τ-MnAl with high coercivity and saturation magnetization

In this paper, high purity τ-Mn54Al46 and Mn54−xAl46Cxalloys were successfully prepared using conventional arc-melting, melt-spinning, and heat treatment process. The magnetic and the structural properties were examined using x-ray diffraction (XRD), powder neutron diffraction and magnetic measurements. A room temperature saturation magnetization of 650.5 kAm-1, coercivity of 0.5 T, and a maximum energy product of (BH)max = 24.7 kJm-3 were achieved for the pure Mn54Al46 powders without carbon doping. The carbon substituted Mn54−xAl46Cx, however, reveals a lower Curie temperature but similar saturation magnetization as compared to the carbon-free sample. The electronic structure of MnAl shows that the Mn atom possesses a magnetic moment of 2.454 μB which results from strong hybridization between Mn-Al and Mn-Mn. We also investigated the volume and c/a ratio dependence of the magnetic moments of Mn and Al. The results indicate that an increase in the intra-atomic exchange splitting due to the cell volume ex...


INTRODUCTION
The current generation of high performance permanent magnets is based on two of the three transition metals (Fe, Co, Mn) capable of carrying a large magnetic moment in metallic compounds.2][3] However, the rare earth elements Nd, Sm, and Dy, etc. are scarce and getting much more expensive.Therefore, developing strong permanent magnets without the rare earth elements is an emerging issue to be addressed. 4,5lthough supporting high magnetic moments, Mn is not used as the major constituent of permanent magnets.The small difference in band structure between Fe and Mn is apparently sufficient to make Mn-Mn coupling antiferromagnetic at the interatomic spacing typical of rare-earth transition metal alloys.6][7][8] Among them, the ferromagnetic τ-phase in MnAl alloys has recently received much attention because of its high magnetic moment of 2.4 µ B /f.u., high magnetic crystalline anisotropy constant, low cost, and low density of 5.2 g/cm 3 . 4,9,10The ferromagnetic τ-phase occurs in the range of Mn composition from 51 to 59 at.%, which has an ordered body-centered tetragonal (L1 0 ) structure with c > a (see Fig. 1). 4 The Mn atoms that enter the (0, 0, 0) site become ferromagnetically coupled while the Al atoms and any Mn atoms in excess of equiatomicity enter the (1/2, 1/2, 1/2) site. 11,1213]19 The saturation magnetization is also found to increase with C with a larger resultant Mn moment. 11,12In addition, it has been reported that the experimental saturation magnetization Ms, coercivity Hc, and the Curie temperature T C for τ-phase Mn 54 Al 46 are 462.8kAm -1 , 0.48 T, and 382 • C, respectively, 13   the theoretical values. 20Thus, most of the studies of this system have been limited to the stabilization and improvement of magnetic performance of τ-phase-based alloys.][33][34][35] However, the theoretical results for the unusual magnetic properties are still in disagreement with the experimental results.In order to exploit the potential of the MnAl τ-phase as a hard magnetic material, a further characterization of the crystal structure and magnetic properties is needed.

EXPERIMENTAL AND THEORETICAL CALCULATION METHODS
Samples of composition Mn 54-x Al 46 C x (x = 0, 1, 2, 3) were produced by argon arc-melting.The as-prepared ingots were used to prepare ribbon samples by a single-roller melt spinning technique under argon atmosphere at a wheel speed of 15 m/s.To get the magnetic MnAl phase, the as-spun ribbons were annealed at 450 • C for 45 min under argon atmosphere, while the as-prepared ingots were annealed at 1050 • C for two days under argon atmosphere and were then cooled in air.The phases in the specimens were identified using an x-ray diffraction technique with Cu-K α radiation (λ = 1.5418Å).Neutron diffraction patterns were collected on the high resolution powder diffractometer at HZB Germany.The measurements were performed on Mn 54 Al 46 and Mn 51 Al 46 C 3 alloys at 273 K using a neutron wavelength of 1.7982 Å with powder specimens of the annealed buttons.Magnetic thermobalance was used to measure the Curie temperatures T C .The saturation magnetizations were determined by a physical properties measurement system (PPMS) using an applied field of 7 T.The hysteresis loops were measured using a vibrating sample magnetometer (VSM) with the maximum applied magnetic field up to 2 T. A density of 5.16 g/cm 3 was used to calculate the magnetization and the maximum energy product (BH)max for Mn 54 Al 46 .
The electronic structure calculations were performed using WIEN2K code.The generalized gradient approximation (GGA) proposed by Perdew, Burke, and Ernzerhof (PBE) was used as exchange and correlation.The scalar relativistic approximation for the valence states and spin-orbit coupling is included by a second-variational procedure with states up to 9 Ry above Fermi energy.The radial parts of the basis function inside the muffin tin were calculated using the full relativistic representation, muffin tin radius are 2.5 (Bohr) for both Mn and Al atoms.The Fourier series were truncated at g min = -8, g max = 12 and a cutoff parameter R*Kmax=8 was used.The energy converge was taken as 10 -8 Ry.

