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METHOD FOR TUNING THE FERROMAGNETIC ORDERING TEMPERATURE OF ALUMINUM IRON BORIDE

20180005736 · 2018-01-04

    Inventors

    Cpc classification

    International classification

    Abstract

    A series of solid solutions AlFe.sub.2.sub._.sub.xMnxB.sub.2 have been synthesized by arc-melting and characterized by powder X-ray diffraction, and magnetic measurements. All the compounds adopt the parent AlFe.sub.2B.sub.2-type structure, in which infinite zigzag chains of B atoms are connected by Fe atoms into [Fe.sub.2B.sub.2] slabs that alternate with layers of Al atoms along the b axis. The parent AlFe.sub.2B.sub.2 is a ferromagnet with T.sub.c=282 K. A systematic investigation of solid solutions AlFe.sub.2.sub._.sub.xMn.sub.x.B.sub.2 showed a non-linear change in the structural and magnetic behavior. The ferromagnetic ordering temperature is gradually decreased as the Mn content (x) increases. The substitution of Mn for Fe offers a convenient method for the adjustment of the ferromagnetic ordering temperature of AlFe.sub.2B.sub.2.

    Claims

    1. A series of solid solutions having the general formula: AlFe.sub.2-xMn.sub.xB.sub.2.

    2. The series of claim 1 wherein x has a value selected from the group consisting of 0.4, 0.65, 0.8, 1.0, 1.2, 1.6, 2.0, and any combination thereof, wherein the value of x may vary by +/−0.06.

    3. A solid solution having the general formula: AlFe.sub.2-xMn.sub.xB.sub.2, wherein x is at least 0.1.

    4. The solid solution of claim 3 comprising Fe-rich phases and Mn-rich phases.

    5. The solid solution of claim 3 wherein x is between 0.1 and 0.3.

    6. The solid solution of claim 3 wherein x is between 0.3 and 0.5.

    7. The solid solution of claim 3 wherein x is between 0.5 and 0.7.

    8. The solid solution of claim 3 wherein x is between 0.7 and 0.9.

    9. The solid solution of claim 3 wherein x is between 0.9 and 1.1.

    10. The solid solution of claim 3 wherein x is between 1.1 and 1.3.

    11. The solid solution of claim 3 wherein x is between 1.3 and 1.5.

    12. The solid solution of claim 3 wherein x is between 1.5 and 1.7.

    13. The solid solution of claim 3 wherein x is between 1.7 and 1.9.

    14. The solid solution of claim 3 wherein x is between 1.9 and 2.0.

    Description

    BRIEF DESCRIPTION OF THE DRAWINGS

    [0009] FIG. 1 is a depiction of the crystal structures of AlFe.sub.2B.sub.2. The [Fe.sub.2B.sub.2] slabs are highlighted (Fe=larger atoms and B=smaller atoms in the highlighted slabs). Al atoms are located between the [Fe.sub.2B.sub.2] slabs.

    [0010] FIG. 2 is X-ray powder diffraction patterns of AlFe.sub.2-xMn.sub.xB.sub.2. The bottom, light-gray pattern was calculated based on the reported crystal structure of AlFe.sub.2B.sub.2. See W. Jeitschko, Acta Crystallogr. Sect. B, 25 (1969) 163-165. In the powder diffraction patterns, the asterisk (*) and rhombus (.diamond-solid.) marks indicate the Al.sub.13Fe.sub.4 and Al.sub.10Mn.sub.3 impurities, respectively.

    [0011] FIG. 3 depicts the Unit cell volume of AlFe.sub.2-xMn.sub.xB.sub.2 as a function of x. The standard deviations for the volume are smaller than the symbol size.

    [0012] FIG. 4A depicts the temperature dependence of magnetic susceptibility for AlFe.sub.2-xMn.sub.xB.sub.2 measured under applied magnetic field of 1 mT; the dependence for x=1.6 is shown as the inset.

    [0013] FIG. 4B depicts the Field dependent magnetization of AlFe.sub.2-xMn.sub.xB.sub.2 measured at 1.8 K.

    DESCRIPTION OF THE EMBODIMENT(S) OF THE INVENTION

    [0014] The present invention is directed to a series of solid solutions having the general formula: AlFe.sub.2-xMn.sub.xB.sub.2. Herein, x has a value between 0 and 2, such as between 0.1 and 2, or between 0.1 and 1.9. In some embodiments, x can have a nominal value of any of 0, 0.4, 0.65, 0.8, 1.0, 1.2, 1.6, and 2.0. The value of x may vary from these nominal values by +/−0.06, preferably by no more than +/−0.03. Accordingly, a nominal value of 0.4, for example, may encompass an x value between 0.34 and 0.46, preferably between 0.37 and 0.43. A nominal value of 0.65 may encompass an x value between 0.59 and 0.71, preferably between 0.62 and 0.68. A nominal value of 0.8 may encompass an x value between 0.74 and 0.86, preferably between 0.77 and 0.83. A nominal value of 1.0 may encompass an x value between 0.94 and 1.06, preferably between 0.97 and 1.03. A nominal value of 1.2 may encompass an x value between 1.14 and 1.26, preferably between 1.17 and 1.23. A nominal value of 1.6 may encompass an x value between 1.54 and 1.66, preferably between 1.57 and 1.63.

