The manganese-antimony (MnSb) alloys with specific stoichiometry, on atomic range from 43% to 50%at.Sb, as cast (with no quenching or annealing), were analyzed using OM, VSM, DSC and XRD characterization methods. The XRD diffractograms were refined using Rietveld method, were possible to identify four different phases: a) Mn1, 09Sb (hexagonal, spatial group P63mmc); a’) MnxSb (1.092<x<1.22, hexagonal P63mmc); b) Mn2Sb (tetragonal P4nmm); and c) Sb (rhombohedral Rm). OM’s images helped us to double check the coherence with XRD refined data, and VSM plus DSC helped us to identify the respective magnetic and thermal phase transitions. The Mn1.09Sb phase has proven to be tunable exclusively through its composition, what is a promising property for specific applications, although its second order transition can limit the application to devices where this behavior is desired. The abnormal presence of Mn2Sb in all studied samples suggests the known phase diagrams can contain an extended and not reported Mn2Sb phase region high temperature.

Keywords: magnetic materials, phase transformation, MnSb metals and alloys, manganese-antimony alloys, curie temperature, magnetic transitions

The discovery of nonferrous magnetic alloys was reported by Heusler¹ in 1898 and since then the investigation and application of these alloys on industrial and scientific devices have been growing continuously. Guillaud² described a variable Curie Temperature (between 90°C and 314°C) for MnSb alloy, obtained exclusively through the variation of the stoichiometry (from 45% to 49% in atomic percentage of Sb (at.%Sb). At least six different phase diagrams were published, besides the long time from its discovery, they still have some uncertain regions related to minimum/maximum stoichiometry of MnSb phase and its respective Tc. Okamoto’s phase diagram³ defines this region being from 45% and 49% atomic Sb at room temperature up to 314°C, and respective Tc varying from 90°C to 314°C. Crystallographic files from ICSD⁴ provide references where the phase is described as Mn_1.092Sb or Mn1.1Sb. Eight samples, from 43% to 50% at. Sb was produced, covering the complete range of Mn_1.092Sb phase. Guillaud² reported a tunable magnetic transition through stoichiometry between 90°C to 314°C, Teramoto & Van Run⁵ confirmed the non dependency of annealing temperature between 400°C and 700°C for 49% atomic of Sb, and plotted a partial phase diagram (Figure 1), where MnSb stable phase varies with temperature from 46 to 50%at of Sb at 400°C, and a single point at 41% atomic Sb (%at.Sb) at 840°C describing a non linear behavior. Teramoto & Van Run⁵ reported quenching from temperatures between 400°C to 700°C didn’t change the Tc, being independent of annealing or quenching temperature, but only related to stoichiometry. Okamoto³ reported a peritectic transition at 840°C and variable Tc related stoichiometry from 44% to 49% at.Sb. at 400°C. Chen⁶ identified the peritectic temperature at 843°C and the stoichiometry between 45% and 49.5% atomic of Sb at 400°C, Vanyarkho⁷ reported the peritectic temperature at 841°C and the MnSb phase from 45% to 49% at 400°C, Williams⁸ reported the peritectic temperature at 853°C and variable magnetic range from 40% to 50% below 573; and Kainzbauer⁹ reported the peritectic temperature at 830°C and limits of MnSb phase from 45.5% to 50.5at% of Sb. Although Guillaud² described the reaction at MnSb alloy as a SOMT (Second Order Magnetic Transition), Nwodo¹⁰ reported a FOMT (first order magnetic transition), AFM-FI (Anti- ferromagnetic→Ferrimagnetic) reaction, attributed to a spin reorientation of Mn₂Sb dropped with Sn (Mn₂Sb_0.9Sn_0.1).

