Intrinsic magnetic properties of single-phase Mn1+xGa (0 < x < 1) alloys

Magnetization measurements have been carried out on a series of carefully prepared single-phase Mn1 + xGa (0 < x < 1) alloys. The saturation magnetization Ms, measured at 5 K, has a value of 92.0 emu/g for x = 0.15. This is the highest value reported in these alloys and is close to the calculated value of 116 emu/g for the stoichiometric compound (x = 0). Ms decreases gradually with x and has a value of 60.7 emu/g for x = 0.86. This behavior is consistent with the extra Mn atoms occupying Ga sites and coupling antiferromagnetically with the rest of the Mn atoms. The intrinsic magnetic properties of the Mn-Ga alloys indicate their great potential as novel, rare-earth free permanent magnetic materials.

Scientific RepoRts | 5:17086 | DOI: 10.1038/srep17086 most complex binary phase diagrams. To avoid complexities and the effects of the microstructure on the magnetic properties, as seen in thin films and nanograin samples [10][11][12] , we need to prepare good quality single-phase samples (see Methods). In the present study, we report on the intrinsic magnetic properties of single-phase Mn 1 + x Ga (0 < x < 1) alloys.

Results and Discussion
The X-ray diffraction results, as shown in Fig. 1, confirm that all the alloys are single phase, and can be fitted to the tetragonal structure. The lattice parameters as well as the volume of the unit cells for all the alloys are displayed in Table 1. The reduction in Mn content is associated with a slight contraction in a and an expansion along c, leading to a small overall volume increase of the unit cell.
The neutron diffraction pattern for Mn 1.33 Ga is shown in Fig. 2. We only show the neutron diffraction pattern to confirm the single-phase nature of the alloy. The refinement results will not be given here as we could not obtain a unique fit to the data. This was due to the number of parameters involved in the fitting. These include: the number and location of possible Ga vacancies for x > 0, possible disorder

Composition
Lattice constants  among the Mn/Ga atoms at different sites, the magnitude and direction of the magnetic moments of Mn on the two sites. Fits to the neutron results could be obtained with the same χ 2 value but with quite different values for the above noted parameters that were consistent with the nominal composition (x) and the experimental M s . For this reason, we cannot make any comment on previously published fitting results on Mn 1 + x Ga. We are in the process of carrying out an extensive neutron diffraction study for all the compositions to obtain a complete description for the structure. The only conclusion we can draw from our diffraction (XRD and NPD) experiments is that the most likely structure for all the alloys is the disordered tetragonal structure with space group I4/mmm as shown in Fig. 3. For MnGa (x = 0), the c-axis is half of that shown in the figure. In the ordered structure, P4/mmm, there are no vacancies and no Ga/Mn substitutions while for the disordered structure, I4/mmm, Mn/Ga substitutions can occur as well as the possibility of vacancies at the Ga 2a sites. Figure 4(a) shows the magnetization as a function of temperature for some of the alloys under an applied field of 0.01 T, and the corresponding field dependence of the magnetization is shown in Fig. 4(b). The values for the saturation magnetization, M s (in both emu/g and, in parentheses, Bohr magnetons per formula unit), the anisotropy field, H A , as well as the Curie temperature, T c , are listed in Table 2.
One can see that for Mn 1.15 Ga the saturation magnetization (M s ) has a value of 92.0 emu/g, which is amongst the highest reported M s of rare earth free permanent magnetic materials.
Before analyzing the results we decided that it would be prudent to recalculate the magnetic moment per unit cell of MnGa using the most accurate DFT calculation with the FLAPW (full potential linearized plane-wave) code. Our result is in very good agreement with that of references 1,2 .
The influence of Mn content on the intrinsic magnetic properties of Mn 1 + x Ga alloys is clearly seen in Table 2   hand, H a and T C of the alloys decrease monotonically with the decrease of Mn content. Even with this decrease, H a and T C of the Mn 1.15 Ga alloy are 66 kOe and 595 K, respectively, which are comparable with those of Nd 2 Fe 14 B. Therefore, the overall intrinsic magnetic properties (M s , H a , and T C ) of the Mn 1 + x Ga alloys (especially for alloys with x ≤ 0.33) indicate their great potential as novel permanent magnetic materials. Figure 5 shows the composition dependence of the saturation magnetization, measured at 5 K, of the Mn 1 + x Ga alloys. The solid red square denotes the calculated value for MnGa (x = 0). A linear extrapolation of the data, green dotted line, predicts a value of M s = 98 emu/g for MnGa. This is 15% lower than that calculated for the ordered MnGa structure. The difference between the extrapolated and the calculated values is most likely due to the extrapolation from the disordered Mn 1 + x Ga structures. Nevertheless, even this reduced M s gives a maximum energy product of 20 MGOe for MnGa.
To achieve a high (BH) max in MnGa alloys, however, major experimental efforts are necessary. For example, magnetic hardening should be accomplished in advance since the high coercivity is crucial for permanent magnets. Equally important is to induce a strong crystallographic texture to provide a high remanence ratio (M r /M s ) leading to a high (BH) max .

Conclusion
In summary, structure and intrinsic magnetic properties were studied for a series of single-phase Mn 1 + x Ga (0 < x < 1) alloys. The net magnetic moment in the Mn 1 + x Ga system increases monotonically with decreasing Mn content. This behavior is consistent with extra Mn atoms occupying Ga sites and coupling antiferromagnetically with the rest of the Mn atoms. We report the highest value of 92 emu/g for the x = 0.15 alloy. Mn 1 + x Ga alloys (x ≤ 0.33) exhibit excellent intrinsic magnetic properties. An extrapolated value of 20 MGOe for (BH) max is predicted for stoichiometric disordered MnGa alloy, demonstrating the great potential of Mn 1 + x Ga alloys as novel permanent magnetic materials.

Methods
A series of tetragonal Mn 1 + x Ga (x = 0.15, 0.20, 0.33, 0.50, and 0.86) alloys were prepared by induction melting high purity gallium (99.9 wt.%) and manganese (99.5%) in an argon atmosphere. To compensate for evaporation losses during melting, an extra 3 wt.% Mn was added to the alloys. The as-melted ingots were then annealed in a tubular vacuum furnace at temperatures, T a , ranging from 700 K to 900 K for annealing times, t a , extending from one day to up to one week. The ingots were subsequently quenched into ice water. The annealing step is crucial for obtaining single-phase alloys. Due to the complexity of  the Mn-Ga binary phase diagram, the annealing temperature rangeis quite narrow and the optimum choice depends critically on the composition of the alloy. The annealing temperatures were 740 K, 825 K and 870K for x = 0.15, 0.20 and 0.33, respectively, and 885K for the alloys with higher Mn content. While the annealing temperature was critically important, the annealing time did not appear to play a major role and we did not observe any significant differences between samples annealed for 2 and 7 days. The crystal structures were determined by x-ray diffraction with Cu-K α radiation. For one composition, x = 0.33, the structure was investigated using neutron powder diffraction (NPD) datacollected on the BT1 high-resolution powder neutron diffractometer located at the NIST Center for Neutron Research with monochromatic neutrons of wavelength 1.5403 Å produced by a Cu (311) monochromator. Rietveld refinement was performed using the GSAS/EXPGUI program 13 . Magnetic properties were measured using a Quantum Design physical properties measurement system (PPMS) magnetometer with a maximum magnetic field of 14 T. Temperature dependence of the magnetization was obtained from a vibrating sample magnetometer (VSM) using a field of 0.01T. From the initial magnetization and corresponding differential susceptibility curves, the values for the anisotropy field, H A , were obtained.