    [0015] The present invention is further directed to a solid solution having the general formula: AlFe.sub.2-xMn.sub.xB.sub.2, wherein x has a value between 0 and 2. In some embodiments, x is at least 0.1. In some embodiments, x is between 0.1 and 2. In some embodiments, x is between 0.1 and 1.9. In some embodiments, x is between 0.1 and 0.3. In some embodiments, x is between 0.3 and 0.5. In some embodiments, x is between 0.5 and 0.7. In some embodiments, x is between 0.7 and 0.9. In some embodiments, x is between 0.9 and 1.1. In some embodiments, x is between 1.1 and 1.3. In some embodiments, x is between 1.3 and 1.5. In some embodiments, x is between 1.5 and 1.7. In some embodiments, x is between 1.7 and 1.9. In some embodiments, x is between 1.9 and 2.0.

    [0016] The present invention reports a detailed study of solid solutions having the general formula AlFe.sub.2-xMn.sub.xB.sub.2. Herein, x has a value between 0 and 2, such as between 0.1 and 2, or between 0.1 and 1.9. We demonstrate the change in the magnetic behavior upon substitution of Mn for Fe.

    [0017] Results and Discussion

    [0018] Synthesis and Crystal Structure

    [0019] A series of solid solutions AlFe.sub.2-xMn.sub.xB.sub.2 (x=0, 0.4, 0.65, 0.8, 1.0, 1.2, 1.6), were prepared by arc-melting. All of them crystallize in the AlFe.sub.2B.sub.2 structure type, as shown by the comparison of the experimental and calculated powder X-ray diffraction patterns. See FIG. 2, which are X-ray powder diffraction patterns of AlFe.sub.2-xMn.sub.xB.sub.2. The bottom, light-gray pattern was calculated based on the reported crystal structure of AlFe.sub.2B.sub.2. See W. Jeitschko, Acta Crystallogr. Sect. B, 25 (1969) 163-165. In the powder diffraction patterns, the asterisk (*) and rhombus (.diamond-solid.) marks indicate the Al.sub.13Fe.sub.4 and Al.sub.10Mn.sub.3 impurities, respectively. AlFe.sub.2B.sub.2 was obtained in phase-pure form after treatment of the reaction products with dilute HCl. Such work up, however, was not possible for Mn-containing phases that turned out to be much more acid-sensitive than AlFe.sub.2B.sub.2. For that reason, the samples of AlFe.sub.2-xMn.sub.xB.sub.2 and AlMn.sub.2B.sub.2 were contaminated with small amounts of Al.sub.13Fe.sub.4 and Al.sub.10Mn.sub.3, respectively.

    [0020] The refinements of PXRD data revealed that substitution of Mn for Fe in AlFe.sub.2B.sub.2 leads to the increase in the unit cell volume, in accord with the larger size of Mn atoms. See FIG. 3, which depicts the unit cell volume of AlFe.sub.2-xMn.sub.xB.sub.2 as a function of x. The standard deviations for the volume are smaller than the symbol size. See also Table 1. The unit cell parameters and unit cell volume change non-linearly with the Mn content (x). As will be shown below, this irregularity is also reflected in the magnetic behavior of AlFe.sub.2-xMn.sub.xB.sub.2.