RESULTS AND DISCUSSION
Since τ-MnAl phase is metastable and cannot be obtained directly by melting, the ϵ-phase needs to be formed first and transformed into τ-phase by subsequent heat treatment.It was found that the amount of τ-MnAl in the final product strongly depends on the starting content of Mn in the alloys. 13Therefore, an atomic ratio of 54-46 for Mn-Al was carefully chosen as a starting composition in order to produce high purity ferromagnetic τ-phase.Fig. 2 shows the X-ray diffraction patterns of Mn 54 Al 46 powders taken at room temperature with different methods of preparation.As shown in Figure 2(a), the as-prepared ingot contains mainly ferromagnetic τ-phase with a small amount of β+γ 2 phases.After annealing at 1050 • C for two days and cooling in air, the β+γ 2 phases disappeared and an almost pure τ-phase was obtained (see Fig. 2(b)).This indicates that the homogenous high temperature ϵ-phase (h.c.p) was formed at 1050 • C, and the cooling rate in air was proper for the transformation from ϵ-phase to ferromagnetic τ-phase.The X-ray TABLE I. Refined structural data and χ 2 for Mn 54-x Al 46 C x (x = 0, 1, 2 and 3) alloys.diffraction measurement (Fig. 2(c)) of the as-spun ribbons also revealed that the Mn 54 Al 46 alloy was quenched completely into the ϵ-phase with a wheel speed of 15 m/s.By annealing the sample at 450 • C for 45 minutes, the τ-phase was formed (Fig. 2(d)).Notably, the XRD peaks of the later τ-phase prepared by melt-spinning method are much sharper than those of the arc-melting method, indicating different microstructures obtained by these two methods.Indeed, the SEM images of the microstructure obtained by arc-melting method (Fig. 3(a)-3(b)) consist of many twin related substructures, correlating with the ϵ-τ phase transformation processes.The ϵ-τ phase transformation occurs via a compositionally invariant, diffusion nucleation and growth process. 23,24Thus, twinning during this process may help to accommodate the localized stress associated with the volume and shape change.In the melt-spun samples (Figs.3(c) and 3(d)), however, a much finer twin structure can be observed, which may lead to much less stress in the sample.Fig. 4 shows the XRD patterns of Mn 54-x Al 46 C x (x = 0, 1, 2, 3) alloys made using the arc-melting and heat treatment followed by air cooling method.All the treated specimens crystallized in the pure magnetic τ-phase.These results are different from that of the reference, 13 in which pure τ-phase Mn 52.5 Al 46 C 1.5 alloys could not be obtained using conventional furnace.This may be due to the different composition and heat treatment process used in the preparation.The pure Mn 54-x Al 46 C x (x = 1, 2, τ-phase can also be obtained by annealing of the as-spun MnAl(C) ribbons at 450 • C for 45 minutes (data not shown).As compared to the carbon-free samples, the XRD peaks of all Mn 54-x Al 46 C x (x = 1, 2, 3) alloys are much higher and narrower.These results implied that C addition can promote the formation of τ-phase due to the fact that the insertion of carbon can help relieve the internal stress. 30The refined lattice parameters, unit cell volumes and densities of the Mn 54-x Al 46 C x result from analyzing XRD patterns are listed in Table I.It can be seen that the lattice parameter a slightly decreases with the increased carbon content, while the lattice parameter c and unit cell volume V slightly increase with the increased carbon content.The increase of the unit cell volume with carbon substitution may suggest that the carbon is present in the interstitial sites in the MnAl structure, since the atomic radius of carbon is much smaller than that of Mn or Al atom.In order to clarify this question, ND measurements were done for Mn 54-x Al 46 C x (x=0 and 3) samples.Two different carbon sites have been assumed during the refinement.(1) Carbon enters the substitution site, namely (0, 0, 0) or (1/2, 1/2, 1/2) site, which is similar to the refinements method used in reference. 30The site occupancies for C, Al and Mn atoms at the two available sites can be specified as follows: at site (0, 0, 0); m, n and (1-m-n), and at site (1/2, 1/2, 1/2); (z-m), ( y-n), and (x-1+m+n), respectively.This refers to an alloy with the unit-cell composition of Mn x Al y C z where x + y + z = 2. (2) Carbon enters the interstitial site, namely (0, 1/2, 1/2).The sites (1/2, 1/2, 0) was not considered because if carbon atoms enter this interstitial site the lattice parameter a would not contract.Fig. 5 is the refined ND patterns of the Mn 54 Al 46 and Mn 51 Al 46 C 3 at RT with different models.The refined occupancy and magnetic moments are listed in Tables II and III, respectively.Regarding to the substitution case, most of the Mn / Al occupy (0, 0, 0) / (1/2, while some (1/2, 1/2, / (0, 0, 0), respectively.Carbon prefers to occupy (1/2, 1/2, 1/2) with some occupying on (0, 0, 0) site.The Al and C prefer to occupy the (1/2, 1/2, 1/2) site, so the tendency of Mn to occupy the (1/2, 1/2, 1/2) site decreases.The magnetic moment of Mn (2.04 µ B ) on (0, 0, 0) site slightly increases to 2.14 µ B due to the C substitution.However, the average moments of Mn on (1/2, 