Method for tuning the ferromagnetic ordering temperature of aluminum iron boride

Abstract

A series of solid solutions AlFe2_xMnxB2 have been synthesized by arc-melting and characterized by powder X-ray diffraction, and magnetic measurements. All the compounds adopt the parent AlFe2B2-type structure, in which infinite zigzag chains of B atoms are connected by Fe atoms into [Fe2B2] slabs that alternate with layers of Al atoms along the b axis. The parent AlFe2B2 is a ferromagnet with Tc=282 K. A systematic investigation of solid solutions AlFe2_xMnx.B2 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 AlFe2B2.

Description

FIELD OF THE INVENTION

The present invention relates to boride compounds, and more specifically to layered-structured borides of the general formula: AlFe_2-xMn_xB₂.

BACKGROUND OF THE INVENTION

Transition metal borides have found a number of technologically important applications, among which the most notable is their use as permanent magnets based on neodymium iron boride, Nd₂Fe₁₄B. See J. F. Herbst, Rev. Mod. Phys., 63 (1991) 819-898. The research on the magnetism of complex intermetallic borides thus has been predominantly focused on the rare-earth containing systems with strong magnetic anisotropy. The latter, when combined with the high saturation magnetization of the transition metal sublattice, offers the highest energy products and thus the strongest permanent magnets known. See O. Gutfleisch, M. A. Willard, E. Bruck, C. H. Chen, S. G. Sankar, J. P. Liu, Adv. Mater., 23 (2011) 821-842. In contrast, the magnetism of rare-earth free borides is far less explored. Such materials usually behave as soft magnets, which could be one of the reasons why their magnetic behavior has not inspired as much research interest as the properties of the rare-earth containing borides. Nevertheless, two recent thrusts poise rare-earth free magnetic materials to gain increased attention. The first is the need to discover novel permanent magnets with decreased rare-earth content. See Critical Materials Strategy, U.S. Department of Energy, Washington, D.C., 2010. The second direction is due to the discovery of giant magnetocaloric effect at room temperature that promises to become the foundation of the future refrigeration technology. See K. A. Gschneidner, Jr., V. K. Pecharsky, A. O. Tsokol, Rep. Prog. Phys., 68 (2005) 1479-1539; B. G. Shen, J. R. Sun, F. X. Hu, H. W. Zhang, Z. H. Cheng, Adv. Mater., 21 (2009) 4545-4564; and V. Franco, J. S. Blazquez, B. Ingale, A. Conde, Annu. Rev. Mater. Res., 42 (2012) 305-342. The latter requires the use of soft magnets with high saturation magnetization to achieve a large cooling effect while avoiding hysteretic energy losses in a quickly alternating magnetic field.

We have recently reported the promising magnetocaloric properties of AlFe₂B₂, a ternary boride with a rather simple layered structure, the magnetic behavior of which went overlooked for more than 40 years. See X. Y. Tan, P. Chai, C. M. Thompson, M. Shatruk, J. Am. Chem. Soc., 135 (2013) 9553-9557 and W. Jeitschko, Acta Crystallogr. Sect. B, 25 (1969) 163-165. Our initial interest in this material was sparked by the high saturation magnetization offered by FeB. The ordering temperature of this ferromagnet, however, is too high for practical purposes (around 600 K). Consequently, we turned to the ternary material that affords a “diluted” magnetic lattice featuring two-dimensional (2-D) [Fe₂B₂] slabs alternating with layers of Al atoms along the b axis of the orthorhombic unit cell. See FIG. 1, which is a depiction of the crystal structures of AlFe₂B₂. The [Fe₂B₂] slabs are highlighted (Fe=larger atoms and B=smaller atoms in the highlighted slabs). Al atoms are located between the [Fe₂B₂] slabs. AlFe₂B₂ shows ferromagnetic ordering at ˜300 K nearly zero coercivity, and a significant magnetocaloric effect. Another attractive feature of this material is its being composed of earth-abundant, lightweight elements.

SUMMARY OF THE INVENTION

Briefly, the present invention is directed to a solid solution having the general formula: AlFe_2-xMn_xB₂, wherein x is at least 0.1.

The present invention is further directed to a series of solid solutions having the general formula: AlFe_2-xMn_xB₂.

Other objects and features will be in part apparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a depiction of the crystal structures of AlFe₂B₂. The [Fe₂B₂] slabs are highlighted (Fe=larger atoms and B=smaller atoms in the highlighted slabs). Al atoms are located between the [Fe₂B₂] slabs.

FIG. 2 is X-ray powder diffraction patterns of AlFe_2-xMn_xB₂. The bottom, light-gray pattern was calculated based on the reported crystal structure of AlFe₂B₂. See W. Jeitschko, Acta Crystallogr. Sect. B, 25 (1969) 163-165. In the powder diffraction patterns, the asterisk (*) and rhombus (♦) marks indicate the Al₁₃Fe₄ and Al₁₀Mn₃ impurities, respectively.

FIG. 3 depicts the Unit cell volume of AlFe_2-xMn_xB₂ as a function of x. The standard deviations for the volume are smaller than the symbol size.

FIG. 4A depicts the temperature dependence of magnetic susceptibility for AlFe_2-xMn_xB₂ measured under applied magnetic field of 1 mT; the dependence for x=1.6 is shown as the inset.