    TABLE-US-00001 TABLE 1 EDX analysis compositions, unit cell parameters, magnetic ordering temperatures (T.sub.C), and saturation magnetization at 1.8 K (M.sub.sat) for AlFe.sub.2−xMn.sub.xB.sub.2. Mn content M.sub.sat, μ.sub.B from EDX per T Sample analysis (x) a, Å b, Å c, Å V, Å.sup.3 T.sub.C, K atom AlFe.sub.2B.sub.2 — 2.945 (4) 11.09 (1) 2.887 (3) 94.39 (1) 282 1.15 AlFe.sub.1.6Mn.sub.0.4B.sub.2 0.37 (8) 2.941 (3) 11.08 (1) 2.895 (3) 94.38 (1) 242 0.87 AlFe.sub.1.35Mn.sub.0.65B.sub.2 0.63 (6) 2.913 (9) 11.07 (4) 2.936 (9) 94.66 (1) 220 0.60 AlFe.sub.1.2Mn.sub.0.8B.sub.2 0.74 (6) 2.912 (8) 11.09 (4) 2.936 (8) 94.77 (1) 188 0.50 AlFeMnB.sub.2 0.95 (5) 2.938 (2) 11.07 (1) 2.919 (4) 94.93 (1) 119 0.38 AlFe.sub.0.8Mn.sub.1.2B.sub.2 1.22 (7) 2.942 (9) 11.05 (2) 2.921 (8) 94.98 (1)  43 0.16 AlFe.sub.0.4Mn.sub.1.6B.sub.2 1.57 (8) 2.937 (5) 11.08 (1) 2.921 (4) 95.01 (1) — 0.07 AlMn.sub.2B.sub.2 — 2.936 (5) 11.12 (1) 2.912 (8) 95.06 (1) — —

    [0021] A detailed description of the crystal structure of AlFe.sub.2B.sub.2 can be found in our recent paper. See X. Y. Tan, P. Chai, C. M. Thompson, M. Shatruk, J. Am. Chem. Soc., 135 (2013) 9553-9557. All AlFe.sub.2-xMn.sub.xB.sub.2 embodiments are isostructural to AlFe.sub.2B.sub.2. All these structures contain 2-D [T.sub.2B.sub.2] slabs alternating with layers of Al atoms along the b axis. T in the formulation may be either of Fe, Mn, or a combination of Fe and Mn (i.e., Fe.sub.2-xMn.sub.x wherein x has a value between 0 and 2). The B atoms form a layer of zigzag chains inside the slabs that are capped above and below by T atoms. Thus, the structure has a distinct 2-D topology, especially in the sense of magnetic exchange interactions between the T sites. Noteworthy, similar zigzag chains of B atoms are found in the structures of binary transition-metal borides, TB, where the transition metal atoms bind the boron chains into an extended 3-D framework. Therefore, the structure of AlT.sub.2B.sub.2 can be viewed as generated from the binary structure by the introduction of Al atoms, which break down the 3-D framework of the binary boride to create the corresponding layered structure of the ternary boride.

    [0022] Magnetic Properties

    [0023] In agreement with the earlier reports, AlFe.sub.2B.sub.2 exhibits an abrupt increase in the magnetic moment associated with the ferromagnetic phase transition at T.sub.C=282 K. See X. Y. Tan, P. Chai, C. M. Thompson, M. Shatruk, J. Am. Chem. Soc., 135 (2013) 9553-9557 and M. El Massalami, D.d. Oliveira, H. Takeya, J. Magn. Magn. Mater., 323 (2011) 2133-2136. The substitution of Mn for Fe gradually suppresses the ferromagnetic behavior (See FIG. 4A), as the magnetic phase transition for the AlFe.sub.2-xMn.sub.xB.sub.2 samples becomes less abrupt with the increase in the Mn content (x) and the 1.8-K saturation magnetization per T atom also gradually decreases (See FIG. 4B), dropping from 1.15 μ.sub.B for x=0 to only 0.07 μ.sub.B for x=1.6 (Table 1). FIG. 4A is a graph depicting the temperature dependence of magnetic susceptibility for AlFe.sub.2-xMn.sub.xB.sub.2 measured under applied magnetic field of 1 mT; the dependence for x=1.6 is shown as the inset. FIG. 4B is a graph depicting field dependent magnetization of AlFe.sub.2-xMn.sub.xB.sub.2 measured at 1.8 K.

    [0024] Conclusions

    [0025] The series of solid solutions AlFe.sub.2-xMn.sub.xB.sub.2, whose structure contains 2-D [Fe.sub.2-xMn.sub.xB.sub.2] slabs alternating with layers of Al atoms, exhibits gradual evolution of magnetic properties with the change in the d-electron count. The itinerant ferromagnetism in the AlFe.sub.2-xMn.sub.xB.sub.2 series becomes most pronounced in AlFe.sub.2B.sub.2, which exhibits ferromagnetic ordering at 282 K. The latter was shown by us to be a promising magnetic refrigerant, and thus the present invention provides a convenient method for varying the magnetic ordering temperature thereof.

    Examples

    [0026] The following non-limiting examples are provided to further illustrate the present invention.