1/2, 1/2) site increases dramatically which leads to almost the same total unit cell moment for different samples.This agrees with the magnetic measurements.For the interstitial carbon case, a similar refined χ 2 value was obtained.The moment of Mn atoms on (0, 0, 0) site is about 2.30 µ B .The total magnetic moments in the unit cell is about 1.9 µ B , which is higher than that of the values obtained from the carbon-free samples.According to the magnetization values in Table IV, the substitution case is more consistent with the experimental observations.Fig. 6 shows the M-H curve for τ-Mn 54 Al 46 measured at room temperature.A room temperature magnetization of 650.5 kAm -1 can be obtained at a magnetic field of 7 T.This value is higher compared to those of the previous reports. 11,13,31In order to further improve the magnetic properties, the alloys were mechanical grinded for different periods of time.The fine powders were then fixed into an epoxy resin and subjected to a magnetic field of about 1.0 T to form magnetically aligned samples of cylindrical shape.The hysteresis loops of the powders with different grinding time are shown in Fig. 7.It can be seen that the coercivity increases quickly with the increased lengths of grinding time and reaches its maximum of 0.5 T at about 90 minutes of grinding, while the magnetization decreases with the increased grinding time.The grain refinement, defects and microstructural changes through the milling should play important role in the determination of the coercivity.This was confirmed by the SEM images and XRD patterns (data not shown here).With milling time increasing, the XRD pattern peaks of Mn 54 Al 46 become broader and the background become higher, which indicates that the grain size decreases and some grains decompose to amorphous phase.That should be responsible for the simultaneous decrease in magnetization.The best magnetic properties of µ 0 T, remanence Mr = 377.5 kAm -1 and max = 24.7 kJm -3 at RT can be obtained for the 15-minute grinded sample.The magnetic properties of coercivity µ 0 Hc = 0.6 T, remanence M r = 410.8kAm -1 and maximum energy product (BH) max = 39.0 kJm -3 were achieved at 10 K.However, relatively poor magnetic properties were obtained at RT for carbon-doped MnAl (data not shown).
Table IV lists the magnetic properties of the Mn 54-x Al 46 C x (x = 0, 1, 2, 3) alloys at RT and 10 K.The addition of C decreases Tc of Mn 54-x Al 46 C x significantly, from 375.1 • C to 231.8 • C. The decrease of the lattice parameter a will decrease the bond distance of Mn-Mn in ab-plane, which makes negative contribution to the exchange coupling between Mn-Mn and leads to the decrease in the Tc.In contrast, the C addition causes slightly decreased saturation magnetization, which is due to the moment of Mn on (1/2, 1/2, 1/2) site increases dramatically with C doping according to the neutron diffraction analysis.Fig. 8 shows the density of states (DOS) of each atom in the τ-Mn 50 Al 50 phase calculated based on our structural data.The Mn has a magnetic moment of 2.44 µ B , whereas Al has a small induced moment of -0.06 µ B .These results agree well with those of references. 31,35It should be pointed out that the calculated magnetic moments are larger than those obtained from ND refinement for Mn 54 Al 46 , which is due to the fact that the extra Mn goes to (1/2, 1/2, 1/2) sites with a negative contribution to the total magnetic moment.There is strong hybridizations between the Mn-d states and the Al-p states as well as the Al-s and the Mn-d states, as can be seen from the identical shape of the partial DOS for Mn and Al atoms.Because of the strong hybridization among Mn-Al and short nearest neighbor (NN) distance of Mn-Mn (∼2.78 Å), the calculated magnetic moment of the Mn atom is less than 2.5 µ B , which is a substantial reduction of the free atom moment of 5 µ B .Therefore, increasing the NN distance of Mn-Mn should be an effective way to increase the magnetic moments of Mn atoms.Fig. 9 shows the DOS of Mn 50 Al 50 with expanded unit cell volume of 10%.The volume dependencies of the magnetic moments of Mn and Al are plotted in Fig. 10.It can be seen from the DOS (Fig. 9), that the increase in the unit cell volume leads to a narrow bandwidth of the states, and an increase in the intra-atomic exchange splitting results in a large magnetic moment.In the meantime, the overlap between Mn-Al atomic states has been reduced.The magnetic moment of Mn increases to a value of 2.91 µ B with the increase of 20% in unit cell volume (Fig. 10).The magnetic moment of the Mn atom decrease to 2.0 µ B under a contraction of ∆V/V = 20%, where smaller exchange splitting is found for the Mn atomic states.The absolute value of the magnetic moment of the Al atoms increases with the expansion of the unit cell, likely due to the high polarization of the Al atoms by the Mn magnetic moments.
FIG. 2. X-ray diffraction patterns of Mn 54 Al 46 powders taken at room temperature with different preparative methods.(a) as-prepared ingot, (b) ingot annealed at 1050 • C for two days then cooling in air, (c) as-spun ribbons, (d) ribbons annealed at 450 • C for 45 minutes.