FIG. 4B depicts the Field dependent magnetization of AlFe_2-xMn_xB₂ measured at 1.8

DESCRIPTION OF THE EMBODIMENT(S) OF THE INVENTION

The present invention is directed to a series of solid solutions having the general formula: AlFe_2-xMn_xB₂. 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.

The present invention is further directed to a solid solution having the general formula: AlFe_2-xMn_xB₂, 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.

The present invention reports a detailed study of solid solutions having the general formula AlFe_2-xMn_xB₂. 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.

Results and Discussion

Synthesis and Crystal Structure

A series of solid solutions AlFe_2-xMn_xB₂ (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 AlFe₂B₂ 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_2-xMn_xB₂. The bottom, light-gray pattern was calculated based on the reported crystal structure of AlFe₂B₂. See W. Jeitschko, Acta Crystallogr. Sect. B, 25 (1969) 163-165. In the powder diffraction patterns, the asterisk (*) and rhombus (♦) marks indicate the Al₁₃Fe₄ and Al₁₀Mn₃ impurities, respectively. AlFe₂B₂ 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 AlFe₂B₂. For that reason, the samples of AlFe_2-xMn_xB₂ and AlMn₂B₂ were contaminated with small amounts of Al₁₃Fe₄ and Al₁₀Mn₃, respectively.

The refinements of PXRD data revealed that substitution of Mn for Fe in AlFe₂B₂ 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_2-xMn_xB₂ 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_2-xMn_xB₂.

TABLE 1
EDX analysis compositions, unit cell parameters, magnetic ordering temperatures (TC), and
saturation magnetization at 1.8 K (Msat) for AlFe2−xMnxB2.
	Mn content						Msat, μB
	from EDX						per T
Sample	analysis (x)	a, Å	b, Å	c, Å	V, Å3	TC, K	atom
AlFe2B2	—	2.945 (4)	11.09 (1)	2.887 (3)	94.39 (1)	282	1.15
AlFe1.6Mn0.4B2	0.37 (8)	2.941 (3)	11.08 (1)	2.895 (3)	94.38 (1)	242	0.87
AlFe1.35Mn0.65B2	0.63 (6)	2.913 (9)	11.07 (4)	2.936 (9)	94.66 (1)	220	0.60
AlFe1.2Mn0.8B2	0.74 (6)	2.912 (8)	11.09 (4)	2.936 (8)	94.77 (1)	188	0.50
AlFeMnB2	0.95 (5)	2.938 (2)	11.07 (1)	2.919 (4)	94.93 (1)	119	0.38
AlFe0.8Mn1.2B2	1.22 (7)	2.942 (9)	11.05 (2)	2.921 (8)	94.98 (1)	43	0.16
AlFe0.4Mn1.6B2	1.57 (8)	2.937 (5)	11.08 (1)	2.921 (4)	95.01 (1)	—	0.07
AlMn2B2	—	2.936 (5)	11.12 (1)	2.912 (8)	95.06 (1)	—	—

A detailed description of the crystal structure of AlFe₂B₂ 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_2-xMn_xB₂ embodiments are isostructural to AlFe₂B₂. All these structures contain 2-D [T₂B₂] 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_2-xMn_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 AlT₂B₂ 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.

Magnetic Properties

In agreement with the earlier reports, AlFe₂B₂ exhibits an abrupt increase in the magnetic moment associated with the ferromagnetic phase transition at T_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_2-xMn_xB₂ 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_B for x=0 to only 0.07 μ_B for x=1.6 (Table 1). FIG. 4A is a graph depicting the temperature dependence of magnetic susceptibility for AlFe_2-xMn_xB₂ 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_2-xMn_xB₂ measured at 1.8 K.

Conclusions

The series of solid solutions AlFe_2-xMn_xB₂, whose structure contains 2-D [Fe_2-xMn_xB₂] 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_2-xMn_xB₂ series becomes most pronounced in AlFe₂B₂, 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

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

Materials and Methods

Synthesis

All manipulations during sample preparation were carried out in an argon-filled dry box (content of O₂<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 H₂ gas for 5 h at 775 K Fused-silica tubes were obtained from National Scientific Corporation, Inc. (Quakertown, Pa.). Phase-pure AlFe₂B₂ 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_2-xMn_xB₂ (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 (<10⁻² 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 Al₁₃Fe₄ 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 Al₁₃Fe₄. The removal of this byproduct by treatment with dilute HCl, however, was impossible, because AlMn₂B₂ reacted with acid swiftly.

Since all bulk samples of AlMn₂B₂ were contaminated with a trace amount of Al₁₀Mn₃, single crystals of AlMn₂B₂ 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 (<10⁻² 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 AlMn₂B₂ (maximum size ˜0.4×0.2×0.02 mm³), as well as small amounts of byproducts, AlB₂ and Al₅₇Mn₁₂, and traces of Al. The crystals of AlMn₂B₂ could be easily distinguished upon visual inspection of the sample and were picked up manually for further measurements.

X-ray Diffraction

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 AlMn₂B₂ 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).

Physical Measurements

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.

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.

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

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.

Claims

1. A series of solid solutions having the general formula: AlFe2-xMnxB2, wherein x has a value between 0.1 and 1.9.

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, and any combination thereof, wherein the value of x may vary by +/−0.06.

3. A solid solution having the general formula: AlFe2-xMnxB2, wherein x is between 0.1 and 1.9.

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.