    [0027] Materials and Methods

    [0028] Synthesis

    [0029] All manipulations during sample preparation were carried out in an argon-filled dry box (content of O.sub.2<1 ppm). Powders of aluminum (99.95%), manganese (99.95%), and iron (98%) were obtained from Alfa Aesar. Boron powder (95-97%) was obtained from Strem Chemicals. Mn and Fe metals were additionally purified by heating in a flow of H.sub.2 gas for 5 h at 775 K. Fused-silica tubes were obtained from National Scientific Corporation, Inc. (Quakertown, Pa.). Phase-pure AlFe.sub.2B.sub.2 was prepared by arc-melting a mixture of elements followed by annealing and post-treatment with dilute HCl, as previously reported. See X. Y. Tan, P. Chai, C. M. Thompson, M. Shatruk, J. Am. Chem. Soc., 135 (2013) 9553-9557. The samples AlFe.sub.2-xMn.sub.xB.sub.2 (x=0.4, 0.65, 0.8, 1.0, 1.2, 1.6, 2.0) were synthesized by arc-melting mixtures of elements that were weighed out in the ratio of Al:Fe:Mn:B=1.5:(2-x):x:2 and pressed into pellets. (The 50 wt. % excess of Al was found to minimize the content of byproducts.) The ingots obtained after arc-melting were sealed under vacuum (<10.sup.−2 mbar) in 10 mm inner diameter (i.d.) silica tubes and annealed at 1073 K for one week. The powder patterns at this point revealed the major target phase contaminated with small amounts of Al.sub.13Fe.sub.4 and MnB. Thus, the ingots were ground, pelletized, sealed under vacuum in 10 mm i.d. silica tubes, and re-annealed at 1073 K for another week. The obtained samples contained the desired product with a trace amount of Al.sub.13Fe.sub.4. The removal of this byproduct by treatment with dilute HCl, however, was impossible, because AlMn.sub.2B.sub.2 reacted with acid swiftly.

    [0030] Since all bulk samples of AlMn.sub.2B.sub.2 were contaminated with a trace amount of Al.sub.10Mn.sub.3, single crystals of AlMn.sub.2B.sub.2 were also grown from Al flux for magnetic property measurements. The starting materials with the Al:Mn:B ratio of 10:1:2 were mixed and placed into a 10 mm i.d. alumina crucible, covered with a piece of silica wool, and sealed into a 13 mm i.d. silica tube under vacuum (<10.sup.−2 mbar). The reaction was heated up to 1423 K in 15 h, held at that temperature for 15 h, and then slowly cooled down at 1 K/min. After reaching 1273 K, the tube was quickly taken out of the furnace, flipped upside down, and placed into a centrifuge for hot filtration through the silica wool to remove the unreacted liquid Al. The obtained sample contained plate-shaped crystals of AlMn.sub.2B.sub.2 (maximum size ˜0.4×0.2×0.02 mm.sup.3), as well as small amounts of byproducts, AlB.sub.2 and Al.sub.57Mn.sub.12, and traces of Al. The crystals of AlMn.sub.2B.sub.2 could be easily distinguished upon visual inspection of the sample and were picked up manually for further measurements.

    [0031] X-Ray Diffraction

    [0032] Room temperature powder X-ray diffraction (PXRD) was carried out on a PANalytical X'Pert Pro diffractometer with an X'Celerator detector using Cu-Kα radiation (λ=1.54187 Å). To avoid the fluorescence of Fe-containing samples, a graphite monochromator was used on the secondary side of the powder diffraction system. The corresponding statement has been added to the text. The patterns were recorded in the 2θ range of 10° to 80° with a step of 0.017° and the total collection time of one hour. The analysis of PXRD patterns was carried out with the HighScore Plus suite. Highscore Plus, PANalytical B.V., Almelo, Netherlands, 2006. The identity of AlMn.sub.2B.sub.2 single crystals was verified by room-temperature unit cell determination on a Bruker AXS SMART diffractometer equipped with an APEX-II CCD detector and Mo-Kα X-ray source (λ=0.71093 Å).

    [0033] Physical Measurements

    [0034] The elemental analyses were performed on a JEOL 5900 scanning electron microscope equipped with energy dispersive X-ray (EDX) spectrometer. Multiple locations on different crystallites were probed to establish the statistically averaged composition of each sample. The elemental ratios established for each sample agreed well with the nominal composition used for the sample preparation. Magnetic measurements were performed with a Quantum Design SQUID magnetometer MPMS-XL. Direct current (DC) magnetic susceptibility measurements were carried out in the field-cooled (FC) mode in the 1.8-300 K temperature range. Additional DC susceptibility measurements were performed on samples with x=1.2 and 1.6 in the zero-field-cooled (ZFC) and FC modes from 320 to 750 K. Isothermal field-dependent magnetization was measured at 1.8 K with the field varying from 0 to 7 T.

    [0035] When introducing elements of the present invention or the preferred embodiments(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

    [0036] In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

    [0037] As various changes could be made in the above compositions and processes without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.