Based on reported and well recognized phase diagram produced by Okamoto & Van Run³ the samples were prepared with stoichiometry from 43% to 50% at.Sb, producing 8 different samples, nominated as Mn₅₇Sb₄₃ (57% atomic of Mn and 43% atomic of Sb), Mn₅₆Sb₄₄, Mn₅₅Sb₄₅, Mn₅₄Sb₄₆, Mn₅₃Sb₄₇, Mn₅₂Sb₄₈, Mn₅₁Sb₄₉ and Mn₅₀Sb₅₀. The samples were prepared from pure elements (Sb - Alfa Aesar 99.99% and Mn Alfa Aesar 99.98%), and were weighed at high precision equipment under atmosphere pressure, and then melted under argon atmosphere in an electric arc furnace (100A). Each sample was melted 6 times, inverting its position after each melt. As the intention of this part of the study was to observe the alloy as cast on electric arc furnace, no quenching or annealing treatment was done. The samples were grinded into powder and separated into 4 groups, to be analyzed at XRD (X-Ray Diffraction) and Rietveld refinement using Topas® software, DSC (Differential Scanning Calorimetry), VSM (Vibrating Sample Magnetometer), Optical Microscopy (OM). For metallographic analysis the fourth group was embedded in Bakelite before sanding and polishing processes. The sanding process was done using abrasive discs grades P120, P220, P320, P400, P600, P1200 and P2000, on sequence polishing process was done using synthetic felt discs and alumina paste (1.0µm, 0.3µm and 0.05µm), the samples were etched using HNO₃+CH₃CH₂OH for 20s, and then finally analyzed at Optical Microscope (Olympus BX41M-LED). The XRD parameters were: 2θ from 20 to 100 degrees, step of 0.01 and rotation speed 10omin^-1. On DSC: samples were separated and evaluated from 80°C to 370°C, with ramp of 5°Cmin^-1. As the alloy can contain metastable phases, only heating curves were considered. VSM samples were analyzed at 200Oe magnetic field from 40°C to 400°C, 5°Cmin^-1 ramp. The samples Mn₅₇Sb₄₃ and Mn₅₀Sb₅₀ were evaluated at ED’s equipment, using magnification scale of 2,000times, and 10 measures of each phase were done to confirm the coherence of phases described at OM.

The images obtained from Optical Microscope (Figure 1), shows dark stripes (phase a) at Figure 1A Mn₅₇Sb₄₃ and Figure 1B Mn₅₆Sb₄₄, related to Mn₂Sb phase, the clear phase (phase b) is attributed to Mn₁, 1Sb phase which is dominant in all the samples. At Figure 1, the other samples showed are: c) Mn₅₅Sb₄₅, (d) Mn₅₄Sb₄₆, (e) Mn₅₃Sb₄₇, and (f) Mn₅₂Sb₄₈ basically with only one phase (Mn₁,09₂Sb). In accord to EDS lectures the dark dots are reminiscent phase Mn₂Sb or oxides. The images g) Mn₅₁Sb₄₉ and h) Mn₅₀Sb₅₀ shows an almost white phase, attributed do Sb. The phases were confirmed through EDS probe analysis.

Figure 1 Optical microscopy images (MO) related to as cast samples from 43 %at.Sb to 50 %at.Sb.

Magnetic analysis using VSM

As the samples were analyzed on “as cast” state, only the heating curve were considered. The data collected were plotted at Origin® software and used the function derivative to determine the transitions temperatures. As the samples were not annealed, the metastable equilibrium was not reached, causing too much noise on magnetic measures. The curves were smoothed and the transition temperatures were defined through derivative function. The magnetic measured data from VSM (MxT) were summarized at Figure 2, it shows a Tc dependence of %at.Sb from 49% to 45%, regressing from 316 °C to 101°C. Two magnetic transitions were observed on samples from 48%at.Sb to 44%at.Sb, the first varying in accord to composition, attributed to Mn1.1Sb phase; the second one, a little above to 310°C, provides evidences there is a second phase. As the samples were not annealed, it is not possible to confirm if this second phase is because of metastable phases or if could be two phase region on phase diagram.

Figure 2 Moment (emu/g) x Temperature diagram – VSM data.

