The magnetocaloric effect (MCE) describes the reversible adiabatic temperature change in a magnetic material through exposure to a changing magnetic field. This is the basis of magnetic refrigeration, analogous to compression cooling, where an applied magnetic field is used as the driving force. Magnetic refrigeration is an environmentally friendly cooling method which boasts higher energy efficiencies than traditional vapor-compression cooling. However, due to economic and geopolitical reasons, there is a growing need in the scientific community to identify and explore inexpensive functional magnetic materials systems which are comprised entirely of earth-abundant constituents (Lewis, L. H. & Jiménez-Villacorta, F. Metall. Mater. Trans. A 44, 2–20 (2012)). To this end, 1-2-2 type intermetallic boride, AlFe2B2, is potentially of interest as a novel rare-earth-free magnetocaloric material for room temperature thermal management applications, providing the opportunity for impactful research concerning energy usage and transformation.
The AT2X2 (A = Al, Ga, Ge; T = Mn, Fe, Ni, Co; X = B, C) system crystallizes in the prototypical AlMn2B2 Cmmm-type orthorhombic structure, as seen in Figure 1. This compound adopts a layered morphology consisting of alternating planes of Al monolayers and Fe-B bilayers, the latter of which consist of 2D zigzag chains that combine to form arrays of T-B polyhedra. At room temperature AlFe2B2 demonstrates itinerant ferromagnetism with a saturation magnetization (Ms) of ~ 1 μB per Fe atom at T = 4.2 K and a Curie temperature (Tc) of 290 K, Figure 2. In the vicinity of the Curie transition, it exhibits an isothermal magnetic entropy change (ΔSmag) and an adiabatic temperature change (ΔTad) of 4.1 J kg−1K−1and 2.2 K respectively.
Figure1. AlFe2B2 prototypical Cmmm-type orthorhombic unit cell consisting of two dimensional Al monolayers which act as spacers between Fe-B layers.
Figure 2. M(T) curve at 2 T upon heating through the phase transition (Tt ~ 290 K)
Prior to our efforts the mechanism underlying the observed magnetocaloric effect was not well understood. Correlations made between crystal structure, microstructure, composition, magnetism, and thermal behavior provide insight into the fundamental driving forces underlying the magnetocaloric response of AlFe2B2, and related transition-metal borides. Contributions of this work include developing trends (Lejeune, B.T., Jensen, B.A., Barua, R., Stonkevitch, E., McCallum, R.W., Kramer, M.J. and Lewis, L.H., 2019. Lattice-driven magnetic transitions in Al (Fe, T) 2X2 compounds. Journal of Magnetism and Magnetic Materials, 481, pp.262-267) for tuning the magnetic transition temperature (260 K < TC < 320 K) in the AT2X2 system through compositional modification and describing the lattice-magnetism interactions, Figure 3, resulting from these substitutions. Additionally, through Bridgman single crystal growth and characterization of AlFe2B2 compound at the Ames Laboratory as part of a DOE Science Graduate Research Fellowship (SCGSR), it was identified that minor Al-Fe antisite substitution was possible and that very minor substitutions (~2 at. %) led to significant changes in the magnetic phase transition temperature (280 K < TC < 315 K) and observed magnetocaloric effect (2.3 < ΔS < 4.0 J/mol-K), as shown in Figure 4 (Lejeune, B.T., Schlagel, D.L., Jensen, B.A., Lograsso, T.A., Kramer, M.J. and Lewis, L.H., 2019. Effects of Al and Fe solubility on the magnetofunctional properties of AlFe 2 B 2. Physical Review Materials, 3(9), p.094411). The anisotropic transport properties of this compound were also investigated in crystallographically oriented samples including the thermal conductivity, electrical resistivity, and Seebeck coefficient as a function of temperature.
Figure 3. Trends in the AlT2B2 (T = Mn, Fe, Ni, Co) system showing the effect of transition metal interatomic distance on the magnetic phase transition temperature
Figure 4. Enhancement in the magnetocaloric effect of AlFe2B2 is observed in Fe-enriched samples relative to their Al-rich and stoichiometric counterparts