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FIG. 3. SEM images of the microstructure for the τ-MnAl prepared by different methods.(a-b) Ingot annealed at 1050 • C for two days then cooling in air.(c-d) as-spun ribbons annealed at 450 • C for 45 minutes.

FIG. 10 .
FIG.10.The volume dependence of the magnetic moments and total energy of the τ-MnAl.

FIG. 11 .
FIG.11.The axial c/a ratio dependence of magnetic moments and total energy of ferromagnetic τ-MnAl.
Fig.8shows the density of states (DOS) of each atom in the τ-Mn 50 Al 50 phase calculated based on our structural data.The Mn has a magnetic moment of 2.44 µ B , whereas Al has a small induced moment of -0.06 µ B .These results agree well with those of references.31,35It should be pointed out that the calculated magnetic moments are larger than those obtained from ND refinement for Mn 54 Al 46 , which is due to the fact that the extra Mn goes to (1/2, 1/2, 1/2) sites with a negative contribution to the total magnetic moment.There is strong hybridizations between the Mn-d states and the Al-p states as well as the Al-s and the Mn-d states, as can be seen from the identical shape of the partial DOS for Mn and Al atoms.Because of the strong hybridization among Mn-Al and short nearest neighbor (NN) distance of Mn-Mn (∼2.78 Å), the calculated magnetic moment of the Mn atom is less than 2.5 µ B , which is a substantial reduction of the free atom moment of 5 µ B .Therefore, increasing the NN distance of Mn-Mn should be an effective way to increase the magnetic moments of Mn atoms.Fig.9shows the DOS of Mn 50 Al 50 with expanded unit cell volume of 10%.The volume dependencies of the magnetic moments of Mn and Al are plotted in Fig.10.It can be seen from the DOS (Fig.9), that the increase in the unit cell volume leads to a narrow bandwidth of the states, and an increase in the intra-atomic exchange splitting results in a large magnetic moment.In the meantime, the overlap between Mn-Al atomic states has been reduced.The magnetic moment of Mn increases to a value of 2.91 µ B with the increase of 20% in unit cell volume (Fig.10).The magnetic moment of the Mn atom decrease to 2.0 µ B under a contraction of ∆V/V = 20%, where smaller exchange splitting is found for the Mn atomic states.The absolute value of the magnetic moment of the Al atoms increases with the expansion of the unit cell, likely due to the high polarization of the Al atoms by the Mn magnetic moments.Fig.11shows the axial FIG. 5. Neutron diffraction patterns of the Mn 54 Al 46 and Mn 51 Al 46 C 3 at RT.

TABLE III .
Obtained Mn magnetic moment M Mn , unit-cell magnetization M cell and saturation magnetization Ms for Mn 54 Al 46 and Mn 51 Al 46 C 3 alloys at 293 K from neutron diffraction analysis.(I) carbon atoms enter the substitutional site 1a and 1d, (II) carbon atoms take the interstitial sites 2e.

TABLE IV .
Curie temperature Tc and saturation magnetization Ms for Mn 54-x Al 46 C x alloys.