Differential scanning calorimetry

Also on DSC, the data was plotted, smoothed and derivated to reduce the impact of noise caused by metastable phases. Even though, the Mn₅₀Sb₅₀ presented only one transition, associated to the richest composition of Mn_1.1Sb phase, indicated at phase diagrams³ as 314°C, but measured at 326°C. Only sample Mn₅₇Sb₄₃ presented a secondary transition close to expected transition of Mn₂Sb phase (pointed at 277°C on Okamoto’s phase diagram,³ indicating only this sample would contain this phase. The other samples indicated there is a dependence of Tc in accord to composition, in most cases with at least two transitions temperature, but significantly different from VSM. The probably reason is the non homogeneization of the sample through annealing (Figure 3).

Figure 3 Heat flow x Temperature diagram – DSC data.

DRX and Rietveld refinement Topas®

The X-ray diffraction (Figure 4) and Rietveld refinement at Topas® software, confirmed the 2θ angles related to “a=Mn_1,1Sb (hexagonal P63mmc)”; “b=Mn2Sb (tetragonal P4nmm); and “c=Sb (rhombohedral Rm)”. All the samples presented predominance of Mn_{1, 1}Sb. The alpha antimony, peak “c”, is only present at Mn52Sb48, Mn₅₁Sb₄₉ and Mn₅₀Sb₅₀ alloys, and is visible only at OM images Figure 1. (g) and (h) as “white phases”. Mn₂Sb phase was identified in all the samples, although in low percentage (below 10.7%), going in opposition to mentioned phase diagrams.

Figure 4 XRD – 2θ angles.

The Mn_1.1Sb phase has proven to be tunable exclusively through its composition, what is a promising property for specific applications, although it is a second order transition, what can limit frequency of devices, what could be compensated increasing the mass of them. The abnormal presence of Mn₂Sb in all studied samples suggests the known phase diagrams can contain an extended and not reported double phase region (Mn₂Sb +Mn_1.1Sb) at high temperatures, indicating the reported “phase precipitation” by Teramoto¹ can be a perithectoid reaction.¹¹

Further studies are in progress to investigate the region of tunable Tc with stoichiometry from 43%at.Sb to 50%at.Sb with objective to better understand the “precipitation effect of Mn₂Sb from Mn_1.1Sb phase”.

Author declares that there is no conflict of interest.

1. Heusler F, Take E. The nature of the Heusler alloys. Trans Faraday Soc. 1912;8:169–184.
2. Guillaud C. Ferromagnetic properties of manganese-antimony and manganese-arsenic alloys. Annales de Physique. 1949;12:671–703.
3. Okamoto H. Manganese-Antimony Binary Phase Diagram (based on 1990 Okamoto H.). 1990.
4. Bergerhoff G, Brown ID. Inorganic Crystal Structure Database. Acta Crystallogr Sect A Found Crystallogr. 1981.
5. Teramoto I, Van Run AMJG. The existence region and the magnetic and electrical properties of MnSb. J Phys Chem Solids. 1968;29(2):347–55.
6. Chen T, Charlan GB, Keezer RC. Growth of MnSb Single Crystals by Pulling With a Seed From non Stoichiometric Molten Solution. J Cryst Growth. 1977;29–36.
7. Vanyarkho VG, Moshchalkova NA, Gunchenko VM, et al. On the existence of the compound MnSb. Inorg Mater. 1988.
8. Williams RS. LI. Über die Legierungen des Antimons mit Mangan, Chrom, Silicium und Zinn; des Wismuts mit Chrom und Silicium und des Mangans mit Zinn und Blei. ZAAC. 1907;55:1–33.
9. https://matdata.asminternational.org/apd/index.aspx
10. Kainzbauer P, Richter KW, Ipser H. Experimental Investigation of the Binary Mn-Sb Phase Diagram. J Phase Equilibria Diffus. 2016;37(4):459–468.
11. Nwodo AN, Kobayashi R, Wakamori T, et al. Quasi-First Order Magnetic Transition in Mn1.9Fe0.1Sb0.9Sn0.1. Mater Trans. 2018;59(3):348–52.