AlFe_2–<i>x</i>Co_<i>x</i>B₂ (<i>x</i> = 0–0.30): <i>T</i>_C Tuning through Co Substitution for a Promising Magnetocaloric Material Realized by Spark Plasma Sintering

AlFe₂B₂ and AlFe_2–<i>x</i>Co_<i>x</i>B₂ (<i>x</i> = 0–0.30) were synthesized from the elements in three different ways. The samples were characterized by powder X-ray diffraction, Rietveld refinements, energy-dispersive X-ray spectroscopy, and magnetic measurements. Using Al flux the formation of AlFe₂B₂ single crystals is preferred. Arc melting enables the substitution of ∼6% Co. This substitution of Fe by Co decreases the Curie temperature <i>T</i>_C from 290 to 240 K. The highest Co substitution up to 15% is achieved by spark plasma sintering (SPS). <i>T</i>_C is reduced to 205 K. In all cases an excess of Al is necessary to avoid the formation of ferromagnetic FeB. Al₁₃Fe_4–<i>x</i>Co_<i>x</i> is the common byproduct. <i>T</i>_C and the cobalt content are linearly correlated. The transition paramagnetic–ferromagnetic remains sharp for all examples. The magnetic entropy change of the Co-containing samples is comparable to AlFe₂B₂. SPS synthesis yields, in short reaction times, a homogeneous and dense material with small amounts of paramagnetic Al₁₃Fe_4–xCo_<i>x</i> as an impurity, which can serve as sinter additive. These properties make AlFe_2–<i>x</i>Co_<i>x</i>B₂ a promising magnetocaloric material for applications between room temperature and 200 K