, the L10 phase becomes stable instead of the γ2-phase, with the β-phases eventually becoming stable. In addition, Mn composition denoted as x corresponds to that of the sample: the averaged Mn content among the product phases. Fig. 3 shows the images of compositional mapping of a Mn0.52Al0.48 sample. The Mn rich phase exists as a secondary phase. On the other hand, the composition of the L10 phase was found to be similar to the average composition within the analytical variation of EDS. A similar trend was found for the other samples: the composition of L10 was the same as the average composition. The highest fraction of L10, which was approximately 90%, was found when the composition was slightly rich in Mn (0.52 at%); this concentration of Mn is still lower than that corresponding to the conventional synthesis route based on the ε-phase (approximately 0.54 at%) (see Fig. 4). In terms of the chemical ordering, the ordering parameter (S) as well as site occupancy at 1a and 1d sites were deduced (Fig. 5 and Table 1). As demonstrated in both analyses, chemical ordering was rationally dependent on the composition—the stoichiometric composition is crucial for the high degree of ordering.
x | 1a (0 0 0) | 1d (1/2 1/2 1/2) | ||
---|---|---|---|---|
Mn | Al | Mn | Al | |
0.47 | 0.932 | 0.068 | 0.008 | 0.992 |
0.50 | 0.993 | 0.007 | 0.007 | 0.993 |
0.52 | 0.995 | 0.005 | 0.045 | 0.955 |
0.53 | 0.994 | 0.006 | 0.066 | 0.934 |
0.55 | 0.986 | 0.014 | 0.114 | 0.886 |
The lattice parameters of the L10 phases are listed in Fig. 6. Although the cell volume was almost constant for all the compositions, the axial length, c, was the maximum at the stoichiometric composition, while the a-axis was the minimum. As a result, the axial ratio, c/a, was the highest value at this composition. This trend is conventionally thought to originate due to the decrease in the extent of Mn occupancy at the antisites in the case of the Mn-rich compositions, resulting in an increase in the c-axis length owing to the reduction in AFM coupling. On the other hand, the c/a value also decreased when the amount of Al was excessive. In an early DFT study, Sakuma pointed out that the tetragonal distortion resulting from the transformation of the cubic B2 structure into the L10 structure results in the splitting of the d bands, owing to the destruction of the degeneracy at the Fermi level, and suggested the existence of the solid-state Jahn–Teller effect in this system.7 Later, the structural stability of this system was investigated based on DFT. It was found that nonmagnetic MnAl preferentially forms a cubic B2 structure, highlighting the importance of magnetic interaction in the structure.18 In short, the FM state prefers the tetragonal L10 structure, and the tetragonality is expected to be weakened with a reduction in the magnetic moment. Thus, the reduction in tetragonality with the increase in the nonmagnetic Al content can be explained on this basis.
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Fig. 6 Lattice constants (c and a), axial ratio (c/a), and cell volume (V) values of the L10 phases. |
The typical magnetization curves of the test samples are shown in Fig. 7. Owing to the difference in the L10 contents, the saturation magnetization (Ms) varied from sample to sample, while the coercive force (Hc) values of the samples were almost similar. These are plotted against the Mn content in Fig. 8. The highest Ms value was observed in the case of a slightly Mn-rich composition, for which the volume extent of the L10 phase in the product was the highest. In order to account for the effect of the secondary phases, the observed Ms value was normalized with respect to the L10 phase weight fraction, which was evaluated as being 1.8μB for the samples. This value corresponds to 75% of the theoretical one. In addition, we expected that the normalization would increase the Ms towards the stoichiometric composition. However, this trend was not observed, so that the ordering parameter did not matter either. On the other hand, the highest Hc value was observed at the stoichiometric composition. A plot of Hc against the axial ratio (c/a) (see Fig. 9) shows that the increase in tetragonality increased Hc. It is interesting to note that the reduction in c/a did not significantly decrease Hc in the case of the Al-rich composition.
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Fig. 9 Coercive force (Hc) as a function of the axial ratio (c/a). The guiding line represents Mn-rich samples. |
The effect of c/a on the MAE value of several L10 compounds has been investigated theoretically using a second-order perturbation formula.13 According to the obtained results, the MAE value of L10 compounds tends to become higher with the c/a value. This can be roughly explained from that the reduction in the dimensionality results in localized valence electrons, leading to the increase in the gradient in the electrostatic potential and subsequent strengthening in the spin–orbit interactions. More specifically, for the MnAl system, an increase in the MAE value of approximately 18% is predicted for an increase in c/a of 5%. The theory reasonably explains the obtained experimental results; however, the degree of quantitative agreement is rather low. In the present work, an increase in Hc of 37% was caused by a mere difference of 0.3% in c/a. This suggests that the Mn present at the antisite was the dominant factor affecting MAE: the ordering was more critical than the c/a in the case of the studied samples. Indeed, we found a linear relationship between Hc and S among the samples (Fig. 10). In addition, the rough estimation of the anisotropic field based on the initial magnetization and corresponding differential curves showed a marginal difference among them, which suggests that the microstructural influence was observed in the Hc of present samples as well.
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