Tailoring magnetocaloric effects in 2D transition-metal phthalocyanines via halogen substitution

The magnetocaloric effect (MCE) offers compact low-temperature refrigeration for quantum and energy technologies, but practical use requires affordable, synthesizable materials with high performance. Here, first-principles calculations and atomistic spin simulations identify halogen substitution as an effective route to tune two-dimensional transition-metal phthalocyanine frameworks (M-XPc; M = Cr, Mn; X = F, Cl, Br, I). Halogen atoms induce lattice expansion and charge redistribution, driving antiferromagnetic-to-ferromagnetic transitions and modifying magnetic anisotropy. This chemical control produces inverse, rotating, and conventional MCE within one rare-earth-free material family. Cr-BrPc shows a large inverse MCE, with a maximum magnetic entropy change of –9.04 µJ m −2 K −1 (–6.21 J kg −1 K −1 ) under a 5 T field change, and a rotating entropy change of 8.73 µJ m −2 K −1 (5.91 J kg −1 K −1 ). Mn-IPc exhibits enhanced conventional MCE near the hydrogen-liquefaction temperature range, reaching 5.47 µJ m −2 K −1 (2.88 J kg −1 K −1 ) and an adiabatic temperature change of 4.65 K. These results establish halogenated MPc frameworks as tunable rare-earth-free candidates for cryogenic magnetic refrigeration.